LED Drive Circuit Stabilizing Current via Negative Feedback
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Solution Overview
Problem
Existing drive circuits for LEDs, such as those used in portable information terminals, experience variations in current supply due to temperature-related changes in forward voltage and transistor characteristics, leading to inconsistent luminance.
Innovation Solution
A drive circuit design that includes an operational amplifier for comparing input voltages, transistors for current supply based on comparison results, and switches for controlling the current flow, forming a negative feedback circuit to maintain a consistent output voltage and prevent current variation, along with resistors and a D/A converter for precise control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If PWM control is used to adjust LED luminance, then luminance control is achieved, but current variation occurs due to temperature changes and LED forward voltage variation
Solution Approach 1:
The patent implements a negative feedback circuit using an operational amplifier that continuously monitors the voltage across the LED and adjusts the transistor gate voltage to maintain constant current flow. The feedback loop compares the actual LED voltage with a reference voltage and compensates for variations caused by temperature changes and forward voltage drift, thereby stabilizing the current despite PWM modulation.
Solution Approach 2:
The patent dynamically adjusts the transistor's operating parameters (gate voltage) in response to changing conditions. By modifying the control voltage applied to the transistor gate based on feedback from the operational amplifier, the system maintains optimal current levels across varying temperatures and LED characteristics while preserving PWM luminance control.
2Device complexity
If a simple switching circuit is used for PWM control, then device complexity is reduced, but current control precision deteriorates
Solution Approach 1:
The patent introduces an operational amplifier as an intermediary component between the PWM switching signal and the LED driving transistor. This intermediary actively regulates the transistor's gate voltage based on real-time LED voltage feedback, providing precise current control without requiring complex multi-component circuits. The op-amp mediates between the simple PWM switch and the LED, achieving high precision with minimal additional complexity.
Data Source
AI summary
A drive circuit (100) includes an operational amplifier (OP1) that compares a voltage applied to an inverting input terminal with a reference voltage applied to a non-inverting input terminal, a MOS transistor (M1) that has an output connected to the inverting input terminal of the operational amplifier (OP1) and supplies a current to a load (3) depending on a comparison result of the operational amplifier (OP1), and a switch (SW1) that switches, based on a control signal, between outputting the comparison result of the operational amplifier (OP1) to the MOS transistor (M1) and outputting a predetermined voltage to the MOS transistor (M1) to turn off the MOS transistor (M1).