Preset control loop for PPG LED driver

Abstract

The embodiment of the invention relates to a preset control loop for a PPG LED driver. According to an embodiment, a driver circuit for a light emitter in a photoplethysmography (PPG) system includes a main control loop having an operational amplifier and a transistor configured as a source follower. During the preset control stage, the preset circuit sets the output voltage of the operational amplifier to be a preset level. The switching circuit activates the preset circuit in a preset phase and activates the main control loop in a main control phase. Before the main control loop is activated, the preset circuit pre-regulates the gate voltage of the source follower transistor to be close to its threshold voltage. This improves startup performance by reducing delay and uncontrolled initial current while avoiding the need for amplifier trimming. The circuit can realize fast stabilization time and accurate control of short current pulse at a very low duty ratio, and is beneficial to PPG measurement accuracy and power efficiency in wearable health care monitoring equipment.

Description

Technical Field

[0001] This disclosure generally relates to electronic devices, and in certain embodiments, to preset control loops for light-emitting diode (LED) drivers. Background Technology

[0002] Photoplethysmography (PPG) is a widely used non-invasive optical technique for detecting changes in blood volume in the microvascular bed of tissues. It is commonly used in healthcare monitoring devices to measure physiological parameters such as heart rate, oxygen saturation, and pressure. PPG technology relies on the principle that blood absorbs more light than surrounding tissue; therefore, changes in blood volume affect the transmission or reflection of light.

[0003] In PPG measurements, a light-emitting diode (LED) illuminates skin tissue, while a photodiode detects small changes in light intensity caused by changes in blood volume. Measurements typically involve short light pulses with very low duty cycles to conserve power, as many PPG devices are battery-powered. The LED driver circuitry responsible for controlling these light pulses plays a crucial role in the accuracy and efficiency of PPG measurements.

[0004] One of the challenges of PPG technology is handling ambient light, which can interfere with optical measurements. PPG systems often incorporate ambient light compensation (ALC) techniques to address this issue. This can involve performing additional optical measurements immediately before and after the main PPG sampling to detect and eliminate the effects of ambient light. Summary of the Invention

[0005] Technical advantages are generally achieved through embodiments of this disclosure, which describe preset control loops for light-emitting diode (LED) drivers.

[0006] The first aspect relates to a system for photoplethysmography (PPG) measurements. The system includes: a light emitter; a photodetector configured to detect light from the light emitter; and a driver circuit coupled to the light emitter, the driver circuit including: a main control loop comprising: an operational amplifier and a first transistor configured as a source follower; a preset circuit coupled to the operational amplifier and configured to set a voltage at the output of the operational amplifier to a preset voltage during a preset control phase; and a switching circuit configured to activate the preset circuit during the preset control phase and activate the main control loop during the main control phase.

[0007] The second aspect relates to a driver circuit for a light emitter in a photoplethysmography (PPG) system. The driver circuit includes: an operational amplifier; a first transistor configured as a source follower, the gate of the first transistor being coupled to the output of the operational amplifier; a preset circuit configured to set a voltage at the output of the operational amplifier to a preset voltage during a preset control phase; and a switching circuit configured to: couple the preset circuit to a non-inverting input of the operational amplifier during the preset control phase, and activate a main control loop during a main control phase by coupling a reference voltage to the non-inverting input of the operational amplifier.

[0008] The third aspect relates to a circuit for driving a light emitter in a photoplethysmography (PPG) system. The circuit includes: a main control loop including an operational amplifier and a first transistor configured as a source follower; a preset circuit configured to set a voltage at the output of the operational amplifier to a preset voltage during a preset control phase, the preset circuit including a second transistor arranged in a diode configuration; and a switching circuit configured to: activate the preset circuit during the preset control phase by coupling the preset circuit to a non-inverting input of the operational amplifier, and activate the main control loop during the main control phase by decoupling the preset circuit and coupling a reference voltage to the non-inverting input of the operational amplifier.

[0009] The implementation can be carried out in hardware, software, or any combination thereof. Attached Figure Description

[0010] To gain a more complete understanding of this disclosure and its advantages, the following description is now taken in conjunction with the accompanying drawings, in which:

[0011] Figure 1 This is a block diagram of an embodiment system for photoplethysmography (PPG) measurements;

[0012] Figure 2 This is a graph of an example electrical signal from the detector;

[0013] Figure 3 This is a simplified schematic diagram of an example driver circuit;

[0014] Figure 4 This is a timing diagram of an embodiment of the PPG measurement system;

[0015] Figure 5 This is a block diagram of an embodiment of the driver circuit; and

[0016] Figure 6 It is a timing diagram for the pulses and sampling time of the driver circuit. Detailed Implementation

[0017] This disclosure provides numerous applicable inventive concepts that can be embodied in a wide variety of specific contexts. Specific embodiments are merely illustrative of particular configurations and do not limit the scope of the claimed embodiments. Unless otherwise indicated, features from different embodiments may be combined to form other embodiments. Various embodiments are illustrated in the accompanying drawings, wherein identical parts and elements are identified by the same reference numerals, and repeated descriptions are omitted for brevity.

[0018] The variations or modifications described in one embodiment may also be applied to other embodiments. Furthermore, various changes, substitutions, and alterations may be made without departing from the spirit and scope of this disclosure as defined by the appended claims.

[0019] While aspects of the invention have been described primarily in the context of photoplethysmography (PPG) measurements, it should be understood that these aspects are generally applicable to light-emitting diode (LED) drivers as well. Specifically, aspects of this disclosure can be similarly applied to other applications requiring precise control of short current pulses with extremely low duty cycles.

[0020] In the embodiments, a method is proposed to improve the performance of LED drivers used in photoplethysmography (PPG) applications to enhance the startup phase of LED driver circuitry for accurate and efficient PPG measurements.

[0021] This disclosure presents a pre-conditioning control loop or preset loop circuit to address the limitations of conventional current sink topologies used in LED drivers. In conventional designs, the main control loop is inactive when the output current is zero, creating potential problems during the startup phase. These problems can include delayed response time and uncontrolled initial current, potentially leading to electromagnetic interference (EMI).

[0022] The pre-conditioning circuit operates by setting the gate voltage of the output stage transistors closer to their threshold voltage before the main control loop is activated. Pre-conditioning allows for a faster and more controlled LED current rise time upon startup. This proposal achieves this without requiring complex trimmings of the operational amplifier (op-amp), which would otherwise increase circuit complexity, area, and test time.

[0023] By implementing a pre-set circuit, LED drivers offer several advantages. They provide a faster settling time for the LED current, which can be particularly beneficial for ambient light compensation in PPG systems to reduce the latency of the driver circuitry. More controlled startup also reduces the likelihood of EMI, thereby improving overall system performance. The system and circuitry also allow for efficient operation at higher output data rates (ODRs) without a significant increase in power consumption.

[0024] The disclosed proposal can be integrated into system-on-chip (SoC) solutions for healthcare monitoring devices utilizing PPG technology. It balances performance improvements with design simplicity, potentially leading to more accurate and power-efficient PPG measurements across a variety of applications. These and other details are further elaborated below.

[0025] Figure 1 A block diagram of an embodiment system 100 for photoplethysmography (PPG) measurements is shown. System 100 includes a processor 102, a memory 104, a sensor 106, a power supply unit (PSU) 108, and an interface 110, which may (or may not) be arranged as shown. Although in Figure 1 One component of each component (i.e., processor 102, memory 104, sensor 106, power supply unit 108, and interface 110) is shown, but the number of components is not limiting, and a similarly larger number is contemplated in other embodiments.

[0026] System 100 may include additional components not shown, such as long-term storage devices (e.g., non-volatile memory), power management circuitry, security and encryption modules (e.g., trusted platform modules (TPM)). System 100 may be an electronic device, such as a smartwatch, fitness tracker, medical device (e.g., pulse oximeter), wristband, exercise band, smart ring, earbud, or any device capable of housing sensor 106.

[0027] In this embodiment, each component can communicate with any other component inside or outside the system 100. For example, each component can communicate using the I2C (Inter-Integrated Circuit) communication protocol (alternatively referred to as I2C or IIC), the I3C (Improved Inter-Integrated Circuit) communication protocol, the Serial Peripheral Interface (SPI) specification, etc.

[0028] Processor 102 can be any component or collection of components suitable for performing computational or other processing-related tasks. In embodiments, processor 102 is an application processor, baseband processor, or microcontroller. In embodiments, processor 102 is configured to provide control signals to timing circuitry 124 to control the timing and detection of light emission by transmitter 120 and detector 122.

[0029] Memory 104 may be any component or collection of components suitable for storing programming, instructions, or calibration settings executed or retrieved by processor 102. In one embodiment, memory 104 includes a non-transitory computer-readable medium.

[0030] Sensor 106 can be any component or assembly of components suitable for PPG measurement. In an embodiment, sensor 106 includes a transmitter 120, a detector 122, a timing circuit 124, a PPG circuit 126, and a driver 128, which may (or may not) be arranged as shown. Sensor 106 may include additional components not shown, such as integrated memory and a dedicated microcontroller.

[0031] Generally, sensor 106 can be configured in either a transmission or reflection mode. In transmission mode, emitter 120 and detector 122 are placed on opposite sides of the measurement site (e.g., fingertip or earlobe), and detector 122 measures light that has passed through the tissue. In reflection mode, emitter 120 and detector 122 are placed on the same side of the measurement site, and detector 122 measures light reflected from the underlying tissue.

[0032] Emitter 120 is configured to emit light of a specific wavelength toward the skin to penetrate the skin and tissue. As the emitted light passes through the skin, tissue, and blood vessels, some of the light is absorbed, while the remainder is scattered or reflected. Due to the cardiac cycle, the amount of light absorbed varies with the pulsation of blood volume. This variation in absorption caused by the changing blood volume (due to the heartbeat) alters the intensity of light transmitted through or reflected from the tissue. This change forms the basis of the PPG signal. Emitter 120 can be, for example, a light-emitting diode (LED) or a laser diode.

[0033] Detector 122 is configured to capture light that passes through (transmission mode) or is reflected (reflection mode) from body tissue. Once the light reaches detector 122, it is converted into an electrical signal using the photoelectric effect, where photons striking detector 122 cause electrons to be released, resulting in a measurable current. Detector 122 may include, for example, a photodiode or photodetector sensitive to a specific wavelength of light emitted by emitter 120.

[0034] Timing circuit 124 is configured to synchronize the light emission of transmitter 120 and the light detection of detector 122. In one embodiment, timing circuit 124 may include a dedicated memory and controller to operate sensor 106. However, in another embodiment, the processor 102 and memory 104 of the host device may be used to control the operation of sensor 106.

[0035] PPG circuit 126 is configured to receive a differential electrical signal from detector 122, generate compensation currents for the DC component and ambient light component of the electrical signal, amplify the electrical signal, and isolate the PPG signal from unrelated signals or noise to improve the signal-to-noise ratio.

[0036] Driver 128 is configured to control the operation of transmitter 120. In an embodiment, driver 128 is implemented as a current sink, where an operational amplifier and a source follower are used as the output stage to implement the control loop. Driver 128 manages current pulses (typically short pulses) and corresponding duty cycles (typically very low), controlling the intensity and timing of the light emitted by transmitter 120. When inactive, driver 128 is typically turned off to save power. To improve startup performance and reduce electromagnetic interference (EMI), driver 128 may be combined with a pre-conditioned control loop. In an embodiment, the pre-conditioned loop sets the gate of the voltage follower closer to the threshold voltage, thereby allowing the main control loop to be activated more quickly when transmitter 120 needs to be activated for PPG measurements.

[0037] PPG circuit 126 includes a digital-to-analog converter and an internal variable current generator (IDAC) that generates current to compensate for ALC components.

[0038] Once detector 122 converts light into an electrical signal, PPG circuit 126 converts the analog electrical signal into a digital signal, which is processed and analyzed, for example, by processor 102 of the host device. Simultaneously, PPG circuit 126 is configured to provide compensation current to remove unwanted ambient light (ALC) and DC components from the PPG signal, as will be described in further detail below.

[0039] In one embodiment, processor 102 receives data from sensor 106, interprets it, and converts it into usable biometric information, such as heart rate, heart rate variability, blood oxygen saturation (SpO2), and blood pressure trends. In various embodiments, processor 102 is configured to alert the user to anomalies related to PPG measurements via interface 110. Processor 102 may apply signal processing algorithms to refine the data, compensating for factors such as ambient light noise or variations in object reflectivity to provide more reliable information.

[0040] The power supply unit 108 can be any component or collection of components that supplies power to one or more components within the system 100. The power supply unit 108 may include various power management circuit devices, charge storage components (i.e., batteries), etc.

[0041] Interface 110 can be any component or set of components that allows processor 102 to communicate with other devices / components or users.

[0042] Figure 2The diagram illustrates a graph of an example electrical signal 200 from detector 122. Following the photoelectric detection process, the electrical signal 200 generated by detector 122 represents various components of the light intensity interacting with blood flow in the tissue. As blood is pumped through the blood vessels by the heart, it causes pulsating changes in the volume of blood within the tissue. These changes modulate the intensity of the light received by detector 122, resulting in an electrical signal with both time-varying (alternating current, AC) and constant (direct current, DC) components.

[0043] The AC component 202 of the electrical signal 200 is of particular interest because it directly corresponds to the pulsatile changes in blood volume—essentially, it reflects the rhythmic beating of the heart. The AC component 202 is typically relatively small relative to the electrical signal 200, usually representing less than 1% of the total detected signal; however, it carries the information needed to assess cardiovascular health and other physiological parameters.

[0044] On the other hand, the DC component 204 represents the baseline light absorption of the non-pulsating elements of the electrical signal 200, including tissue, skin, bone, and non-pulsating blood. The DC component 204 is typically much larger than the AC component 202, but does not carry information about heart pulsation. To isolate the AC component 202 of the electrical signal 200 and improve measurement accuracy, the PPG circuit 126 can be configured to cancel or minimize the effect of the DC component 204.

[0045] Ambient light component (ALC) 206 also contributes to electrical signal 200. Ambient light component 206 includes external light from the environment that is not generated by transmitter 120 but still reaches detector 122. Ambient light component 206 can introduce measurement error because it can vary with ambient lighting conditions and can add noise to electrical signal 200. To isolate the AC component 202 of electrical signal 200 and improve measurement accuracy, PPG circuit 126 can be configured to cancel or minimize the influence of ambient light component 206.

[0046] Figure 3 A simplified schematic diagram of an example driver circuit 300 is shown, which can be implemented as driver 128 in system 100. Driver circuit 300 includes a current source 302 and a reference resistor (R... REF 304. Filter capacitor (C) FILTER 306, Operational amplifier 308, First switch (SW1) 310, Second switch (SW2) 312, First diode (D1) 314, First inductor (L1) 316 (i.e., the parasitic inductance of the path connection to the first diode (D1) 314), First transistor (Q1) 318, Digital analog resistor (R DACThe driver circuit 300 includes a second diode (D2) 320, a third switch (SW3) 322, a fourth switch (SW4) 324, a second diode (D2) 326, a second inductor (L2) 328 (i.e., the parasitic inductance of the path connection to the second diode (D2) 326), a second transistor (Q2) 330, and an optional controller 350, which may (or may not) be arranged as shown. The driver circuit 300 may include additional components not shown, such as an integrated microcontroller, a digital signal processor, a memory, etc.

[0047] In this embodiment, the driver circuit 300 is characterized by having a first output (OUT). A ) and second output (OUT) B The dual-output configuration is shown. It should be noted that the dual-output configuration is a non-limiting example, and fewer or more outputs may be expected in other embodiments.

[0048] This configuration allows for flexible operation because the driver circuit 300 can selectively and independently drive either output. For example, this type of configuration is particularly useful when two different LEDs need to be driven. The versatility of the driver circuit 300 lies in its ability to be driven at the first output (OUT) based on specific application requirements. A ) and the second output (OUT) B The ability to choose between ( ).

[0049] The operation of the driver circuit 300 can be characterized as similar to a conventional current sink. At the reference voltage node (V REF A voltage pulse is created at the inverting input of the operational amplifier 308. The operational amplifier 308, through a first transistor (Q1) 318 arranged as a voltage follower or source follower, reports the reference voltage node (V0) at the inverting input of the operational amplifier 308. REF The voltage at the feedback node (V) of the inverting input of operational amplifier 308 is the feedback voltage at the feedback node. FEED ) is coupled to a digital-to-analog resistor (R DAC The source terminals of transistors 320 and 318 (Q1).

[0050] This configuration forms a feedback loop that maintains the voltage at the source terminal of the first transistor (Q1) 318 equal to the reference voltage node (V). REF The voltage at the reference voltage node (V) effectively reduces the voltage at the reference voltage node. REF The voltage pulse at point ) is converted through a digital analog resistor (R) DAC 320 ohms current.

[0051] The driver circuit 300 effectively converts the reference voltage node (V) REFThe voltage pulse at the output is converted into a current pulse. The current pulse is generated outside the load, specifically at the drain terminal of the first transistor (Q1) 318, which is coupled to the first diode (D1) 314 and the first parasitic inductor (L1) 316. This configuration allows for precise control of the current delivered to the output.

[0052] In an embodiment, the driver circuit 300 provides a multiplier to adjust the output current. For example, one method involves modifying the digital-to-analog resistor (R... DAC The value of 320. By changing this resistor, the driver circuit 300 can directly affect the first output (OUT). A The magnitude of the output current at the reference voltage node (V). Alternatively, this can be achieved by changing the reference voltage node (V). REF The output current is adjusted by the voltage at the reference resistor (R). REF 304 and digital analog resistors (R DAC )320 can be a variable resistor that is adjustable using, for example, controller 350.

[0053] For example, in an embodiment, the reference voltage node (V REF The voltage can be used for fine-tuning or adjustment purposes. Simultaneously, a digital variable resistor (R) can be used. DAC The baseline output current is set using a value of 320. However, it's important to note that other methods can also be used for current adjustment. For example, to increase the amplitude of the output signal, the digital variable resistor (R) can be decreased. DAC A 320Ω resistor is used to enhance the current level. The flexibility of current control allows for precise output customization to meet specific application requirements.

[0054] The pulse generation in the driver circuit 300 is mainly composed of current source 302 and reference resistor (R). REF )304 and filter capacitor (C FILTER The interaction control of 306. Current source 302 is coupled to a regulated power supply (V). REG The controller 350 provides control signals to set the operation of the current source 302. In embodiments, the control signals may be generated by an external or internal controller (e.g., controller 350) synchronized with the operation of system 100. Reference resistor (R) REF )304 and filter capacitor (C FILTER The 306 is arranged as an RC filter, which helps determine the ramp behavior of the pulse. Reference resistor (R) REF )304 and filter capacitor (C FILTER Each of the 306 has a reference voltage node (V REFThe first terminal of the current source 302 is coupled to the output of the current source 302 and the non-inverting input of the operational amplifier 308.

[0055] These components work together to provide a reference voltage node (V REF The characteristics of the output pulse at point ( ) are shaped. The initial input for pulse generation can be provided by a digitally controlled current source 302. This is achieved by a reference resistor (R). REF )304 and filter capacitor (C FILTER The RC filter formed by 306 modifies the input, thereby defining the specific ramp characteristics of the pulse. In this arrangement, the filter capacitor (C... FILTER The 306 component smooths the pulse slope. Smoothing helps reduce abrupt changes in pulse shape, potentially minimizing electromagnetic interference and improving the overall quality of the output signal. The combination of these components allows for precise control over the pulse shape, enabling the circuit to generate well-defined current pulses suitable for applications such as LED driving in PPG measurements.

[0056] The output of operational amplifier 308 is coupled to the gate terminal of first transistor (Q1) 318 via a first switch (SW1) 310. A second switch (SW2) 312 is coupled between ground and the gate terminal of first transistor (Q1) 318. In this embodiment, first transistor (Q1) 318 is implemented as an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET). The drain terminal of first transistor (Q1) 318 is coupled to an external voltage (V) via a first diode (D1) 314 and a first inductor (L1) 316 arranged in series. BATT ).

[0057] The driver circuit 300 supports various digital interfaces, allowing for easy programming and adjustment of operating parameters. The driver circuit 300 offers significant flexibility in its operation, allowing for dynamic configuration based on specific application requirements and operating modes. The current source 302 can be set by an external source to adapt to various input conditions. The output current is configurable, enabling fine-tuning under different conditions to achieve optimal performance.

[0058] Such as current source 302, reference resistor (R) REF 304 and digital analog resistors (R DACThe components of driver circuit 320 can be controlled by controller 350, which may be an integrated digital signal processor (DSP). Controller 350 may be internal to driver circuit 300 or integrated within host device or system 100. This arrangement allows for dynamic adjustment of various parameters, enabling real-time control of the driver circuit's output. In embodiments, driver circuit 300 incorporates programmable features to enhance its versatility. For example, if a change in current level is required during operation, this can be achieved through in-chip programming. This functionality may require a minimal amount of memory and DSP capabilities.

[0059] Reference voltage node (V) REF The pulse generation at the point can be synchronized with the overall system operation. For example, controller 350 can manage the sampling operation of the first cycle. This means that the current pulses during PPG sampling can be controlled by controller 350, which also manages the synchronization between sampling events. This integrated approach ensures that the function of driver circuitry 300 is precisely timed and coordinated with broader system requirements.

[0060] In one embodiment, the driver circuit 300 can maintain zero output during the off-time period. This can be achieved using a first switch (SW1) 310 and a second switch (SW2) 312. These switches are used to ground the gate of the first transistor (Q1) 318 during the OFF (off) mode. The operation of these switches can be synchronized with the entire system, thereby ensuring that, for example, the first output (OUT) is grounded by grounding the gate terminal of the first transistor (Q1) 318 during the OFF (off) time of the cycle. A The output of the driver circuit at point ) remains at zero.

[0061] It is important to note that dual-output functionality is an optional feature of the driver circuit 300. With the second output (OUT) B The associated components, namely the third switch (SW3) 322, the fourth switch (SW4) 324, the second diode (D2) 326, the second inductor (L2) 328, and the second transistor (Q2) 330, are not essential for the essential operation of the driver circuit 300. In embodiments requiring only a single output, these components can be omitted, resulting in a focus solely on driving the first output (OUT). A A simpler circuit. Although this description is based on the first output (OUT) A The operation can be performed, but it should be understood that it can be a second output (OUT). B Set up similar operations.

[0062] Figure 4A timing diagram 400 of an embodiment of a PPG measurement system is illustrated. As shown, timing diagram 400 illustrates the operation and sampling process of driver circuit 300, which may correspond to driver 128 of system 100. Timing diagram 400 depicts two measurement cycles representing the periodic nature of the PPG sampling process.

[0063] The total cycle time (T) 402 represents the time from the start of one PPG sampling period to the start of the next PPG sampling period. The periodic nature allows for continuous monitoring while maintaining low-load cyclic operation. The driver circuit 300 generates periodic current pulses, which typically have a low duty cycle of 1% to 10%. The off-time between pulses (i.e., the OFF period (TX)... OFF During (404), the transmitter is shut off via driver circuit 300 to save power and improve overall efficiency. This power-saving strategy is particularly beneficial in energy-intensive applications such as battery-operated devices.

[0064] The operation of the driver circuit 300 involves turning it ON and OFF to generate each pulse. Periodic activation helps create the desired pulse pattern. The sampling process associated with these pulses is divided into two distinct regions: PPG (photoplethysmography) sampling, which occurs during the pulse duration and during which PPG measurements are being performed; and ALC (ambient light compensation) sampling, which occurs before and after the main PPG sampling. The activation periods can be characterized by short, well-defined current pulses, allowing for precise control of the light emission from the PPG readings. The proximity of these ALC samples to the PPG samples allows for effective elimination and more accurate compensation of ambient light interference by capturing ambient light conditions as close as possible to those of the actual PPG measurement.

[0065] To optimize performance, it is advantageous to maintain zero current flowing through the driver circuit 300 outside the PPG sampling period. This condition ensures that the pulse is precisely zero when no PPG data is actively sampled. Achieving this zero-current state outside the PPG sampling duration allows for accurate measurement and effective power management.

[0066] Besides maintaining zero current outside the sampling period, another consideration is the settling time of the driver circuit 300. A fast settling time is advantageous, allowing for more efficient power utilization. Minimizing the settling time makes it possible to reduce the interval between the PPG and ALC sampling periods. Reducing the transition time improves the overall system performance and enables more frequent or accurate measurements without significantly increasing power consumption.

[0067] Combining these features—zero current outside the sampling period and fast settling time—allows for an optimized balance between accurate PPG measurement, effective ambient light compensation, and efficient power management. This design approach is particularly valuable in applications where both measurement accuracy and power efficiency are critical, such as wearable health monitoring devices or other battery-operated PPG systems.

[0068] The driver circuit 300 offers a wide range of customization options to suit various operational requirements. For example, users can specify the sampling frequency used for PPG measurements, such as setting it to 100 Hz. User-defined parameters can be programmed into the system, and subsequently, the entire system is synchronously parameterized to accommodate this selection.

[0069] Another customizable aspect is the number of PPG samples acquired during the on-time and the number of ALC samples acquired during the off-time. For example, the user can select eight PPG samples and eight ALC samples; these numbers are programmable parameters. If the user decides to increase the signal-to-noise ratio by averaging the eight samples, the system can automatically adjust the length of the sampling period or pulse duration to suit the needs. During this phase, the receiver circuitry remains fully active, typically turning on shortly before the first sampling to optimize power consumption.

[0070] Operation of the driver circuit 300 can begin after this initial setup, where all synchronization is based on parameters defined within the system configuration parameters. These parameters may include the number of averages, periodic repetition time, current level, and other factors. System 100 also provides the flexibility to use different light sources and receivers for repeated operation to obtain ratio measurements between photodiodes or LEDs.

[0071] Although many parameters can be adjusted during operation, users typically select all configuration parameters and input them directly into System 100. The resulting data is often provided to the user externally and is usually processed by the microcontroller. The data may undergo further post-processing to extract additional information, such as through Fast Fourier Transform (FFT) analysis or other techniques.

[0072] The highly configurable approach allows the system to be customized to specific application needs while maintaining effective operation and synchronization across all components. It allows users to fine-tune the PPG measurement process to their specific requirements, balancing signal quality, power consumption, and measurement complexity.

[0073] The driver circuit 300 operates in periodic switching cycles, which introduces certain limitations. When the gate voltage (V...) GATE The main challenge arises during the start-up of each pulse when the current is zero and the control loop is inactive. At this time, no current flows through the digital analog resistor (RA). DACWhen the source of the first transistor (Q1) 318 is in the case of 320 and the first transistor (Q1) 318, the first transistor (Q1) 318 is completely turned off, and the feedback voltage (V) in the loop to the operational amplifier 308 is... FEED The current in the first transistor (Q1) 318 is essentially zero, and therefore the current in the first transistor (Q1) 318 is essentially zero. In this case, the control loop is practically non-existent.

[0074] This situation creates a delay in the response. When the reference voltage (V) REF When the ramp begins, the feedback voltage (V) FEED The voltage will not immediately follow the rise. Instead, it remains stationary for a few microseconds, resulting in a delay. This is because of the low initial reference voltage (V). REF A very small current is generated through the digital analog resistor (R). DAC ) 320, so a delay occurs. This keeps the voltage follower current minimal and effectively turns off the loop. When the gate voltage (V GATE When sufficient feedback is generated at the point, current begins to flow, thus allowing the feedback voltage (V) to be generated. FEED Start tracking the reference voltage (V) REF ).

[0075] The behavior of the driver circuit 300 can be understood through the transfer function of the source follower. This transfer function can be expressed as: In this equation, Indicates gain (i.e., G0-DC GAIN). Terminology express zero( -zero), and express Principal pole ( -MainPole). and These are the equivalent resistance and capacitance at the source of the source follower, obtained from the DAC input admittance. Output voltage. It is the voltage at the source terminal of the source follower, the input voltage. It is the gate voltage of the source follower. Coupled between the gate and source of the source follower, and It is the transconductance of a transistor.

[0076] When the output current (I) OUT When ) equals zero, transconductance The value is zero, causing the source follower's gain to become zero. Therefore, the control loop does not follow the reference voltage (V) during startup. REF This results in the delay observed in the circuit response.

[0077] The second challenge in the driver circuit 300 arises from the potential offset in the operational amplifier 308. This offset is particularly problematic because it forces the inverting input of the operational amplifier 308 higher than its non-inverting input, resulting in a feedback voltage (V0). FEED Exceeding the reference voltage (V) REF ).

[0078] This situation can create problems in the operation of the driver circuit 300 in system 100. For example, when the feedback voltage is lower than the reference voltage (V) due to offset. REF At a voltage 10 millivolts higher, it can achieve a feedback voltage (V FEED The remaining load current is derived before the expected ramp-up. The effect of this offset can be quite significant, as even a small difference of ten millivolts can generate several milliamps of current during circuit startup, when the first switch (SW1) 310 or the third switch (SW3) 322 is turned on, due to the gate voltage (V GATE This can result in potentially uncontrolled high values. For example, if the gate voltage is forced to zero during OFF turn-off operation, the driver can slow down at a higher latency. As another example, if the gate voltage is not forced to zero due to offset and remains at the last operating point (e.g., to accelerate circuit turn-on), the gate voltage value can exceed the final value, resulting in higher current spikes and thus increased EMI.

[0079] Unwanted current can result in incorrect ALC sampling. This introduces an unwanted component that causes error in ALC measurements, potentially harming the accuracy of the entire system. A conventional approach to this problem involves trimming the operational amplifier 308. However, this solution requires additional circuitry and increases test time, manufacturing complexity, and cost. Alternatively, ALC sampling must be shifted upwards, increasing operating time and power consumption.

[0080] Embodiments of the present invention provide a solution that eliminates the need for trimming circuitry and the associated testing time. Advantageously, the proposed solution reduces circuit footprint and circuit and system testing time.

[0081] Figure 5 A block diagram of an embodiment of driver circuit 500 that can be implemented as driver 128 in system 100 is shown. Figure 6 The timing diagram 600 illustrates the pulse and sampling time of the driver circuit 500.

[0082] For the sake of brevity, I will not repeat what I said before. Figure 3 The components discussed in the driver circuit 300 are... Figure 5The driver circuits 500 in the circuit are common to each other; unless otherwise stated, these common components maintain the same structure and function.

[0083] In addition to the components previously discussed in driver circuit 300, driver circuit 500 also includes preset circuit 502, fifth switch (SW5) 504, and seventh switch (SW7) 504, which may or may not be arranged as shown. Preset circuit 502 includes second current source 508, sixth switch (SW6) 510, and third transistor (Q3) 512, which may (or may not) be arranged as shown. Driver circuit 500 may include additional components not shown.

[0084] In this embodiment, the operation of the driver circuit 500 includes a preset control mode, also referred to as a preset control phase. The preset control mode combines a preset circuit 502, a second current source 508, and a third transistor (Q3) 512. The preset circuit 502 uses a regulated voltage (V... REG The derived preset reference voltage (V) PRESET To implement this, the third transistor (Q3) 512 is arranged in a simple diode configuration (i.e., a diode-connected transistor), wherein the drain of the third transistor (Q3) 512 is coupled to its gate. A preset reference voltage (V) PRESET This is related to the output stage of driver circuit 500.

[0085] In this embodiment, preset circuitry 502 is implemented using a scaled copy of the output stage operating in subthreshold mode. This configuration is selected to set the gate voltage (V0) before the start of main operation. GATE The value is set to be not exactly zero, but just below the threshold voltage (V) of the first transistor (Q1) 318. TH The value of ). By operating in the subthreshold region, the preset circuit 502 can fine-tune the initial conditions of the preset circuit 502, solving the startup delay problem discussed earlier.

[0086] The preset control mode is designed to improve the response of driver circuit 500 during the startup phase and alleviate the limitations observed in driver circuit 300. It provides a mechanism to pre-adjust driver circuit 500, specifically the gate voltage (V0.05) of the first transistor (Q1) 318, before full operation begins. GATE ).

[0087] The driver circuit 500 is configured with a faster settling time, which can be achieved through the preset circuit 502, which sets the gate voltage (V) of the source follower. GATE Set to be closer to the threshold voltage (V) TH Pre-conditioning allows for faster activation of the main control loop at the start of each PPG sampling period, resulting in a controlled and rapid rise of the LED current pulse.

[0088] The optimized driver circuit 500 offers several advantages over the driver circuit 300. It eliminates the need to trim the operational amplifier 308, which would otherwise increase circuit complexity, area, and test time. Furthermore, it avoids the drawback of simply extending the operating time, which would increase power consumption and reduce the maximum achievable output data rate (ODR).

[0089] The driver circuit 500 provides an effective solution for PPG measurement, balancing the needs for accurate sampling, ambient light compensation, low power consumption, and fast response time. The proposed driver circuit 500 and preset control modes are particularly suitable for applications in wearable health monitoring devices where power efficiency and measurement accuracy are critical.

[0090] The operation of the driver circuit 500 can be divided into different stages, such as Figure 6 As shown. The preset mode occurs between time T0 and time T2. At time T0, the switches behave as follows: when the driver is off, the first switch (SW1) 310 is open and the second switch (SW2) 312 is closed, thereby grounding the gate terminal of the first transistor (Q1) 318. At time T0, the enable signal (ENABLE)... SW6 SW604 transitions to logic high, activating the sixth switch (SW6) 510 and the seventh switch (SW7) 506.

[0091] At time T0, the preset circuit 502 is activated and coupled to the non-inverting input of the operational amplifier 308, while the main circuit components (i.e., current source 302, reference resistor (R)) are also activated. REF )304 and filter capacitor (C FILTER 306) decoupling. During the preset phase, the seventh switch (SW7) 506 configures operational amplifier 308 as a voltage follower, effectively converting the preset voltage (V) PRESET ) is transferred to the gate voltage (V GATE )602. A preset voltage is carefully set by the second current source 508 and the third transistor (Q3) 512 to maintain the output pulse (I LOAD 606 is close to 0 μA.

[0092] At time T2, the preset circuit 502 stabilizes the gate voltage (V). GATE )602 and after setting it to the subthreshold time, enable signal (ENABLE) SW6 SW604 switches back to logic low. This action deactivates the sixth switch (SW6) 510 and the seventh switch (SW7) 506, setting the output of operational amplifier 308 to subthreshold (e.g., approximately 450 mV).

[0093] When the ramp-up begins, the main control loop is activated at time T2. Preset circuit 502 is decoupled from operational amplifier 308, and the main loop is reconnected. First switch (SW1) 310 is activated, and second switch (SW2) 312 is deactivated.

[0094] The preset mechanism offers several advantages. It allows the gate voltage to start at the controlled point at time T2 very close to the operating voltage of the first transistor (Q1) 318. Furthermore, the gate voltage is appropriately set independently of any operational amplifier 308 offset conditions. This results in time savings because the feedback voltage (V... FEED The value is 610, and therefore the output pulse (I) is 610. LOAD )606 closely follows the reference voltage (V) REF 608. Furthermore, it eliminates the need for operational amplifier trimming to compensate for voltage offsets, as this can be achieved by trimming the reference resistor (R). REF )304 is used to compensate for all system errors that affect current accuracy.

[0095] Furthermore, by selecting a preset voltage (V) that can adapt to potential changes caused by the offset voltage of the operational amplifier. PRESET The driver circuit 500 becomes more robust. For example, if the target operating voltage at the first transistor (Q1) 318 is 600 mV, the preset voltage (V) will be increased. PRESET Setting it to 500 mV can absorb offset errors in the range of 10 mV to 50 mV without affecting transistor operation.

[0096] The timing of time T2 can be determined in different ways. It can be preset to a fixed duration, or it can be determined by adjusting the gate voltage (V). GATE The circuit 602 compares the threshold voltage with the threshold voltage to determine the value dynamically. In the latter case, when the gate voltage (V... GATE When the specified threshold is reached, the preset circuit is decoupled and the main circuit is engaged.

[0097] The first aspect relates to a system for photoplethysmography (PPG) measurements. The system includes a light emitter; a photodetector configured to detect light from the light emitter; and a driver circuit coupled to the light emitter, the driver circuit including: a main control loop comprising: an operational amplifier and a first transistor configured as a source follower; a preset circuit coupled to the operational amplifier and configured to set a voltage at the output of the operational amplifier to a preset voltage during a preset control phase; and a switching circuit configured to activate the preset circuit during the preset control phase and the main control loop during the main control phase.

[0098] In a first embodiment of the system, according to the first aspect itself, the preset circuit includes a current source; a second transistor arranged in a diode configuration; and a third switch coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

[0099] In a second embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the preset circuit is configured to set the voltage at the output of the operational amplifier to be close to the threshold voltage of the first transistor.

[0100] In a third embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the switching circuit includes a first switch coupled between the output of the operational amplifier and the gate of a first transistor; a second switch coupled between the gate of the first transistor and ground; and a third switch configured to couple a preset circuit to the non-inverting input of the operational amplifier during a preset control phase.

[0101] In a fourth embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the switching circuit further includes a fourth switch configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during a preset control phase.

[0102] In a fifth embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the driver circuit further includes a digital analog resistor coupled between the source of the first transistor and ground.

[0103] In a sixth embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the main control loop further includes a current source; a reference resistor coupled to the current source; and a filter capacitor coupled in parallel with the reference resistor.

[0104] In a seventh embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the driver circuit is configured to generate current pulses for driving the optical emitter, the duty cycle of which is between 1% and 10%.

[0105] In an eighth embodiment of the system, according to the first aspect itself or any of the aforementioned embodiments of the first aspect, the system is further configured to perform ambient light compensation (ALC) sampling immediately before and after the PPG sampling period.

[0106] The second aspect relates to a driver circuit for a light emitter in a photoplethysmography (PPG) system. The driver circuit includes an operational amplifier; a first transistor configured as a source follower, the gate of which is coupled to the output of the operational amplifier; a preset circuit configured to set a preset voltage at the output of the operational amplifier during a preset control phase; and a switching circuit configured to couple the preset circuit to a non-inverting input of the operational amplifier during the preset control phase and to activate a main control loop during a main control phase by coupling a reference voltage to the non-inverting input of the operational amplifier.

[0107] In a first embodiment of the driver circuit, according to the second aspect itself, the preset circuit includes a current source; a second transistor arranged in a diode configuration; and a switch coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

[0108] In a second embodiment of the driver circuit, according to the second aspect itself or any of the aforementioned embodiments of the second aspect, the switching circuit includes a first switch coupled between the output of the operational amplifier and the gate of a first transistor; a second switch coupled between the gate of the first transistor and ground; and a third switch configured to couple a preset circuit to the non-inverting input of the operational amplifier during a preset control phase.

[0109] In a third embodiment of the driver circuit, according to the second aspect itself or any of the aforementioned embodiments of the second aspect, the switching circuit further includes a fourth switch configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during a preset control phase.

[0110] In a fourth embodiment of the driver circuit, according to the second aspect itself or any of the aforementioned embodiments of the second aspect, the driver circuit further includes a digital analog resistor coupled between the source of the first transistor and ground.

[0111] In a fifth embodiment of the driver circuit, according to the second aspect itself or any of the aforementioned embodiments of the second aspect, the main control loop includes a current source; a reference resistor coupled to the current source; and a filter capacitor coupled in parallel with the reference resistor.

[0112] The third aspect relates to a circuit for driving a light emitter in a photoplethysmography (PPG) system. The circuit includes a main control loop comprising an operational amplifier and a first transistor configured as a source follower; a preset circuit configured to set a preset voltage at the output of the operational amplifier during a preset control phase, the preset circuit including a second transistor arranged in a diode configuration; and a switching circuit configured to activate the preset circuit during the preset control phase by coupling the preset circuit to a non-inverting input of the operational amplifier, and to activate the main control loop during the main control phase by decoupling the preset circuit and coupling a reference voltage to the non-inverting input of the operational amplifier.

[0113] In a first embodiment of the circuit, according to the third aspect itself, the preset circuit further includes a current source; and a switch coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

[0114] In a second embodiment of the circuit, according to the third aspect itself or any of the aforementioned embodiments of the third aspect, the switching circuit includes a first switch coupled between the output of the operational amplifier and the gate of a first transistor; a second switch coupled between the gate of the first transistor and ground; a third switch configured to couple a preset circuit to the non-inverting input of the operational amplifier during a preset control phase; and a fourth switch configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during the preset control phase.

[0115] In a third embodiment of the circuit, according to the third aspect itself or any of the aforementioned embodiments of the third aspect, the circuit further includes a digital-to-analog resistor coupled between the source of the first transistor and ground.

[0116] In a fourth embodiment of the circuit, according to the third aspect itself or any of the aforementioned embodiments of the third aspect, the main control loop further includes a current source; a reference resistor coupled to the current source; and a filter capacitor coupled in parallel with the reference resistor.

[0117] Although this specification has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of this disclosure as defined by the appended claims. In the various drawings, the same elements are indicated by the same reference numerals. Furthermore, the scope of this disclosure is not intended to be limited to the specific embodiments described herein, as it will be readily understood from this disclosure by those skilled in the art that existing or future processes, machines, manufactures, compositions of matter, means, methods, or steps can perform substantially the same functions or achieve substantially the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to include such processes, machines, manufactures, compositions of matter, means, methods, or steps within their scope.

[0118] Therefore, the specification and drawings are to be considered merely as a description of this disclosure as defined by the appended claims, and are intended to cover any and all modifications, variations, combinations or equivalents that fall within the scope of this disclosure.

Claims

1. A system for photoplethysmography (PPG) measurement, comprising: Light emitter; A photodetector is configured to detect light from the light emitter; as well as A driver circuit, coupled to the light emitter, includes: The main control circuit includes an operational amplifier and a first transistor configured as a source follower. A preset circuit, coupled to the operational amplifier and configured to set the voltage at the output of the operational amplifier to a preset voltage during a preset control phase, and A switching circuit is configured to activate the preset circuit during the preset control phase and the main control loop during the main control phase.

2. The system according to claim 1, wherein the preset circuit comprises: Current source; The second transistor is arranged in a diode configuration; as well as A third switch is coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

3. The system of claim 1, wherein the preset circuit is configured to set the voltage at the output of the operational amplifier to be close to the threshold voltage of the first transistor.

4. The system according to claim 1, wherein the switching circuit comprises: A first switch is coupled between the output of the operational amplifier and the gate of the first transistor; The second switch is coupled between the gate of the first transistor and ground; as well as A third switch is configured to couple the preset circuit to the non-inverting input of the operational amplifier during the preset control phase.

5. The system of claim 4, wherein the switching circuit further comprises a fourth switch configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during the preset control phase.

6. The system of claim 1, wherein the driver circuit further comprises a digital-to-analog resistor coupled between the source of the first transistor and ground.

7. The system according to claim 1, wherein the main control loop further comprises: Current source; A reference resistor is coupled to the current source; as well as A filter capacitor is coupled in parallel with the reference resistor.

8. The system of claim 1, wherein the driver circuit is configured to generate current pulses for driving the optical transmitter, the duty cycle of the current pulses being between 1% and 10%.

9. The system of claim 1, wherein the system is further configured to perform ambient light compensated ALC sampling immediately before and after the PPG sampling period.

10. A driver circuit for a light emitter in a photoplethysmography (PPG) system, the driver circuit comprising: Operational amplifier; The first transistor is configured as a source follower, and the gate of the first transistor is coupled to the output of the operational amplifier; A preset circuit is configured to set the voltage at the output of the operational amplifier to a preset voltage during a preset control phase. as well as The switching circuit is configured as follows: During the preset control phase, the preset circuit is coupled to the non-inverting input of the operational amplifier, and The main control loop is activated during the main control phase by coupling a reference voltage to the non-inverting input of the operational amplifier.

11. The driver circuit of claim 10, wherein the preset circuit comprises: Current source; The second transistor is arranged in a diode configuration; as well as A switch is coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

12. The driver circuit of claim 10, wherein the switching circuit comprises: A first switch is coupled between the output of the operational amplifier and the gate of the first transistor; The second switch is coupled between the gate of the first transistor and ground; as well as A third switch is configured to couple the preset circuit to the non-inverting input of the operational amplifier during the preset control phase.

13. The driver circuit of claim 12, wherein the switching circuit further comprises a fourth switch configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during the preset control phase.

14. The driver circuit of claim 10, further comprising a digital-to-analog resistor coupled between the source of the first transistor and ground.

15. The driver circuit of claim 10, wherein the main control loop comprises: Current source; A reference resistor is coupled to the current source; as well as A filter capacitor is coupled in parallel with the reference resistor.

16. A circuit for driving a light emitter in a photoplethysmography (PPG) system, the circuit comprising: The main control circuit includes an operational amplifier and a first transistor configured as a source follower; A preset circuit is configured to set the voltage at the output of the operational amplifier to a preset voltage during a preset control phase, the preset circuit including a second transistor arranged in a diode configuration; as well as The switching circuit is configured as follows: The preset circuit is activated during the preset control phase by coupling the preset circuit to the non-inverting input of the operational amplifier. The main control loop is activated during the main control phase by decoupling the preset circuit and coupling the reference voltage to the non-inverting input of the operational amplifier.

17. The circuit of claim 16, wherein the preset circuit further comprises: Current source; as well as A switch is coupled between the gate terminal of the second transistor and the non-inverting input of the operational amplifier.

18. The circuit of claim 16, wherein the switching circuit comprises: A first switch is coupled between the output of the operational amplifier and the gate of the first transistor; The second switch is coupled between the gate of the first transistor and ground; A third switch is configured to couple the preset circuit to the non-inverting input of the operational amplifier during the preset control phase; as well as A fourth switch is configured to couple the output of the operational amplifier to the inverting input of the operational amplifier during the preset control phase.

19. The circuit of claim 16 further includes a digital-to-analog resistor coupled between the source of the first transistor and ground.

20. The circuit of claim 16, wherein the main control loop further comprises: Current source; A reference resistor is coupled to the current source; as well as A filter capacitor is coupled in parallel with the reference resistor.

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