Mixed-Mode LED Driver for Uniform Brightness Control
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Solution Overview
Problem
Existing display technologies face challenges in achieving uniform color and brightness control, particularly at high PWM frequencies, due to limitations in current switching techniques and the need for complex circuitry or analog control signals.
Innovation Solution
A mixed-mode LED backlight circuit that utilizes both PWM and analog control signals, generated from a phase locked loop (PLL), to control light output levels, ensuring increased linearity, accuracy, and reduced complexity across various light output ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If PWM dimming is used to control brightness, then brightness control is achieved, but color uniformity deteriorates due to drive current changes causing wavelength shifts
Solution Approach 1:
The patent dynamically switches between PWM dimming and analog dimming modes based on the desired brightness level. For higher brightness levels, PWM mode is used for efficient power control, while for lower brightness levels, analog mode is used to maintain color uniformity. This dynamic mode switching resolves the contradiction by adapting the control method to the specific operating conditions.
Solution Approach 2:
The patent changes the control parameter from duty cycle (PWM) to drive current amplitude (analog) depending on the brightness range. By adjusting which parameter is controlled, the system achieves both efficient brightness control at high levels and color uniformity at low levels, resolving the trade-off between these two requirements.
2Manufacturing precision
If analog dimming is used to maintain color uniformity, then color consistency is improved, but circuit complexity increases due to requirement for analog control signals
Solution Approach 1:
The patent introduces a digital-to-analog converter (DAC) as an intermediary component that translates digital brightness control signals into analog drive current signals. This DAC enables analog dimming functionality without requiring complex external analog control circuitry, thus maintaining color uniformity while limiting the increase in overall system complexity to a single standardized component.
3Reliability
If high PWM frequencies are used to avoid visible pulsing, then display quality is improved, but linearity and accuracy of brightness control deteriorate
Solution Approach 1:
The system dynamically selects the dimming mode based on the PWM frequency and desired brightness level. At high PWM frequencies where linearity issues occur, the system switches to analog dimming mode which provides superior brightness linearity and accuracy. This dynamic adaptation resolves the contradiction by using each mode in its optimal operating range.
Data Source
AI summary
LED backlight circuits for a display and methods for operating the circuits are disclosed. The LED backlight circuit includes a set of drivers and a set of LED strings. A driver can control a light output level of the LED strings. The LED strings can be controlled by a mixed-mode LED driver that utilizes a PWM control signal over a first range of light output levels and an analog control signal over a second range of light output levels. Clock signals used for PWM control and for frequency-to-current or frequency-to-voltage conversion for analog control can both be generated from a phase locked loop (PLL).