Light Emitting Device Driver with PWM Extension Circuit
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Solution Overview
Problem
Conventional light emitting device driving apparatuses struggle to maintain constant current control at low dimming ratios, leading to response delay and flickering issues, especially in automotive displays where high dimming ratios are required.
Innovation Solution
An apparatus with a constant current control circuit and an extension circuit that generates a post-extending PWM signal, extending the ON period of the PWM signal to ensure the light emitting device reaches the set current even at low dimming ratios, using a configuration that includes a ring oscillator, D-type flip flops, inverters, and a switching control circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the PWM signal ON period is shortened to achieve high dimming ratios, then brightness adjustment range is improved, but the light emitting device cannot reach the set current due to response delay
Solution Approach 1:
The extension circuit generates a post-extending PWM signal that extends the ON period of the original PWM signal. This preliminary extension ensures that the light emitting device has sufficient time to reach the set current even when the original PWM ON period is very short, thereby maintaining current control precision across the full dimming ratio range.
2Object-affected harmful factors
If the PWM signal cycle is reduced to eliminate flicker, then visual comfort is improved, but the ON period becomes even shorter exacerbating response delay
Solution Approach 1:
By pre-extending the ON period through the post-extending PWM signal generation circuit, the system can use shorter PWM cycles without flicker while ensuring the light emitting device still receives sufficient ON time to reach the target current, thus eliminating both flicker and response delay issues.
3Device complexity
If conventional constant current control is used at low dimming ratios, then circuit simplicity is maintained, but flickering occurs due to insufficient current establishment time
Solution Approach 1:
The extension circuit acts as an intermediary between the original PWM signal and the constant current control circuit. It processes the PWM signal to extend the ON period, ensuring stable current control at low dimming ratios without requiring complex modifications to the existing constant current control architecture.
Data Source
AI summary
An apparatus for driving a light emitting device includes a constant current control circuit configured to control a current flowing through the light emitting device to be constant, and an extension circuit configured to generate a post-extending PWM signal obtained by extending a period of an input PWM signal having a first level. The constant current control circuit is turned on during the period of the post-extending PWM signal having the first level, and the constant current control circuit is turned off during a period of the post-extending PWM signal having a second level.