LED Dimming Circuit Using PMOS Switch Driver
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Solution Overview
Problem
Conventional LED dimming circuits using Buck converters face issues with voltage maintenance across the LED load after regulator turn-off, leading to unregulated current flow and delayed restoration, which is unacceptable for precision applications requiring accurate control of LED intensity and color.
Innovation Solution
A high side disconnect switch driver circuit using a PMOS switch with a pull-up and pull-down switch configuration, driven by a comparator and logic NOR gate, allows precise control of the PWM signal level shift independent of the supply voltage, reducing RC constant delays and power dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a conventional Buck switching regulator with capacitor is used to generate pulsed current, then the regulator can maintain voltage across the LED load after turn-off, but this causes delayed current restoration due to capacitor discharge and prevents precise LED intensity control
Solution Approach 1:
The patent extracts the capacitor from the LED current path by introducing a separate output capacitor that does not directly couple to the LED load. The regulator capacitor is removed from the circuit entirely, eliminating the RC time constant that caused delayed current restoration. This allows the LED current to follow the PWM signal edges precisely without being influenced by capacitor charge/discharge dynamics.
Solution Approach 2:
The patent segments the power supply function into two independent parts: the regulator provides continuous power to an output capacitor, while a separate switch (PMOS or NMOS) directly controls LED current based on PWM signals. This segmentation decouples the voltage regulation function from the current switching function, allowing precise current control without capacitor-induced delays.
2Ease of operation
If a high-side PMOS switch is used for LED dimming, then the LED load can be disconnected from the high voltage side, but resistor dividers introduce RC time constants and excessive power dissipation
Solution Approach 1:
The patent removes the resistor divider network from the high-side PMOS gate control circuitry. Instead of using resistors to divide voltage and control the PMOS gate, the invention uses a direct voltage control approach where the PWM signal is level-shifted or directly applied to the gate through a capacitor-coupled or voltage-follower circuit, eliminating the power-dissipating resistor divider.
Solution Approach 2:
The patent replaces the passive resistor-based voltage division mechanism with an active voltage control mechanism using operational amplifiers, voltage followers, or direct digital-to-analog conversion. This substitution eliminates the continuous power dissipation inherent in resistor dividers while maintaining precise voltage control for the PMOS gate.
3Adaptability or versatility
If resistor dividers are used to control PMOS gate voltage in high-side dimming, then the switch can be driven with inverted PWM signal, but this introduces RC time constants that affect switching speed
Solution Approach 1:
The patent replaces the passive RC-based signal inversion and level-shifting circuitry with active electronic components such as operational amplifiers, voltage followers, or dedicated level-shifter circuits. These active circuits provide instantaneous signal transformation without the time delays inherent in RC networks, thereby maintaining fast switching speeds while achieving the required signal inversion and level shifting.
Solution Approach 2:
The patent introduces an intermediary circuit stage between the PWM signal source and the PMOS gate that actively transforms the signal characteristics. This intermediary uses transistors, operational amplifiers, or dedicated level-shifter ICs to provide the necessary signal inversion and level shifting without relying on passive RC time constants, thus eliminating the speed limitation.
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
The solution enables precise control of pulsed current width to LEDs, minimizing color shift and power loss, and improving dimming accuracy by eliminating resistor divider-induced delays and current draw, thus maintaining consistent LED color across a wide range of brightness.
Implementation Method 1
A high side disconnect switch driver circuit using a PMOS switch with a pull-up and pull-down switch configuration
Implementation Method 2
A control circuit is configured for shifting the level of the PWM signal to cause the PWM signal to vary between a voltage of the first electrode of the switch and a prescribed fixed voltage
Implementation Method 3
A variety of regulator topologies has been implemented to generate pulsed current of varying duty ratio, namely Buck, Boost, and Buck-Boost converters, each of which employs an inductor and filter capacitor to generate regulated voltage
Implementation Method 4
each of which employs an inductor and filter capacitor to generate regulated voltage
Implementation Method 5
An example is in a circuit for driving a light emitting diode (LED) with a pulsed current source, in which the pulse width is varied in order to control light intensity produced by the LED
Data Source
AI summary
A circuit for controlling pulsed current to a load, one application of which is in LED dimmer circuitry, comprises first and second reference nodes for receiving a supply voltage, an input node for receiving a timing signal such as a PWM signal, and a controlled switch coupled between the first and second reference voltage nodes for supplying current to the load. Pull-up circuitry may be coupled between a control electrode of the controlled switch and first reference voltage node, and a pull-down switch coupled between the control electrode and second reference voltage node. A control circuit coupled between the input node and control electrode of the controlled switch is configured to control the controlled switch in response to the timing signal. The circuit may further include a reference voltage source configured for producing a voltage of magnitude independent of supply voltage magnitude. The control circuit is coupled to the reference voltage source and operative to control the controlled switch in response to the timing signal and reference voltage.