LED Current Matching Circuit for Uniform Backlighting
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Solution Overview
Problem
Display panel technologies requiring backlighting face challenges in achieving uniform brightness due to variations in forward voltage drops across LED strings, leading to mismatched currents and non-uniform backlighting.
Innovation Solution
A current matching circuit that calibrates multiple LED strings by using a reference current load to adjust the output currents of each string, ensuring they match within a certain tolerance, thereby maintaining uniform brightness across the display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If multiple LED strings are used to provide backlighting, then the backlighting coverage and brightness are improved, but variations in forward voltage drops cause current mismatch and non-uniform brightness
Solution Approach 1:
The patent applies preliminary action by performing current calibration during a test/trimming phase before normal operation. A calibration circuit measures the forward voltage drop of each LED string and stores compensation values in non-volatile memory. During normal operation, these pre-calculated compensation values are applied to adjust the current to each string, eliminating the need for real-time measurement and ensuring uniform brightness without adding complex real-time control circuitry.
Solution Approach 2:
The patent introduces an intermediary calibration circuit that acts as a mediator between the LED strings and the power converter controller. This calibration circuit includes current sources, measurement circuits, and memory elements that store compensation data. The intermediary circuit processes the forward voltage variations of different LED strings and translates them into compensatable signals, allowing the main power converter to operate uniformly across all strings without direct complex control of each individual string.
2Manufacturing precision
If current calibration circuitry is added to match LED string currents, then brightness uniformity is improved, but device complexity increases
Solution Approach 1:
The calibration is performed once during manufacturing or initial setup, and the results are stored in non-volatile memory. This preliminary action eliminates the need for continuous complex real-time measurement and adjustment circuitry during normal operation. The system uses simple lookup tables or stored compensation values that require minimal processing power and circuit complexity during actual LED operation.
Solution Approach 2:
The system performs self-calibration during a dedicated test phase without requiring external intervention or complex external equipment. The calibration circuit uses internally generated test signals and automatically stores the compensation data in its own memory elements. This self-service approach reduces the need for external calibration equipment and simplifies the overall system architecture.
Data Source
AI summary
A current matching circuit includes a plurality of LED driver circuits. A current to voltage converter circuit is coupled to the plurality of LED driver circuits to generate a plurality of voltage signals. Each one of the plurality of voltage signals is representative of a respective output current through a corresponding one of the plurality of LED driver circuits. A comparison circuit is coupled to the current to voltage converter circuit to compare the plurality of voltage signals. An adjustment circuit is coupled to the comparison circuit and the plurality of LED driver circuits. The adjustment circuit is configured to trim the plurality of LED driver circuits in response to the comparison circuit such that each respective output current through the plurality of LED driver circuits is substantially equal.