LED Backlight Driver Circuit Offset Voltage Cancellation
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Solution Overview
Problem
The existing LED backlight devices for liquid crystal displays suffer from uneven brightness due to offset voltage in differential amplifiers, leading to differences in current flow through LED circuits and resulting in non-uniform illumination.
Innovation Solution
The proposed solution involves a backlight device with a differential amplifier configuration where the input transistors and load transistors are swapped periodically, utilizing an exchange controller to swap the detection voltage and reference voltage inputs to the transistors, and a brightness adjusting switch to control the connection state between the amplifier and driving transistor, ensuring equal current flow through LED circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a differential amplifier is used to control current flow in LED circuits, then the brightness uniformity should be improved, but offset voltage in the amplifier causes error voltage that leads to uneven brightness
Solution Approach 1:
The patent applies periodic action by alternately swapping the connections of detection voltage and reference voltage to the first and second input transistors at regular intervals. This periodic swapping causes the offset voltage errors to alternate in sign, and when averaged over time, the errors cancel out, eliminating the uneven brightness problem while maintaining precise current control.
Solution Approach 2:
The patent converts the harmful offset voltage error into a beneficial effect by deliberately swapping the voltage inputs periodically. The offset voltage that originally caused uneven brightness is transformed into a canceling error through the swapping mechanism, where the error in one half-cycle is compensated by the opposite error in the next half-cycle, ultimately eliminating the harmful effect.
2Illumination intensity
If input transistors are swapped periodically to cancel offset voltage errors, then brightness uniformity is improved, but the circuit complexity increases due to the exchange controller and switching mechanisms
Solution Approach 1:
The patent applies universality by designing the amplifier circuit to perform multiple functions: it not only amplifies the difference between detection voltage and reference voltage but also automatically compensates for offset voltage errors through the periodic swapping mechanism. The same amplifier circuit structure handles both the normal amplification function and the error compensation function, reducing the need for separate compensation circuits.
Solution Approach 2:
The amplifier circuit performs self-service by using its own internal components (input transistors and load transistors) to compensate for its own offset voltage errors. The periodic swapping of voltage inputs within the amplifier causes the offset errors to cancel out automatically, without requiring external calibration or additional compensation circuits, making the amplifier self-correcting.
Data Source
AI summary
A backlight device that is not affected by offset voltage. Driving transistor (41) operates such that current flows in LED circuit (11), and the detection voltage generated by current detector (42) and the reference voltage are input alternately to first input transistor (30a) and second input transistor (30b) of amplifier (20). Brightness adjusting switch (43) operates such that when driving transistor (41) is turned OFF during the OFF period of LED circuit (11), for example, during the horizontal blanking interval or vertical blanking interval, the detection voltage and the reference voltage are swapped, so that the average voltage generated in current detector (42) is equal to the reference voltage, and the effect of the offset voltage can be eliminated.