LED Pixel PWM Circuit for Stable Brightness and Gray Levels
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Solution Overview
Problem
Conventional analog dimming methods for LEDs face issues such as unstable luminescence characteristics and accuracy dependence on analog control signals, particularly related to color temperature and gray level accuracy.
Innovation Solution
A pixel circuit utilizing pulse-width modulation (PWM) with a latch circuit, pass switch, PWM circuit, and current source to control the duty cycle of a constant current flowing through an LED, allowing for precise brightness control independent of color temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If analog dimming method is used to control forward current through LED, then brightness control is achieved, but luminescence characteristics become unstable and color temperature varies
Solution Approach 1:
The patent applies pulse-width modulation (PWM) to generate periodic pulse signals for controlling LED brightness. Instead of using continuous analog current control, the system switches the LED on and off at high frequency with varying duty cycles, achieving stable brightness control without affecting luminescence characteristics or color temperature, since the LED operates at full current during on-states
Solution Approach 2:
The patent replaces the analog electrical control system with a digital pulse control system. By substituting continuous analog voltage/current control with discrete digital PWM signals, the system eliminates the instability and color temperature variation issues inherent in analog control, achieving more reliable and precise brightness regulation
2Illumination intensity
If analog control signal is used for dimming, then brightness adjustment is possible, but gray level accuracy depends on analog signal precision
Solution Approach 1:
The patent replaces analog control signals with digital PWM control signals. The gray level accuracy is no longer dependent on analog signal precision but on digital counter precision and duty cycle calculation, which can be achieved with high accuracy using standard digital circuits and microcontrollers, eliminating the accumulation of analog errors
Solution Approach 2:
The patent changes the control parameter from continuous analog voltage/current to discrete digital duty cycle percentage. This parameter transformation allows for precise control through digital counting and timing, where the gray level accuracy is determined by the resolution of the PWM timer (e.g., 8-bit, 10-bit, or 12-bit resolution) rather than analog signal stability
3Use of energy by moving object
If low forward current is used for dimming, then energy consumption is reduced, but luminescence characteristics become unstable
Solution Approach 1:
The patent uses PWM to switch the LED on and off periodically. During the on-period, the LED receives full forward current ensuring stable luminescence characteristics, while the off-period reduces average power consumption. The duty cycle controls the ratio of on-time to total period, enabling energy efficiency without compromising luminescence stability during active emission
Solution Approach 2:
The patent maintains continuous switching action at high frequency to ensure the LED always operates at optimal forward current when on, avoiding the instability associated with low continuous current. The rapid switching ensures the LED remains in its stable luminescence region during on-states while achieving reduced average power through duty cycle control
Data Source
AI summary
A pixel circuit for illuminating an LED unit with a level of brightness is provided, which includes a latch circuit, a pass switch, a PWM circuit, and a current source. The latch circuit latches control data according to a latch signal to generate a control signal. The pass switch provides the control data from a data signal to the latch circuit according to a scan signal. The PWM circuit generates a PWM signal according to the control signal and an enable signal. The current source supplies a constant current flowing through the LED unit according to the PWM signal.