Sample-and-Hold LED Driver Feedback for Short PWM Dimming
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Solution Overview
Problem
Existing PWM-based driver circuits for current sensitive loads like LEDs face challenges in maintaining constant current regulation due to delays in DC/DC converter response, leading to nonlinear dimming and restricted minimum PWM pulse timing, which can increase system cost and introduce EMI noise.
Innovation Solution
A circuit with a sample and hold feedback control scheme that samples the cathode voltage during the PWM active phase and uses a capacitor to store and provide feedback during the inactive phase, allowing continuous adjustment of the anode voltage and extending the feedback control loop with a timer circuit, thereby maintaining steady-state operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a PWM signal is used to dim the LED string, then the LED brightness can be controlled, but the forward voltage switches between nominal on-voltage and off-voltage, causing constant current regulation to be lost for PWM pulses shorter than the converter response time
Solution Approach 1:
The feedback voltage is sampled and stored on a capacitor during the PWM active phase before the converter needs to respond. This preliminary action ensures that the feedback information is ready and available when the converter needs to adjust the output, allowing constant current regulation to be maintained even during very short PWM pulses where the converter response time would otherwise be insufficient
Solution Approach 2:
A sample and hold circuit with a capacitor is introduced as an intermediary between the PWM signal and the feedback loop. This intermediary stores the feedback voltage during the active phase and releases it during the inactive phase, bridging the time gap between PWM transitions and converter response, thereby maintaining regulation reliability
2Use of energy by moving object
If the feedback loop is opened during the PWM inactive phase, then power consumption is reduced, but the voltage at the cathode is no longer valid for determining the system feedback error signal
Solution Approach 1:
Instead of directly using the real-time cathode voltage during the inactive phase, the system creates a copy of the feedback voltage by sampling and storing it on a capacitor during the active phase. This copied voltage information is then used to generate the error signal during the inactive phase, maintaining feedback functionality without requiring the actual cathode voltage to be valid
Solution Approach 2:
The feedback sampling is performed periodically during the PWM active phase, and the stored feedback information is held and used during the inactive phase. This periodic sampling approach allows the system to refresh the feedback information only when needed (during active phase) while maintaining continuous control capability throughout both phases
3Loss of time
If a DC/DC converter with faster response is used, then the minimum PWM pulse timing can be reduced, but the system cost increases and additional EMI noise may be introduced
Solution Approach 1:
The system uses a feedback mechanism where the output voltage is sensed and used to generate an error signal that controls the converter. By sampling this feedback voltage during the active phase and holding it during the inactive phase, the system maintains effective feedback control without requiring the converter to respond instantaneously to every PWM edge, thereby allowing the use of slower, lower-cost converters with reduced EMI
Solution Approach 2:
The feedback voltage is sampled and stored in advance during the active phase, preparing the control information before the converter needs to act. This preliminary preparation of feedback information reduces the effective response time requirement, allowing standard-speed converters to achieve the required performance without incurring additional cost or EMI penalties
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 solution enhances PWM dimming performance by ensuring the switching regulator can respond to energy demands during both active and inactive PWM phases, maintaining constant current regulation and accommodating shorter PWM pulse durations without additional cost or EMI issues.
Implementation Method 1
A circuit with a sample and hold feedback control scheme that samples the cathode voltage during the PWM active phase and uses a capacitor to store and provide feedback during the inactive phase
Data Source
AI summary
A drive circuit and a method for maintaining an operating state of the drive circuit. The drive circuit includes a capacitor connected to an inverting input terminal of an operational amplifier and to a terminal of a current sensitive load through a switch. The output of the operational amplifier is connected to a switching regulator which has an output terminal connected to another terminal of the current sensitive load. An energy storage element is connected to the inverting input terminal of the operational amplifier. Energy is stored in the energy storage element during a first portion of a PWM pulse which is used during a second portion of the PWM pulse to generate the error signal. A drive signal is generated from the error signal where the drive signal is used to generate a voltage that biases the current source during the second portion of the PWM pulse.