Modifying the duty cycles of PWM driver signals to compensate for LED driver mismatches in a multi-channel LED system

Abstract

LED (Light Emitting Diode) control device comprising the following: a first LED driver (320-a) to switch on or off a first current through a first LED chain (103-a) in response to a first PWM driver signal (390-a); a first compensation circuit (345-a) for generating the first PWM driver signal (390-a) in response to a first "duty set" or work set signal (305-a), wherein the first work set signal (305-a) is a first predetermined target duty cycle set for the first PWM driver signal (390-a), wherein the first compensation circuit (345-a) comprises the following: a first fault detection circuit (350-a) for receiving a first feedback signal (370-a) indicating the first current and for generating a first fault signal (380-a) indicating a difference between a first predetermined setpoint and the first feedback signal (370-a), wherein the first predetermined setpoint corresponds to the first predetermined target duty cycle and the first feedback signal (370-a) represents a measured duty cycle value of the first current; and a first PWM signal generation circuit (310-a) for generating the first PWM driver signal (390-a) with a first duty cycle corresponding to the work set signal (305-a) and further adapted by the first fault signal (380-a).

Description

BACKGROUND 1. Technical field

[0001] The present disclosure relates generally to a multi-string light-emitting diode (LED) system, and more particularly to feedback configurations for brightness adjustment in multi-string LED systems. 2. Description of related technology

[0002] LEDs are used in a wide variety of electronics applications, such as architectural lighting, automotive headlights and taillights, backlights for liquid crystal display devices, including personal computers and high-definition televisions, flashlights, and the like.

[0003] LEDs are current-controlled devices, and thus, regulating the current through the LEDs is an important control technique. In liquid crystal display (LCD) applications using LED backlights, it is often necessary for a controller to control multiple strings of LEDs with independent current settings for each string. The controller can then independently control the brightness of different sections of the LCD.

[0004] US 2012 / 0019154 A1 describes a control circuit for controlling LEDs and a dimmer. US 2013 / 0099684 A1 describes a LED control circuit. SUMMARY

[0005] Some applications of multi-string LED systems (for example, in LCD displays) benefit from illuminating one or more LED strings of the multiple LED channels at a substantially matched or equal brightness level (for example, matched within a predefined match threshold, such as within 1% brightness match). For example, maintaining a uniform brightness level across the LED strings representing rows or columns of pixels of an LCD display provides improved aesthetic quality and viewability of the LCD display.

[0006] To provide a desired brightness level for an LED string and, optionally, a desired brightness level match between LED strings, the controller in the multi-string LED system must maintain a desired (e.g., uniform) and optionally adjusted level of average current between the one or more LED strings of the multi-string LED system. Conventional LED controllers are limited in their ability to provide such current matching due to mismatches between drivers in the controllers that provide control signals to each of the one or more LED strings.

[0007] In one or more embodiments, an LED (Light Emitting Diode) control device has the features defined in the appended main claim (claim 1).

[0008] In one or more embodiments, a method for controlling a multi-channel LED system has the features of the appended independent claim 6 (patent claim 6).

[0009] The subclaims represent preferred further developments of the invention.

[0010] The features and advantages described in the specification are not exhaustive, and in particular, many additional features and advantages will be apparent to those skilled in the art in light of the drawings, the specification, and the claims. It should also be noted that the language used in the specification was chosen primarily for readability and instructional purposes and was not chosen to define or circumscribe the subject matter of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The teachings of the embodiments of the present invention will be readily understood by considering the following detailed description in conjunction with the accompanying drawings. Figure ( Fig. 1) shows a multi-channel LED system according to an embodiment. Fig. Figure 2A shows a conventional feedback configuration for controlling a brightness of an LED string of a multi-channel LED system. Fig. Figure 2B shows the conventional feedback configuration of Fig. 2A for controlling the brightness of a plurality of LED chains of a multi-channel LED system. Fig. 3 shows a feedback configuration for regulating current through an LED string (and optionally adjusting brightness between LED strings) of a multi-channel LED system based on duty cycle feedback according to one embodiment. Fig. 4 shows a feedback configuration for regulating current through an LED string (and optionally adjusting brightness between LED strings) of a multi-channel LED system based on average current feedback according to an unclaimed embodiment. Fig. 5A-5B show comparative time courses of currents through two different LED chains of the multi-channel LED system without feedback control ( Fig. 5A) and with feedback control ( Fig. 5B) according to some embodiments. DETAILED DESCRIPTION OF EMBODIMENTS

[0012] The figures and the following description relate to preferred embodiments of the present disclosure by way of example. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be apparent as viable alternatives that may be employed without departing from the principles of the disclosure.

[0013] Reference will now be made in detail to various embodiments of the present invention(s), examples of which are shown in the accompanying figures. It should be noted that, wherever practical, similar or like reference numerals may be used throughout the figures and may indicate similar or like functionality. The figures show embodiments of the present invention for purposes of illustration only. Those skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods shown herein may be utilized without departing from the principles of the invention described herein.

[0014] Fig. Figure 1 shows a multi-channel LED system 100 according to one embodiment. A multi-channel LED system 100 includes a DC-DC power converter 101 (e.g., a boost converter), multiple LED channels 102, and an LED controller 120. DC-DC converter (boost converter)

[0015] To drive a large array of LEDs from a direct current (DC) voltage source (which may result from rectifying an AC voltage), DC-DC switching power converters, such as boost or buck-boost power converters, are often used to provide a supply voltage to properly bias multiple strings of LEDs. As described in Fig. 1, the boost converter 101 receives an input voltage Vin and supplies a regulated voltage (VLED) to the plurality of LED channels 102. In one embodiment, the boost converter 101 includes an inductor L1, a diode D1, a capacitor C1, and a switch Q B (for example, an NMOS transistor). The switching mechanism (for example, frequency or duty cycle of a switching) of the switch Q B is controlled by the LED control device 120. This control mechanism allows the boost converter 101, V LED at a programmed level set by the LED controller 120. In one embodiment, the LED controller 120 detects the voltage V LED (or a representation of V LED ) to perform feedback control to V LED to the programmed level. When the switch Q Bis turned on, the diode D1 is reverse biased, and the input power received from a source of supply voltage Vin is stored in the inductor L. On the other hand, when the switch Q B is turned off, the diode D1 is forward biased and the input power is transferred to the capacitor C1, charging the capacitor C1 and producing a voltage V LED to supply power to the multiple LED channels 102. The output voltage V LED applied to the LED channels 102 provides current through the LED channels 102. LED channels

[0016] As in Fig. As shown in Figure 1, a plurality of LED channels 102 (e.g., including LED channel 102-a, LED channel 102-k, LED channel 102-n, and the like) are connected in parallel (e.g., between the voltage VLED and a voltage ground). Each LED channel 102 includes a series connection of an LED string 103 (including LED strings 103-a, 103-k,..., 103-n), a PWM transistor 104 (including transistors 103-a, 104-k,..., 104-n) (e.g., an n-type MOSFET), and a sense resistor R (Ra, Rk,..., Rn).

[0017] Each LED string 103 comprises a series connection of a plurality of LEDs. The output voltage VLED of the boost converter 101 is coupled to the anode of a first LED in each LED string 103. The cathode of a last LED in each LED string 103 for a given LED channel 102 is coupled to the drain of the PWM transistor 104 of that given LED channel 104. The source of the PWM transistor 104 is connected to one terminal of the sense resistor R for that given LED channel 102, and the other terminal of the sense resistor R is connected to voltage ground.

[0018] The current through the sense resistor R (e.g., indicating a current through the respective LED chain 103) of a given LED channel 102 is measured as a voltage drop (I SENSE) is detected via the sense resistor R for this LED channel 102 and provided as a feedback signal to the LED controller 120. The LED controller 120, in turn, generates a control signal that drives the gate terminal of the PWM transistor 104 for this given LED channel 102 to control a current through this LED channel 102 and consequently the brightness of the LED chain 103 of this LED channel 102. Although Fig. 1 shows only three LED channels, the multi-channel LED system 100 may include any number of LED channels 102 (for example, 9 LED channels) and corresponding control / regulation circuits. LED control device

[0019] As described above, the LED controller 120 regulates a voltage VLED to a desired setpoint voltage to power the multiple LED channels 102. Accordingly, the LED controller 120 controls the switching (e.g., frequency or duty cycle of switching) of the switch QB of the boost converter 101. The LED controller 120 may employ any of a number of well-known modulation techniques, such as pulse width modulation (PWM) or pulse frequency modulation (PFM), to control the on and off states and duty cycles of the switch QB. PWM and PFM are conventional techniques used to control switching power converters by controlling the widths and frequencies, respectively, of an output drive pulse driving the switch QB. B to control the voltage V LED to achieve.

[0020] In addition, the LED control device 120 supplies control signals individually to each of the LED channels 102 to regulate a brightness of LED strings 103 of the plurality of LED channels 102 and to adjust a brightness between them. For this purpose, as described above, the LED control device 120 detects a current through the sense resistor R (e.g., indicating a current through the respective LED string 103) for each LED channel 102 as a voltage drop (I SENSE ) across the sense resistor R for that LED channel 102 and generates a control signal that drives the gate terminal of the PWM transistor 104 for the respective LED channel 102 to control a current through (for example, and consequently a brightness) of the corresponding LED chain 103.

[0021] In some embodiments, the PWM transistor 104 for a given LED channel 102 receives a pulse width modulation (PWM) control signal from the LED controller 120 to control an average current through and the brightness of the LED string 103 for that LED channel 102 according to a duty cycle of the PWM control signal.

[0022] In some embodiments, as in Fig. 1, the LED controller 120 receives the Iset and Duty Set signals. The Iset signal specifies a target (e.g., desired or setpoint) voltage drop across the sense resistor R corresponding to a target peak current level to be achieved for each LED channel 102 during an on-state of the respective PWM transistor 104 for each LED channel 102. Similarly, the Duty Set signal specifies a target (e.g., desired) duty cycle to be achieved for each LED channel 102 for switching the respective PWM transistor 104. The same Iset and Duty Set signals are provided for all LED channels 102 to provide the same target peak current and duty cycle for all LED channels 102.

[0023] A target average current through each LED channel 102 for a given PWM cycle is a product of the target peak current through the LED channel 102 during that switching cycle and the target duty cycle of switching the PWM transistor 104 for the LED channel 102 for that switching cycle. LEDs are current-controlled devices; the brightness of the LED chain 103 for a given LED channel 102 is a function of an average current through that LED channel 102. Thus, by controlling an average current through each LED channel to the product of Iset and DutySet, via the feedback configurations described below (for example, Fig. 3 and Fig. 4), the brightness control device 120 adjusts the brightness between the LED strings 103 to a substantially equal brightness level (for example, within a predetermined brightness adjustment range, such as within 1%, 2%, 5%, or the like). LED controller architectures for brightness control

[0024] Fig. Figure 2A shows a conventional feedback configuration 200 for controlling the brightness of an LED chain of a multi-channel LED system. As in the configuration 200 of Fig. 2A, the LED control device 120 includes a PWM generator 210 (alternatively referred to herein as a PWM generation circuit) and a driver 220 corresponding to each LED channel 102. Although Fig. 2A shows a feedback configuration (including PWM generator 210 and driver 220) for one LED channel (LED channel 102-k) that can be used in practice and as described with reference to Fig. 2B, the LED controller 120 may include an independent and separate set of feedback components for each of the LED channels 102 (e.g., including LED channel 102-a, LED channel 102-k, LED channel 102n, and the like) present in the multi-string LED system 100. The LED controller 120 regulates current through the LED string 102-k according to programmed current and duty cycle levels (e.g., specified by Iset and Duty Set signals) for the LED channel 102-k.

[0025] Accordingly, the PWM generator 210 receives the Iset and Duty Set signals at reference (Ref) and input (In) terminals of the PWM generator 210, respectively, and generates a PWM signal 290 at the output (Out) terminal of the PWM generator 210.

[0026] Driver 220 is optionally an operational amplifier (op-amp). The output of driver 220 is coupled to the gate of PWM transistor 104-k to control a current through LED channel 102-k. Driver 220 receives a PWM signal 290 from PWM generator 210 at the non-inverting terminal and receives signal I SENSE at the inverting terminal via a feedback loop from the source of the PWM transistor 104-k. A feedback loop 240 is thus configured to control the current through the LED chain 103-k via the voltage I SENSE and provide a control signal to the gate of PWM transistor 104-k to control the voltage I SENSE at the source of PWM transistor 104-k, which follows PWM signal 290.

[0027] However, the driver 220 may be limited in its ability (e.g., operating speed as characterized by a limited slew rate, input-to-output propagation delay time, etc.) to provide a control signal at the gate of the PWM transistor 104-k that is I SENSE enables the PWM signal 290 to follow appropriately. In some cases, the influence of a slow driver is particularly evident at lower duty cycles (e.g., lower than 1%) and affects the capability of the voltage signal I SENSE , or its corresponding current waveform, to follow a desired PWM pulse of the PWM signal 290. For example, as in Fig. 5A, a first channel current 520 (corresponding to the measured voltage I SENSE) to follow a target PWM pulse 510, a delayed rise time, a slower rise time, an inability to reach a target voltage or current value (due to the slower rise time), and a delayed fall time relative to the target PWM pulse 510. The inability of a channel current to follow a target PWM pulse results in the average current for that channel during that PWM cycle being lower than a desired or target average current value. This, in turn, results in a brightness of the LED string for that channel being lower than a desired brightness.

[0028] Furthermore, characteristics of the driver 220 that drive different LED channels 102 can vary across the LED channels 102.

[0029] For example, Fig. 2B the conventional feedback configuration 200 of Fig. 2A for controlling the brightness of two LED chains 103-a and 103-k of the same multi-channel LED system. In this example, note that driver 220-a, which drives LED channel 102-a, is slower than driver 220-k, which drives LED channel 102-k. Faster drivers can follow a target PWM pulse better than slower drivers. As graphically illustrated, as in the waveforms of Fig. 5A is a driver that drives the first channel of Fig. 5A (for example, the driver 220-a from Fig. 2A) slower in operating speed than a driver driving the second channel of Fig. 5A (for example, the 220-k driver from Fig. 2A). As shown in the first and second channel current waveforms 520 and 522 of Fig. As shown in Figure 5A, these different driver speeds affect, to varying degrees, the ability of the LED drivers to follow the same target PWM pulse. This, in turn, results in a higher average current through LED strings driven by faster drivers than the average current through LED strings driven by slower drivers, especially at lower duty cycles. This, in turn, results in greater brightness from LED strings driven by faster drivers than from LED strings driven by slower drivers. Thus, different driver operating speeds affect brightness consistency across LED strings.

[0030] In other words, variations or mismatches between drivers (e.g., different driver operating speeds) in the configuration 200 result in mismatched current waveforms across the LED channels (see, for example, the discrepancies between the first channel current 520 and the second channel current 522, as shown in Fig. 5A). This mismatch of the current waveforms through the LED channels results in a channel-to-channel current mismatch across LED channels 102 and consequently leads to insufficient brightness matching across LED strings 103. A disadvantage of this conventional configuration 200 is that any influence of these driver mismatches on channel currents is neither measured, zeroed, nor otherwise compensated for, resulting in poor current and brightness matching across LED strings 103. LED controller feedback configurations for brightness adjustment across LED strings based on duty cycle feedback

[0031] To provide a desired degree of brightness adjustment between the one or more LED strings, the control device in the multi-string LED system must maintain a uniform and consistent level of average current between the one or more LED strings of the multi-string LED system.

[0032] To overcome the limitations associated with driver mismatches in LED control devices (such as those described in the Fig. 2A and Fig. 2B), embodiments herein provide a feedback configuration in the multi-channel LED system that compensates for such driver mismatch by adjusting a duty cycle of a pulse width modulated (PWM) LED driver signal received by the driver for turning on or off a corresponding one of the one or more LED strings. This adjustment to the duty cycle of the LED driver signal for a channel is performed based on an error or discrepancy between a measured duty cycle of the LED current through that channel (influenced by the driver characteristics) and a desired setpoint or target value of a duty cycle of LED current to be maintained for that channel.By adjusting the duty cycle of the drive signal to reduce the error between measured and desired duty cycle of the LED current to a substantially zero value across the one or more channels, the measured duty cycles of LED currents through the one or more LED channels are adjusted, resulting in channel-to-channel current and brightness adjustment.

[0033] For example, Fig. 3 shows a feedback configuration 300 for adjusting the brightness between LED chains in a multi-channel LED system based on duty cycle feedback according to one embodiment.

[0034] As in configuration 300 in Fig. 3, the multi-channel LED system includes multiple LED channels, including LED channel 102-a and LED channel 102-k. The LED controller 120 senses a current through the sense resistor R (e.g., R a ,..., R k), which indicates a current through the respective LED chain 103 for each LED channel 102, as a voltage drop (I SENSE ) across the sense resistor R for that LED channel 102, and drives the gate terminal of the PWM transistor 104 for the respective LED channel 102 to control current through (e.g., and hence brightness) the corresponding LED chain 103. Further, the LED controller 120 regulates an average current through the LED chains 103-a and 103-k over one or more clock cycles or over a predetermined time period according to target or desired peak current and duty cycle levels (e.g., specified by the Iset and Duty Set signals 305-a and 305-k) for the LED channels 102-a and 102-k, respectively.

[0035] The LED control device 120 includes an LED driver for controlling each of the LED channels based on a corresponding PWM (Pulse Width Modulation) drive signal. As shown in Fig. 3, the LED control device 120 includes a first LED driver 320-a for generating a first PWM (Pulse Width Modulation) drive signal to turn on or off a first current through a first LED chain 103-a in response to the first PWM drive signal 390-a, and a second LED driver 320-k for generating a second PWM drive signal to turn on or off a second current through a second LED chain 103-k in response to the second PWM drive signal 390-k. In some cases, different drivers of the same multi-channel LED system may have different characteristics (e.g., different operating speeds). As shown in the waveforms of Fig. 5A, a driver that controls the first channel of Fig. 5A (for example, the driver 320-a from Fig. 3), a slower operating speed than a driver that uses the second channel of Fig. 5A (for example, the 320-k driver from Fig. 3). This mismatch in driver characteristics results in different or inconsistent current levels and brightness levels between the LED strings driven by these drivers.

[0036] Thus, the LED controller 120 further includes a compensation circuit 345 for each LED channel that generates PWM signals 390 for each of the channel drivers (including the PWM signal 390-a for driver 320-a and the PWM signal 390-k for driver 320-k). Duty cycles of the PWM driver signals are adjusted or modified to compensate for driver mismatches. For example, the compensation circuits 345-a and 345-k generate the first and second PWM driver signals 390-a and 390-k, respectively, in response to the first and second duty set signals 305-a and 305-k, respectively. The first duty set signal 305-a indicates a first duty cycle set for the first PWM drive signal 390-a, and the second duty set signal 305-k indicates a second duty cycle set for the second PWM drive signal 390-k.

[0037] A PWM driver signal for a given channel is configured to have a duty cycle that corresponds to a duty set signal for that given channel, but is further adjusted (increased or decreased) based on an error signal representing a degree of current or brightness mismatch for that given channel. For example, if a first LED driver is slower than a second LED driver, then the first duty cycle of the first PWM driver signal, with respect to the first duty set signal, is adjusted to be greater than the second duty cycle of the second PWM driver signal, adjusted with respect to the second duty set signal, while the first and second duty set signals have substantially the same value. With reference to the Fig. 5A-5B, if a first driver that controls the first channel of Fig. 5A (for example, the driver 320-a from Fig. 3), has a slower operating speed than a second driver driving the second channel of Fig. 5A (for example, the 320-k driver from Fig. 3), the PWM driver signal 390-a provided to the first driver 320-a has a larger duty cycle (for example, first channel adjusted pulse 550 has a longer ON time, in Fig. 5B) than the duty cycle of the PWM driver signal 390-k provided to the second faster driver 320-k (e.g., second channel-matched pulse 552 has a shorter ON time, in Fig. 5B). Thus, by adjusting the PWM pulse width or duty cycle, at given duty setpoints based on a

[0038] Depending on the degree of mismatch between a predetermined setpoint and an actual measured value of a duty cycle, the compensation circuit 345 provides a desired or target current waveform (first channel current 560 or second channel current 562) for a given channel. Further, by adjusting the PWM pulse width or duty cycle, at the same duty setpoints, based on the degree of mismatch, the compensation circuits 345-a and 345-k of different LED channels provide matching current waveforms between the different channels (first channel current 560 and second channel current 562) between the drivers and can thus additionally provide inter-channel current and brightness adjustment.

[0039] For this purpose, each compensation circuit 345 includes an error amplifier 350 (alternatively referred to herein as an error determination circuit 350) and a PWM generator 310 (alternatively referred to herein as a PWM signal generation circuit 310). For example, as shown in Fig. 3, the compensation circuit 345-a includes an error amplifier 350-a and a PWM generator 310-a; and the compensation circuit 345-k includes an error amplifier 350-k and a PWM generator 310-k. The error determination circuit 350 for each channel receives a corresponding feedback signal representing a current flowing through the respective LED channel. For example, the error determination circuit 350-a receives a first feedback signal 370-a corresponding to LED channel 103-a, and the error determination circuit 350-k receives a second feedback signal 370-k corresponding to LED channel 103-k, indicating the first current and the second current, respectively. The error determination circuit 350 for each channel generates a corresponding error signal in response to the feedback signal for that respective LED channel.Each error signal represents a measure of discrepancy between a specified target value (corresponding to a target average current value for the current through the channel) and an actually measured current through the LED channel, caused by driver characteristics or non-idealities for that LED channel. For example, as in . Fig. 3, the error determination circuit 350-a generates a first error signal 380-a (for the LED channel 102-a) indicative of a difference between a predetermined setpoint (e.g., duty set signal 305-a) and the first feedback signal 370-a, and the error determination circuit 350-k generates a second error signal 380-k (for the LED channel 102k) indicative of a difference between the predetermined setpoint (e.g., duty set signal 305-k) and the second feedback signal 370-k.

[0040] The PWM signal generation circuit 310 for each channel generates a PWM drive signal for that channel, with each individual PWM drive signal 390 driving a corresponding LED driver 320. Referring to Fig. 3, the PWM signal generation circuit 310-a generates the first PWM drive signal 390-a so that the first duty cycle corresponds to the first duty set signal 305-a and is further adjusted by the first error signal 380-a. Further, the PWM signal generation circuit 310-k generates the second PWM drive signal 390-k so that the second duty cycle corresponds to the second duty set signal 305-k and is further adjusted by the second error signal 380-k.

[0041] In one or more embodiments, the first and second PWM signal generating circuits 310-a and 310-k respectively generate the first and second PWM drive signals 390-a and 390-k to further have a predetermined peak magnitude value (Iset).

[0042] In this embodiment, the feedback signals 370 provided to each of the error amplifiers 350 for each LED channel 120 correspond to measured duty cycles of current flowing through that channel. In other words, the first feedback signal 370-a provided to the error amplifier 350-a for the first LED channel 102-a represents a measured duty cycle of current flowing through the first LED channel 102-a, and the second feedback signal 370-k provided to the error amplifier 350-k for the second LED channel 102-k represents a measured duty cycle of current flowing through the second LED channel 102-k.In one or more embodiments, the LED controller 120 includes a measurement circuit 325 for each channel (e.g., measurement circuit 325-a for channel 102-a and measurement circuit 325-k for channel 102-k) to generate feedback signals (including first and second feedback signals 370-a and 370-k) representing a measured duty cycle of currents through each of the LED channels by scaling input signals (I. SENSE ) corresponding to the respective currents (including the input signals 105-a and 105-k for the channels 102a and 102-k, respectively) by a predetermined scaling constant that is proportional to the predetermined peak magnitude value (Iset) for the PWM driver signals 390 (including the first and second PWM driver signals 390-a and 390-k).

[0043] Each measurement circuit 325 includes an analog-to-digital converter or ADC (for example, an ADC 330-a for channel 102-a and an ADC 330-k for channel 102-k, as shown in Fig. 3) and optionally a digital filter 340 (including digital filter 340-a for channel 102-a and filter 340-k for channel 102-k). In some embodiments, ADCs 330-a and 330-k are sigma-delta analog-to-digital converters. In some embodiments, ADCs 330-a and 330-k generate first and second feedback signals 370-a and 370-k for first and second LED channels 102-a and 102-k, respectively. This is achieved by digitizing analog input signals I SENSE105-a and 105-k, respectively, received from each of the respective LED channels and representing current flowing through each of those channels. Each of the feedback signals 370 generated by the ADCs represents a measured duty cycle of current through a respective LED channel 102. For example, the first and second feedback signals 370-a and 370-k represent measured duty cycle values ​​("Measured Duty Cycle" 370) of the currents through channels 102-a and 102-k, respectively. This is achieved by setting an input signal range of the ADC to a predetermined value proportional to (or equal to) the predetermined peak magnitude value (Iset) of the PWM drive signals 390 for the respective channels.

[0044] The ADCs 330-a, 330-k for each channel receive the analog signal I SENSEat the input (In) terminal and the signal Iset at the reference (Ref) terminal, including the input signals 105-a and 105-k for the channels 102-a and 102-k, respectively.

[0045] The ADC reference signal (Ref) sets the dynamic input range of an ADC. As in Fig. As shown in Figure 3, when the target setpoint signal Iset is used as an ADC reference, the ADC 330 generates or outputs digital signals corresponding to "measured duty cycles" 370 for the respective LED channel. A digital output of the ADC 330 (generated at the output terminal of the ADC 330) is mathematically obtained as: Output= <lsense>∗LSB where < I SENSE > a time average of signal I SENSE received at the In terminal of the ADC 330, and LSB is the least significant bit (LSB) or resolution of the ADC 330.

[0046] By definition, the LSB, or resolution, of the ADC is obtained by dividing the number of ADC bits (Nbit) by the ADC reference signal (Ref). Thus: Output= <lsense>∗Nbit / Ref

[0047] When the ADC reference signal (Ref) is Iset (as in Fig. 3 shown), then Output=Nbit∗ <lsense> / lset

[0048] By definition, a duty cycle of a waveform is a ratio of an on-time (T ON ) of a pulse of the waveform to a total time period (T) of the waveform, which in configuration 300 is also represented as: Duty cycle=TON / T= <lsense> / lset

[0049] Thus, if the ADC reference signal (Ref) is Iset, then the output of the ADC is mathematically a representation of the duty cycle (or the "measured duty cycle" 370). Thus, corresponding to each channel, a respective ADC 330 converts the analog signal I SENSE into a digital representation at an output terminal (Out), which is optionally received and processed by the digital filter 340 to generate the band-limited (e.g., digitally low-pass filtered) digital feedback signal "Measured Duty Cycle" 370 at the output (Out) terminal of the filter 340 for that channel.

[0050] The feedback signals 370 for each of the individual LED channels, representing "measured duty cycles" 370 for the various channel currents, are received by the corresponding error amplifier 350. The error amplifier 350 for each channel compares each of the "measured duty cycles" 370 with corresponding duty set signals 305 (e.g., the desired or target duty cycle corresponding to each channel) to generate an error signal 380 for the respective channel (e.g., at the Out terminal of the error amplifier 350). The error signal 380 for a given channel is representative of a difference over one or more clock cycles or over a predetermined time interval (e.g., over 5 clock cycles or over a period of 10 milliseconds) between the measured duty cycle and the target duty cycle of the current through that respective LED channel.For example, for the first LED channel 102-a, a feedback signal 370-a is compared with a duty set signal 305-a to generate an error signal 380-a. Similarly, for the second LED channel 102-k, the feedback signal 370-k is compared with the duty set signal 305-k to generate an error signal 380-k.

[0051] The PWM generator 310 for each channel receives each of the error signals 380 at the input (In) terminal of the PWM generator 310 and the setpoint or target signal Iset at the reference (Ref) terminal of the PWM generator 310. The PWM generator 310 generates a PWM signal 390 for the respective channel that switches between the setpoint Iset and a voltage ground at a particular duty cycle for each switching cycle. The duty cycle of each of the PWM signals 390 is, in turn, modified (e.g., relative to a setpoint duty cycle or relative to a duty cycle value for a previous switching cycle for that LED channel) based on a value of the error signal 380 for that respective LED channel. This duty cycle adjustment is performed separately for the PWM signal driving each channel.For example, for LED channel 102-a, the duty cycle of PWM signal 390-a during a given switching cycle is adjusted relative to a duty set signal 305-a based on a value of error signal 380-a from the prior switching cycle. Similarly, for LED channel 102k, the duty cycle of PWM signal 390-k during a given switching cycle is adjusted relative to the duty set signal 305-k based on a value of error signal 380-k from the prior switching cycle. In some embodiments, the duty cycle of PWM signal 390 for a given LED channel is increased (e.g., relative to the setpoint, Duty Set) when a value of error signal 380 for that given LED channel is substantially non-zero and positive (e.g., when "measured duty cycle" 370 for that channel is less than the "Duty Set" signal for that channel, as shown in FIG. Fig. 5A) until the error signal 380 for that channel is reduced to zero or to a value below a predetermined threshold (for example, until "measured duty cycle" 370 is substantially equal to Duty Set for the channel).

[0052] In other words, in some embodiments, a width (or duty cycle) of the PWM signal 390 is adjusted for each individual LED channel 102 to minimize (e.g., reduce to below a predetermined threshold or to substantially zero) an average measure of the error signal 380 for that channel over a predetermined time period or time interval.

[0053] The drivers 320-a and 320-b compare the PWM signals 390-a and 390-b with respective voltages I SENSE to generate control signals that drive the gates of PWM transistors 104-a and 104-k to provide an I SENSE -waveform following the corresponding PWM signals 390-a and 390-b (for example, as shown in Fig. 5B). As described above with reference to Fig. 2A, due to a slow operating speed (e.g., insufficient slew rate, driver propagation delay time, and so on), the drivers 320 may be limited in their ability to provide a control signal to the PWM transistor 104 that corresponds to the voltage signal I SENSE enables precise tracking of the PWM signal 390, especially for low duty cycles.

[0054] However, as an improvement over the conventional configuration 200 of Fig. 2A, in configuration 300, a measure of the inability of drivers 320 to provide a signal I SENSE , which tracks a setpoint drive signal, measured (for example, in the form of an error signal 380, which represents a difference between a measured duty cycle of the signal I SENSE and the desired or target duty cycle, Duty Set); and is compensated by modifying a duty cycle of the PWM signal 390 provided to the driver 320 based on the error signal 380 until the error signal 380 is substantially zero or reduced to below a predetermined threshold.

[0055] Conversely, if the error signal 380 is substantially zero due to the compensation mechanism of configuration 300, the measured duty cycle of the signal I SENSE set substantially equal to the desired or target duty cycle (Duty Set) for the given LED channel 102. Further, when this compensation mechanism of configuration 300 is similarly applied to each of the one or more LED channels 102 of the multi-channel LED system 100, the measured duty cycles of switching of each of the LED channels are adjusted with respect to the common desired or target duty cycle "Duty Set." For a substantially equal level of peak current (determined based on a commonly applied setpoint of Iset) across the LED channels, the average currents through each of the LED channels 102 are sufficiently matched, thereby providing the desired channel-to-channel current and brightness adjustment.

[0056] In one embodiment, the error amplifier 350 is a proportional-integral (PI)-type amplifier that integrates the error between the measured duty cycle and the target duty cycle over one or more clock cycles. In some embodiments, the sigma-delta ADC 330 is a continuous-time sigma-delta ADC. Although the ADC 330 shown here is described as a sigma-delta ADC (for example, for simplicity of hardware and ease of integration), various alternative ADC configurations may be used in practice without departing from the scope of the disclosure. In some embodiments, the PWM generator 310, the sigma-delta ADC 330, and the digital filter 340 are driven or synchronized by a clock signal. In one embodiment, the clock signal has a frequency of 20 MHz.In some embodiments, the PWM signals 390 have a frequency between 20 kHz and 25 kHz (for example, to exclude the human-audible frequency band). In some embodiments, the PWM signals 390 have a duty cycle between 1% and 100%. If the frequency of the clock signal input to the PWM generator 310 is 20 MHz and the frequency of a generated PWM signal 390 is 20 kHz, the resolution of the PWM generator 310 is 0.1%. In some embodiments, if the frequency of the clock signal received by the PWM generator 310 is not constant but varies (for example, due to dithering), the PWM signal 390 is corrected for variability in the frequency of the clock signal.

[0057] In some embodiments, a current through an LED string during an ON time of its PWM driver signal 390 is between 20 mA and 200 mA. In some embodiments, the average current through one or more LED strings 103 is adjusted within a predetermined current adjustment range, such as 1%, 2%, 5%, and the like. In some embodiments, the brightness levels between LED strings 103 are adjusted within a predetermined brightness adjustment range, such as 1%, 2%, 5%, and the like. LED controller feedback configurations for brightness adjustment across LED strings based on average current feedback

[0058] In alternative embodiments, a feedback configuration in the multi-channel LED system compensates for the driver mismatch by adjusting a duty cycle of a pulse width modulated (PWM) signal received from the driver to drive a corresponding LED string of the one or more LED strings based on an error or discrepancy between a measured average current through the one or more LED strings (a product of the measured peak current and measured duty cycle, influenced by the driver characteristics) and a desired setpoint or target value of an average current that must be consistently maintained by the one or more LED strings to achieve the setpoint brightness level.By adjusting the duty cycle to reduce the error between the measured and desired average current to a substantially zero value across the one or more channels, the average currents through the one or more LED channels are adjusted (and set substantially equal to the desired or target average current), resulting in channel-to-channel current and brightness adjustment.

[0059] For example, Fig. 4 illustrates a feedback configuration for regulating current through an LED channel (and optionally adjusting brightness between a plurality of LED strings) of a multi-channel LED system using average current feedback according to a non-claimed embodiment.

[0060] As in configuration 400 in Fig. 4, corresponding to the LED channel 102-a and 102-k, the LED controller 120 includes the drivers 420-a and 420-k, compensation circuits 445-a and 445-k (including PWM generators 410-a and 410-k and error amplifiers 450-a and 450-k), and measurement circuits 425-a and 425-k. Various characteristics of the drivers 420-a and 420-k, the compensation circuits 445-a and 445-k (including the PWM generators 410-a and 410-k and the error amplifiers 450-a and 450-k), and the measurement circuits 425-a and 425-k are similar or identical to corresponding characteristics of the drivers 320-a and 320-k, the compensation circuits 345-a and 345-k (including the PWM generators 310-a and 310-k and the error amplifiers 350-a and 350-k), and the measurement circuits 325-a and 325-k described with reference to Fig. 3 are described.

[0061] A difference between the Fig. 3 and Fig. 4 configurations is that in Fig. 4, the various feedback signals 470 (including the first and second feedback signals 470-a and 470-k corresponding to the first and second LED channels 102-a and 102-k, respectively) represent measured average current values ​​corresponding to the currents flowing through the respective LED channels (including the first and second currents flowing through the LED channels 102-a and 102-k, respectively), rather than the measured duty cycles. The measuring circuits 425-a and 425-k of Fig. 4 generate the respective feedback signals 470 for the corresponding LED channels (including the first and second feedback signals 470-a and 470-k for the first and second LED channels 102-a and 102-k) by scaling input signals corresponding to each of the respective channel currents (including the input signals 105-a and 105-k for the channels 102-a and 102-k, respectively) with a predetermined scaling constant that is different from and independent of the predetermined peak magnitude value (Iset) for the PWM driver signals 390 (e.g., the first and second PWM driver signals 390-a and 390-k).

[0062] For this purpose, each measurement circuit 425 includes an ADC 430 (including an ADC 430-a for channel 102-a and an ADC 430-k for channel 102-k) and optionally a digital filter 440 (including a filter 440-a for channel 102-a and a filter 440-k for channel 102-k). The ADC 430 for each channel receives a fixed or constant reference voltage V REF , which is different and optionally independent of the Iset voltage reference (as the ADC reference in Fig. 3). The reference V REF , which in Fig. 4, does not necessarily vary with variations in Iset. Since V REF sets the input analog signal range of the ADC, in configuration 400, the input signal range and resolution of the ADC have values ​​(for example, preset values) that are independent of the preset peak magnitude value (Iset) for the PWM drive signal 490.

[0063] Consequently, in configuration 400, for each of the LED channels in the multi-channel system (including the first and second channels 102-a and 102-k), the ADC 430 (and optionally the filter 440) generates a digital representation of the measured voltage drop across the sense resistor R k due to a measured average current through each LED channel as a digital feedback signal I Measured 470 for each LED channel, rather than a digital representation of a measured duty cycle of the current through each LED channel (discussed above with reference to configuration 300 of Fig. 3 explained).

[0064] Accordingly, for each LED channel 102, the error amplifier 450 receives a corresponding feedback signal (I Measured ) 470 and compares the feedback signal I Measured 470 for this channel with a nominal voltage drop across R k corresponding to the target average current lavg (for example, obtained as a product of Iset and Duty Set for this channel, calculated by a multiplier 475) for this channel. For example, as in Fig. 4, for the first channel 102-a, the error amplifier 450-a compares a feedback signal 470-a with the voltage signal representing a desired average current 478-a (obtained by multiplying Iset by the "Duty Set" signal 405-a from the multiplier 475-a) to generate an error signal 480-a. Similarly, for the second channel 102-k, the error amplifier 450-k compares a feedback signal 470-k with the voltage signal representing a desired average current 478-k (obtained by multiplying Iset by the "Duty Set" signal 405-k from the multiplier 475-k) to generate an error signal 480-k.Each error amplifier 480 similarly generates an error signal 480 for a corresponding channel, representative of a difference, over one or more clock cycles or over a predetermined time interval, between the measured average current through that LED channel and the desired average current through that LED channel in the form of voltages.

[0065] Corresponding to each LED channel, the PWM generator 410 generates a PWM signal 490 that switches between the setpoint Iset and a voltage ground with a particular duty cycle for each switching cycle; wherein the duty cycle of each PWM signal 490 is in turn modified (e.g., relative to a setpoint duty cycle or relative to a duty cycle value for a previous switching cycle) based on a value of the error signal 480 for that given LED channel. In a manner analogous to that described with reference to the configuration 300, the duty cycle of the PWM signal 490 for a given channel is increased (e.g., relative to the setpoint, Duty Set) when a value of the error signal 480 for that given channel is substantially non-zero and positive (e.g., when I Measured 470 is lower than lavg for this given channel) until the error signal 480 for the given channel is reduced to zero or to a value below a predetermined threshold (for example, when I Measured 470 is substantially equal to lavg for that given channel). In other words, the feedback mechanism of configuration 400 minimizes an average amount of the error signal 480 separately for each LED channel over a predetermined number of switching cycles or over a predetermined time interval by adjusting a width (duty cycle) of the PWM signal 490 for each individual LED channel.

[0066] The driver 420 for a given channel compares the PWM signal 490 for that channel with the signal I SENSE corresponding to that channel to generate a control signal that drives the gate of the PWM transistor 104 of that channel to generate an I SENSE -waveform corresponding to the PWM signal 490 (for example as shown in Fig. 5B) for this channel follows.

[0067] As an improvement over the conventional configuration 200 of Fig. 2A, in the configuration 400, a measure of the inability of the driver 420 to provide a signal I SENSE by a given LED channel tracking a setpoint signal (for example, in the form of the error signal 480, which represents a difference between a measured voltage drop due to a measured average current through the LED chain 103 as a digital signal I Measured 470 and a nominal voltage drop across R k corresponding to a desired average current lavg for that LED channel) and is compensated by modifying a duty cycle of the PWM signal 490 provided to the driver 420 of that LED channel based on the error signal 480 for that LED channel until the error signal 480 for that LED channel is substantially zero or reduced below a predetermined threshold.

[0068] Conversely, when the error signal 480 for a given LED channel is set to substantially zero by the compensation mechanism of configuration 400, the measured voltage drop due to the measured average current through that LED channel is used as a feedback signal I Measured 470 for this channel is essentially set equal to the nominal voltage drop across R k corresponding to the target average current lavg for this given LED channel 102.

[0069] Furthermore, when this compensation mechanism of configuration 400 is similarly applied to each of the multiple LED channels 102 of the multi-channel LED system 100, the measured voltage drops due to the measured average current through each of the LED chains 103 of each of the LED channels with respect to the common desired or target voltage drop across R k adjusted according to the desired average current lavg. Thus, the average currents through each of the LED channels 102 are sufficiently adjusted, providing the desired channel-to-channel current and brightness adjustment.

[0070] It should be noted that the PWM generator 410, the driver 420, the analog-to-digital converter (for example, sigma-delta ADC 430), the digital filter 440 and the error amplifier 450 described with reference to Fig. 4, may have one or more properties in common with the PWM generator 310, the driver 320, the analog-to-digital converter (for example, a sigma-delta ADC 330), the digital filter 340 and the error amplifier 350 described with reference to Fig. 3. For the sake of brevity, these details are not repeated here.

[0071] The feedback configurations 300 and 400 of the described embodiments provide compensation for driver mismatches in the controller by adjusting, based on a degree of driver mismatch, the widths or duty cycles of pulse-width modulated (PWM) pulses that control the operation of the LED strings or the current through the LED channel. For example, if a first LED driver operates at a first operating speed and a second LED driver operates at a second operating speed (greater than the first speed), the first duty cycle of the first PWM driver signal is adjusted with respect to the first duty set signal to be greater than the second duty cycle of the second PWM driver signal adjusted with respect to the second duty set signal, while the first and second duty set signals have substantially the same value.Such feedback configurations thus allow for adjustment of an average current between the one or more LED strings to provide better uniformity or adjustment in brightness levels across the LED strings.

[0072] The Fig. 5A-5B show comparative time courses of currents through two different LED channels of the multi-channel LED system, without feedback control ( Fig. 5A) and with feedback control ( Fig. 5B) according to some embodiments.

[0073] In Fig. 5A, the first channel and the second channel are configured to be controlled by drivers that differ from each other in characteristics. As shown in Fig. As shown in Figure 5A, due to the driver mismatches between the first channel and the second channel, in the absence of feedback configurations 300 or 400, in response to receiving identical control signals (collectively represented as command pulse 510 for a single duty cycle), a current flowing through the first channel (first channel current 520) is different from a current flowing through the second channel (second channel current 522). Accordingly, a brightness of an LED string of the first channel is different from, or mismatched with, the brightness of an LED string of the second channel.

[0074] Fig. Figure 5B includes examples of time courses showing a result of the feedback process (here with reference to Fig. 3 or Fig. 4) to compensate for a driver mismatch between the first and second channels, to adjust the brightness between the LED chains of the first and second channels. As described in Fig. As shown in Figure 5B, the duty cycles of the control signals provided to the first channel and the second channel are modified (represented as first-channel adjusted pulse 550 and second-channel adjusted pulse 552 for the given duty cycle) to compensate for the effects of a driver mismatch between the first channel and the second channel based on a degree of mismatch. Accordingly, a current through the first channel and a current through the second channel (represented, for example, as first channel current 560 and second channel current 562) are substantially matched and adjusted to be equivalent to the target or desired average current value.< / lsense> < / lsense> < / lsense> < / lsense>

Claims

[1] LED (Light Emitting Diode) control device comprising: a first LED driver (320-a) for switching on or off a first current through a first LED chain (103-a) in response to a first PWM driver signal (390-a); a first compensation circuit (345-a) for generating the first PWM drive signal (390-a) in response to a first duty set signal (305-a), the first duty set signal (305-a) being a first predetermined target duty cycle set for the first PWM drive signal (390-a), the first compensation circuit (345-a) comprising: a first error determination circuit (350-a) for receiving a first feedback signal (370-a) indicative of the first current and for generating a first error signal (380-a) indicative of a difference between a first predetermined target value and the first feedback signal (370-a), wherein the first predetermined target value corresponds to the first predetermined target duty cycle and the first feedback signal (370-a) represents a measured duty cycle value of the first current; and a first PWM signal generating circuit (310-a) for generating the first PWM drive signal (390-a) having a first duty cycle corresponding to the duty set signal (305-a) and further adjusted by the first error signal (380-a). [2] The LED control device according to claim 1, further comprising: a second LED driver (320-k) for switching on or off a second current through a second LED chain in response to a second PWM driver signal (390-k); and a second compensation circuit (345-k) for generating the second PWM drive signal (390-k) in response to a second duty set signal (305-k), wherein the second duty set signal (305-k) is a second predetermined target duty cycle set for the second PWM drive signal (390-k), the second compensation circuit (345-k) comprising: a second error determination circuit (350-k) for receiving a second feedback signal (370-k) indicative of the second current and for generating a second error signal (380-k) indicative of a difference between a second predetermined setpoint and the second feedback signal (370-k); and a second PWM signal generating circuit (310-k) for generating the second PWM drive signal (390-k) having a second duty cycle corresponding to the second duty set signal (305-k) and further adjusted by the second error signal (380-k). [3] The LED control device according to claim 1, wherein: the first error determination circuit (350-a) generates the first error signal (380-a) during a first switching cycle of the first PWM drive signal (390-a); and wherein the first PWM signal generating circuit generates the first PWM drive signal (390-a) so that the first duty cycle corresponds to the first duty set signal (305-a) and is further adjusted by the first error signal (380-a), wherein for a second switching cycle following the first switching cycle, based on the first error signal (380-a) generated during the first switching cycle: the first PWM signal generating circuit decreases the first duty cycle with respect to the first duty set signal (305-a) when the first feedback signal (370-a) generated during the first switching cycle exceeds the first predetermined set value; and the first PWM signal generating circuit increases the first duty cycle with respect to the first duty set signal (305-a) when the first feedback signal (370-a) generated during the first switching cycle is less than the first predetermined set value. [4] The LED control device according to claim 1, wherein: the first PWM signal generating circuit generates the first PWM drive signal (390-a) to further have a predetermined peak magnitude value; and the LED control device further comprises an analog-to-digital converter (ADC) for generating the first feedback signal (370-a) by digitizing an analog input signal corresponding to the first current, wherein an input signal range of the ADC is proportional to the predetermined peak magnitude value. [5] The LED control device according to claim 2, wherein: the first LED driver (320-a) operates at a first operating speed; the second LED driver (320-k) operates at a second operating speed, wherein the second operating speed is higher than the first operating speed; and the first duty cycle of the first PWM driver signal (390-a) is adjusted with respect to the first duty set signal (305-a) to be greater than the second duty cycle of the second PWM driver signal (390-k) adjusted with respect to the second duty set signal (305-k). [6] A method for controlling an LED (Light Emitting Diode) system having a first LED chain, the method comprising: Switching on or off a first current through the first LED chain in response to a first PWM driver signal (390-a), wherein the first PWM driver signal (390-a) is responsive to a first duty set signal (305-a), wherein the first duty set signal (305-a) is a first predetermined target duty cycle set for the first PWM driver signal (390-a), the method further comprising: Receiving a first feedback signal (370-a) indicative of the first current through the first LED string; Generating a first error signal (380-a) indicative of a difference between a first predetermined setpoint and the first feedback signal (370-a), wherein the first predetermined setpoint corresponds to the first predetermined desired duty cycle and the first feedback signal (370-a) represents a measured duty cycle value of the first current; and Generating the first PWM drive signal (390-a) having a first duty cycle corresponding to the first duty set signal (305-a) and further adjusted by the first error signal (380-a). [7] The method of claim 6, wherein the LED system further comprises a second LED chain, the method further comprising: Switching on or off a second current through the second LED chain in response to a second PWM driver signal (390-k), wherein the second PWM driver signal (390-k) is responsive to a second duty set signal (305-k), wherein the second duty set signal (305-k) is a second predetermined target duty cycle set for the second PWM driver signal (390-k), the method further comprising: Receiving a second feedback signal (370-k) indicative of the second current through the second LED chain; Generating a second error signal (380-k) indicating a difference between a second predetermined setpoint and the second feedback signal (370-a); and Generating the second PWM drive signal (390-k) having a second duty cycle corresponding to the second duty set signal (305-k) and further adjusted by the second error signal (380-k). [8] Method according to claim 6, wherein the first error signal (380-a) is generated during a first switching cycle of the first PWM driver signal (390-a); and wherein the generation of the first PWM driver signal (390-a) is further adjusted by the first error signal (380-a), wherein the generation for a second switching cycle following the first switching cycle based on the first error signal (380-a) generated during the first switching cycle comprises: Reducing the first duty cycle with respect to the first duty set signal (305-a) when the first feedback signal (370-a) generated during the first switching cycle exceeds the first predetermined setpoint; and Increasing the first duty cycle with respect to the first duty set signal (305-a) if the first feedback signal (370-a) generated during the first switching cycle is less than the first predetermined setpoint. [9] A method according to claim 6, wherein: the first PWM drive signal (390-a) has a predetermined peak magnitude value; and generating the first feedback signal (370-a) comprises digitizing an analog input signal corresponding to the first current via an analog-to-digital converter (ADC), wherein an input signal range and a resolution of the ADC are independent of the predetermined peak magnitude value for the first PWM drive signal (390-a). [10] A method according to claim 6, wherein: the first PWM drive signal (390-a) has a predetermined peak magnitude value; and generating the first feedback signal (370-a) comprises digitizing an analog input signal corresponding to the first current via an analog-to-digital converter (ADC), wherein an input signal range of the ADC is proportional to the predetermined peak magnitude value.

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