LED Drive Circuit Feedback Diode for LCD Backlight
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Solution Overview
Problem
Conventional LED drive circuits for liquid crystal display devices using white light-emitting diodes as backlights are costly due to the need for multiple transistors in parallel or high-power transistors, which also result in a larger circuit board size, especially in small-sized displays.
Innovation Solution
The implementation of a constant voltage circuit with a diode feedback path in the LED drive circuit, allowing for reduced transistor power loss and size by compensating for temperature-dependent forward voltage changes, enabling the use of fewer and less expensive transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple transistors are used in parallel or transistors large in allowable loss are used as final-stage transistors for driving the white light-emitting diodes by current, then the white light-emitting diodes can be driven reliably, but the cost of parts for the LED drive circuit becomes higher
Solution Approach 1:
The patent employs a feedback mechanism where the output voltage is fed back to the inverting input terminal of the operational amplifier. This feedback loop automatically regulates the voltage across the white light-emitting diodes, ensuring stable operation without requiring multiple transistors or high-power transistors. The feedback principle enables reliable LED driving with reduced component complexity and lower part costs.
2Reliability
If multiple transistors are used in parallel or transistors large in allowable loss are used as final-stage transistors for driving the white light-emitting diodes by current, then the white light-emitting diodes can be driven reliably, but the size of the circuit board equipped with the LED drive circuit becomes larger
Solution Approach 1:
The feedback circuit using an operational amplifier provides reliable LED driving control within a compact configuration. The feedback mechanism ensures stable voltage regulation across the white light-emitting diodes, eliminating the need for multiple transistors or large-power transistors that would occupy excessive board space. This approach maintains reliability while significantly reducing the circuit board area.
Solution Approach 2:
The patent replaces the traditional transistor-based current drive mechanism with an operational amplifier-based voltage feedback system. This substitution eliminates the need for multiple transistors and large-power components, thereby reducing the overall circuit board size while maintaining reliable LED driving performance.
3Stability of the object's composition
If a constant voltage circuit is used to control the voltage output, then the voltage becomes constant, but the temperature-dependent forward voltage changes of LEDs are not compensated
Solution Approach 1:
The patent utilizes the temperature-dependent characteristics of the operational amplifier's input offset voltage to compensate for the temperature-dependent forward voltage changes of the white light-emitting diodes. By carefully selecting the operational amplifier with appropriate temperature coefficients, the circuit automatically adjusts the feedback voltage to maintain constant LED voltage across temperature variations, achieving both voltage stability and temperature compensation.
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 reduces the cost of parts and minimizes the size of the circuit board by maintaining a constant voltage across the LEDs, even at varying temperatures, allowing for the use of fewer and less expensive transistors, thereby decreasing the overall cost and size of the LED drive circuit.
Implementation Method 1
compensating for temperature-dependent forward voltage changes
Implementation Method 2
at least one diode provided in a feedback path lying between the output terminal of the constant voltage circuit and the feedback terminal
Implementation Method 3
a constant voltage circuit for controlling a voltage outputted from an output terminal in such a manner that a voltage inputted to a feedback terminal becomes a constant voltage
Implementation Method 4
at least one light-emitting diode
Implementation Method 5
means for driving the at least one light-emitting diode connected to the output terminal of the constant voltage circuit by a constant current
Data Source
AI summary
The present invention provides a liquid crystal display device comprising a liquid crystal display panel, a backlight having at least one light-emitting diode, and an LED drive circuit for driving the at least one light-emitting diode. The LED drive circuit includes a constant voltage circuit for controlling a voltage outputted from an output terminal in such a manner that a voltage inputted to a feedback terminal becomes a constant voltage, and means for driving the at least one light-emitting diode connected to the output terminal of the constant voltage circuit by a constant current. The LED drive circuit has at least one diode provided in a feedback path lying between the output terminal of the constant voltage circuit and the feedback terminal.