Logarithmic PWM Dimming Circuit for Smooth LED Brightness
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Solution Overview
Problem
Switching regulator circuits used to drive light emitting diodes (LEDs) typically control brightness linearly, which does not appear as a smooth transition to the human eye, as perceived brightness is better represented on a logarithmic scale.
Innovation Solution
An exponential-scale pulse width modulation (PWM) controller is employed, featuring a waveform generator circuit that produces a logarithmic waveform signal, and a comparator circuit that generates a PWM control signal with a duty cycle that changes exponentially, allowing for smooth transitions in LED brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If linear current control is used to change LED brightness, then the control mechanism is simple, but the brightness transition does not appear smooth to the human eye
Solution Approach 1:
The patent transforms the linear current control parameter into a logarithmic current control parameter. By changing the control law from linear to logarithmic, the brightness transition becomes smooth to the human eye while maintaining a relatively simple control circuit implementation through standard PWM techniques.
Solution Approach 2:
The patent employs pulse width modulation (PWM) technique where the LED is driven by periodic pulses rather than continuous current. By adjusting the duty cycle of these periodic pulses according to a logarithmic scale, smooth brightness transitions are achieved while keeping the average current control simple through periodic switching action.
2Ease of operation
If analog dimming is used to control LED brightness, then the circuit is simple, but color shifts occur during dimming
Solution Approach 1:
The patent uses PWM periodic switching to control LED brightness instead of analog current adjustment. This periodic action keeps the LED operating at full current during on-pulses, maintaining stable color characteristics, while the duty cycle adjustment provides simple brightness control without color shifts.
Solution Approach 2:
The patent maintains continuous operation of the LED at its optimal current level during the on-pulses of the PWM cycle, ensuring the LED always operates in its stable color region. The dimming effect is achieved through temporal averaging rather than reducing the instantaneous current, thus preserving color stability.
3Device complexity
If PWM dimming with linear duty cycle is used, then the control is simple, but the brightness range is limited
Solution Approach 1:
The patent changes the duty cycle parameter from linear progression to logarithmic progression. This parameter transformation extends the effective dimming range by distributing the duty cycle values more effectively across the perceptible brightness spectrum, allowing for a wider dimming range while maintaining simple control implementation.
Data Source
AI summary
An exponential scale pulse width modulation (PWM) controller comprises a waveform generator circuit configured to generate a logarithmic waveform signal that has the shape of an increasing logarithm function; and a first comparator circuit including a first input to receive the logarithmic waveform signal, a second input to receive an input signal, and an output that provides a PWM control signal that includes signal pulses having a duty cycle that changes exponentially with respect to the input signal.