Constant Current Drive Circuit for LED PWM Dimming
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Solution Overview
Problem
Existing constant current drive circuits for LED elements in PWM dimming applications are affected by variations in offset voltage, leading to inaccurate stop timing of quick charge, resulting in overshoot or insufficient luminance due to delays in the LED's rise from an off state to an on state.
Innovation Solution
A constant current drive circuit with a second resistor connected between the output side of the gm amplifier and the capacitor, utilizing a charge stopping circuit with a comparison voltage generated by a second gm amplifier, reduces the influence of offset voltage variations and allows for precise adjustment of quick charge stop timing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a quick charge circuit is used to accelerate the rise of the output transistor when switching from OFF to ON, then the response speed is improved, but the stop timing becomes inaccurate due to offset voltage variations
Solution Approach 1:
The patent implements a feedback mechanism where the charge stop circuit monitors the gate voltage of the output transistor and stops the quick charge current when the voltage reaches a predetermined level. This feedback control eliminates the impact of offset voltage variations in the comparator, ensuring accurate stop timing while maintaining fast response speed.
Solution Approach 2:
The patent introduces an intermediary voltage detection mechanism that uses the gate voltage of the output transistor as an intermediate signal to control the stop timing of the quick charge circuit. This intermediary approach decouples the stop timing accuracy from the comparator's offset voltage, allowing precise control independent of comparator imperfections.
2Speed
If the quick charge current is increased to reduce delay time, then the rise time is improved, but overshoot occurs due to inaccurate stop timing
Solution Approach 1:
The feedback control in the charge stop circuit prevents overshoot by accurately detecting when the gate voltage reaches the target level and immediately stopping the quick charge current. This precise feedback control allows the use of higher quick charge currents to reduce rise time without causing overshoot, as the stopping point is accurately controlled regardless of comparator offset voltage.
3Adaptability or versatility
If PWM dimming is performed with short ON time, then the dimming range is improved, but the LED does not reach required luminance due to time delay
Solution Approach 1:
The patent applies preliminary action by pre-charging the gate capacitor of the output transistor using the quick charge circuit before the normal operation begins. This preliminary charging action ensures that the transistor reaches full conduction quickly, allowing the LED to achieve required luminance even during short ON periods in PWM dimming, thereby extending the effective dimming range to include very low duty cycles.
Data Source
AI summary
Provided is a constant current drive circuit that can improve accuracy of a stop timing of quick charge and prevent an overshoot, an insufficient quick charge, and the like from occurring. The constant current drive circuit includes a resistor R5 connected between the output side of the gm amplifier 6 and the input side of the transistor M1. The comparator 9 serving as a charge stopping circuit includes a voltage Vof, and stops charging by the quick charge circuit 8 based on a comparison result of the voltage Vof and the voltage Vs2 generated in the resistor R5.