Adaptive Switched LED Driver with Boosted PWM and Trimming Circuit
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Solution Overview
Problem
Conventional LED driver architectures experience high power consumption and thermal issues in high current mode applications due to fixed resistors and poor ground noise rejection, particularly in 3D mode or scan mode operations for LCDs.
Innovation Solution
An LED driver utilizing a boosted PWM signal to maintain a constant gate-source voltage for a transistor controlling the LED string, combined with a current mirror circuit and a programmable trimming circuit to manage current and reduce resistance, thereby improving power efficiency and ground noise rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed resistor is used in the LED power path to limit peak current, then current control is achieved, but power consumption increases and thermal issues occur
Solution Approach 1:
The patent removes the fixed resistor from the LED power path entirely, replacing it with a transistor-based current control mechanism. This extraction eliminates the continuous power loss associated with resistive current limiting while maintaining peak current control through the transistor's switching action.
Solution Approach 2:
The patent replaces the passive resistive current limiting mechanism with an active transistor-based switching system. The transistor acts as a controlled switch that can precisely limit peak current without the continuous power dissipation inherent in fixed resistor designs.
2Ease of operation
If PWM-controlled transistor is used to control LED on and off times, then brightness control is achieved, but turn-on impedance increases at high currents reducing power efficiency
Solution Approach 1:
The patent employs dynamic current control where the transistor's gate voltage is actively adjusted based on real-time conditions. The current control signal dynamically modulates the transistor's resistance, optimizing the balance between brightness control capability and power efficiency across varying operating conditions.
Solution Approach 2:
The patent implements a feedback mechanism where the actual LED current is sensed and used to adjust the transistor's gate voltage. This feedback loop ensures that the transistor operates at optimal points, maintaining low on-resistance even at high current levels while preserving precise brightness control.
3Device complexity
If conventional LED driver architecture is used, then simple structure is maintained, but ground noise rejection is poor limiting performance
Solution Approach 1:
The patent introduces dedicated current sensing circuitry and separate signal paths as intermediaries between the PWM control signal and the LED driver transistor. This intermediary structure isolates the control logic from the high-current power path, significantly improving ground noise rejection while maintaining overall system simplicity.
Data Source
AI summary
An LED driver controls current through an LED string. The LED driver generates a boosted PWM signal to drive a PWM transistor in the LED current path such that the PWM transistor maintains a substantially constant VGS, thus minimizing turn-on impedance of the PWM transistor. A current mirror circuit controls peak LED current when the PWM transistor is on. A trimming circuit includes a set of programmable switches to couple or decouple trimming transistor from the LED current path, and allowing for fine calibration of the LED current. By maintaining a low resistance and compensating for current mismatch in the LED current path, the LED driver provides efficient power performance and robustness that is particularly beneficial in high current applications.