LED Pixel Circuit Amplitude and Pulse Width Control
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Solution Overview
Problem
Existing pixel circuits for LED displays face challenges in achieving high luminance uniformity and low color shift due to the color shift phenomenon caused by forward voltage deviations and limitations in expressing grayscales, particularly when applying active matrix driving circuits designed for OLEDs to LEDs.
Innovation Solution
A pixel circuit design that operates the driving transistor in the saturation region, utilizing an amplitude setting circuit to control the drive current amplitude and a pulse width control circuit to linearly change the drive current duration, ensuring consistent luminance and reduced color shift by managing the drive current's amplitude and pulse width.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If PAM driving method is applied to LED display, then grayscale expression is improved, but color shift problem occurs
Solution Approach 1:
The patent changes the driving parameters from simple amplitude modulation to a combination of amplitude control and pulse width modulation. By controlling both the amplitude and duration of the drive current, the system achieves accurate grayscale expression while compensating for forward voltage deviations, thereby reducing color shift.
Solution Approach 2:
The patent introduces dynamic control mechanisms including an amplitude setting circuit that adjusts the drive current amplitude based on grayscale values, and a pulse width control circuit that dynamically adjusts the current duration. This dynamic adjustment allows the system to maintain color consistency across different grayscale levels.
2Measurement precision
If PWM driving method is applied to LED display, then grayscale expression is improved, but luminance difference occurs due to forward voltage deviation
Solution Approach 1:
The patent implements a feedback mechanism where the actual drive current is monitored and compared with the target current. The amplitude setting circuit adjusts the drive current amplitude based on the grayscale value and forward voltage compensation, ensuring that the LED receives the correct current regardless of forward voltage deviations, thus maintaining luminance uniformity.
Solution Approach 2:
The patent modifies the PWM driving approach by introducing amplitude control in addition to pulse width modulation. By dynamically adjusting both the amplitude and duration of the drive current, the system compensates for forward voltage variations and achieves consistent luminance across different grayscale levels.
3Device complexity
If digital PWM method is applied, then grayscale expression is simplified, but number of grayscales is limited and false contour problem occurs
Solution Approach 1:
The patent employs periodic pulse width modulation where the drive current is applied in controlled pulses with varying durations. By precisely controlling the pulse width and amplitude according to grayscale values, the system achieves high grayscale resolution without the false contour problems associated with digital PWM methods.
4Use of energy by stationary object
If AM driving circuit for OLED is applied to LED, then low power consumption is achieved, but color shift and luminance non-uniformity occur
Solution Approach 1:
The patent adapts the AM driving circuit for LED by introducing parameter control mechanisms that adjust the drive current amplitude and pulse width based on grayscale values and forward voltage compensation. This allows the circuit to maintain low power consumption characteristics while achieving color consistency and luminance uniformity through dynamic parameter adjustment.
Data Source
Figure 1
Figure 2A~2B
Figure 3A~3A(b)
AI summary
A pixel circuit of a display panel is provided, which includes a light emitting element configured to emit light in accordance with a drive current, a current source including a driving transistor connected to the light emitting element, and the current source is configured to provide the drive current having a different amplitude to the light emitting element in accordance with a level of a voltage applied to a gate terminal of the driving transistor, an amplitude setting circuit configured to apply a voltage having a different level to the gate terminal of the driving transistor, and a pulse width control circuit configured to control a duration of the drive current by controlling the voltage applied to the gate terminal of the driving transistor.