PPG LED Driver Circuit With NMOS Gating for Smaller Switch Area
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Solution Overview
Problem
Existing LED drivers for photoplethysmography applications require large switches to handle high driving currents, leading to circuit miniaturization challenges and increased costs due to the large circuit area needed for these switches.
Innovation Solution
The proposed solution involves an operational amplifier (OP) coupled with N current driving circuits and a resistor circuit, utilizing an n-channel metal oxide semiconductor (NMOS) transistor and a switch circuit to control the current flow without the need for large switches, allowing the LED driver to operate in enablement and disablement modes, with the NMOS transistor directly coupled to the LED current path and the switch circuit controlling the transistor with a control voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a switch is used to control each LED in the current driving circuit, then the LED can be enabled or disabled, but the switch requires a large circuit area to withstand the large driving current
Solution Approach 1:
The patent introduces an intermediary p-channel MOS transistor (PMOS) between the control switch and the LED current path. The PMOS transistor acts as a current buffer that can handle the large LED driving current, while the main switch only needs to control the gate of the PMOS, requiring minimal circuit area. This mediator approach resolves the contradiction by separating the high-current handling function from the low-area control function.
Solution Approach 2:
The patent replaces the direct mechanical/electrical switch connection with a field-effect transistor (PMOS) controlled by an electric field at the gate. Instead of using a switch that directly carries the large LED current, the system uses voltage control at the PMOS gate to regulate the current, substituting a high-current mechanical switch with a low-area field-effect device controlled by small signals.
2Adaptability or versatility
If N switches are used between N LEDs and N current driving circuits, then each LED can be independently controlled, but the total circuit area increases significantly
Solution Approach 1:
The patent merges multiple LED control functions into a single current driving circuit that can sequentially or simultaneously drive multiple LEDs. Instead of having separate complete control circuits for each LED, the design uses one current driving circuit with multiplexing capability, reducing the total number of switches and overall circuit area while maintaining the ability to independently control each LED through time-division or current-steering techniques.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration reduces the need for large switches, minimizing circuit area and cost while maintaining efficient current control, allowing for more compact and cost-effective LED drivers.
Implementation Method 1
The OP includes an OP input end, an OP inverting input end, and an OP output end, wherein the OP input end is for receiving a reference voltage, the OP inverting input end is for receiving a feedback voltage, and the OP output end is for output a control voltage
Implementation Method 2
Each of the N current driving circuit(s) is configured to operate in one of an enablement mode and a disablement mode and includes an n channel metal oxide semiconductor (NMOS) transistor and a switch circuit
Implementation Method 3
The photoplethysmography (PPG) technology involves illuminating skin with a controllable light source (e.g., light emitting diode (LED))
Implementation Method 4
light emitting diode (LED)
Implementation Method 5
The resistor circuit is coupled between the feedback node and a low voltage terminal and configured to determine a total current passing through the N current driving circuit(s) in conjunction with the feedback voltage
Data Source
AI summary
An LED driver includes an operational amplifier (OP), N current driving circuits, and a resistor circuit. The OP compares a reference voltage with a feedback voltage to generate a control voltage. Each current driving circuit is coupled with an LED, and includes: an NMOS transistor including a drain, a source, and a gate, and being turned on according to the control voltage in an enablement mode and turned off according to the voltage of a ground terminal in a disablement mode, wherein the drain is coupled with the LED and the voltage at the source is the feedback voltage; and a switch circuit coupling the OP with the gate in the enablement mode, and coupling the ground terminal with the gate in the disablement mode. The resistor circuit is coupled between the source and the ground terminal and controls the current passing through the N current driving circuits.