Backlight Control Circuit Modulating PWM Duty Ratio for LCD Brightness Consistency
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Solution Overview
Problem
The brightness of liquid crystal display devices decreases during a frame data disable period due to the leakage characteristic of thin film transistors, which is not effectively addressed by existing Content Adaptive Backlight Control methods.
Innovation Solution
A control circuit and method that modulates the duty ratio of the PWM signal to generate a second PWM signal, which increases the brightness of the backlight during the frame data disable period by adjusting the output current based on the second PWM signal, ensuring consistent brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the PWM signal duty ratio is kept constant during frame data disable period, then the control circuit is simple, but the brightness decreases due to transistor leakage
Solution Approach 1:
The patent applies dynamics by making the PWM signal duty ratio adjustable rather than fixed. The control circuit dynamically changes the duty ratio based on the frame data enable signal state: using a first duty ratio during frame data enable periods and a second, higher duty ratio during frame data disable periods to compensate for transistor leakage and maintain consistent brightness.
Solution Approach 2:
The patent changes the duty ratio parameter of the PWM signal based on operational conditions. By detecting the frame data enable signal state and adjusting the PWM duty ratio accordingly (increasing it during disable periods), the system compensates for brightness degradation caused by transistor leakage without requiring additional hardware components.
2Illumination intensity
If the backlight brightness is increased during frame data disable period, then the brightness consistency is improved, but the energy consumption increases
Solution Approach 1:
The patent applies periodic action by rhythmically adjusting the PWM duty ratio according to the frame data timing. The control circuit periodically switches between the first duty ratio (during enable periods) and the second duty ratio (during disable periods), synchronizing with the display refresh cycle. This ensures brightness consistency is maintained only when necessary (during disable periods) while consuming minimal energy during enable periods.
3Illumination intensity
If a higher PWM signal duty ratio is used during frame data disable period, then the brightness compensation is effective, but the contrast between enable and disable periods decreases
Solution Approach 1:
The patent applies local quality by applying different duty ratio values to different time periods within the display refresh cycle. The first duty ratio is used during frame data enable periods when normal brightness control is sufficient, while the second, higher duty ratio is applied specifically during frame data disable periods where leakage compensation is needed. This localized adjustment maintains overall brightness stability while providing targeted compensation.
Data Source
AI summary
The disclosure shows a control circuit for backlight, a control method and a liquid crystal display device. The control circuit includes a display driving chip used to generate a video frame data enable signal and a first PWM signal; a pulse width modulation control circuit for receiving the video frame data enable signal and the first PWM signal, and modulating a duty ratio of the first PWM signal under the video frame data enable signal control to get a second PWM signal; a backlight driving chip for receiving the second PWM signal, and controlling brightness of the backlight based on the second PWM signal to increase brightness of the backlight during a frame data disable period of the video frame data enable signal. Decreasing brightness in liquid crystal display device caused by leakage characteristic of thin film transistor is solved through the disclosure to achieve consistent brightness.