LED Driver Delay Module for High PWM Dimming Ratio
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Solution Overview
Problem
LED driver circuits face challenges in achieving high PWM dimming ratios beyond 10,000:1 and reducing output ripple voltage, especially during short PWM on-times, leading to loss of regulation and audible noise due to inductor current not reaching steady state quickly.
Innovation Solution
The implementation of a 'pre-emptive boost' technique involving a time delay module that delays the current signal to the LEDs with respect to the input signal, allowing the inductor current to ramp up to a steady state, thereby reducing output ripple and improving regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If PWM on-time is reduced to achieve high dimming ratios, then dimming resolution is improved, but inductor current cannot reach steady state causing loss of regulation
Solution Approach 1:
The patent applies preliminary action by extending the boost converter switching cycle beyond the PWM on-time to allow inductor current to reach steady state. The boost converter operates for a duration equal to PWM on-time plus an additional period, ensuring current stabilization before LED current activation, thus maintaining voltage regulation precision even at ultra-low PWM duty cycles.
2Manufacturing precision
If PWM on-time is reduced to achieve high dimming ratios, then dimming resolution is improved, but output ripple voltage increases
Solution Approach 1:
The patent uses preliminary action by pre-charging the inductor during an extended boost converter operation period before the actual LED current pulse. This pre-charging phase allows the inductor to store sufficient energy and reach steady-state current, reducing the ripple effect when the LED current is subsequently activated, even at extremely short PWM on-times.
3Object-generated harmful factors
If inductor current is allowed to ramp up to steady state, then output ripple is reduced, but PWM on-time must be extended reducing dimming ratio capability
Solution Approach 1:
The patent implements preliminary action by separating the inductor current ramp-up phase from the LED current activation phase. The boost converter operates in advance to establish steady-state inductor current, and only after this preliminary current establishment does the LED current pulse begin, allowing both ripple reduction and high dimming ratio to coexist.
Solution Approach 2:
The patent applies segmentation by dividing the overall operation into distinct phases: a preliminary boost converter operation phase for inductor current establishment, and a subsequent LED current pulse phase. This temporal segmentation allows the inductor to reach steady state without extending the actual PWM on-time that controls LED brightness, thereby maintaining high dimming ratios while reducing output ripple.
Data Source
AI summary
In one aspect, an integrated circuit (IC) is configured to receive an input signal. The IC includes a boost switch driver configured to provide a switching operation to a boost converter to drive a string of light emitting diodes (LEDs), a current sink driver connected to a current source and configured to provide a current signal to the string of LEDs, and a delay module configured to delay the current signal to the string of LEDs with respect to the input signal.