Sequential Calibration of LED Current Sources
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Solution Overview
Problem
Large arrays of White Light Emitting Diodes (WLEDs) in LCD displays face challenges in maintaining constant current across multiple strings, leading to brightness mismatches due to variations in current references between integrated circuits, which are costly to correct and prone to errors from voltage drops and resistor mismatches.
Innovation Solution
A system of programmable current source circuits (CSCs) is used, where each CSC is calibrated sequentially using a single reference signal, allowing for precise control of LED currents across multiple integrated circuits distributed across the LCD display, ensuring consistent brightness without the need for external resistors or precise voltage sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If multiple integrated circuits are used to control large arrays of WLED strings, then the system can handle higher current requirements and larger array sizes, but current reference mismatches between integrated circuits cause brightness non-uniformity across the display
Solution Approach 1:
The patent applies preliminary action by calibrating each current source circuit sequentially before normal operation. During initialization, each CSC is switched into calibration mode where it receives a precise reference current and adjusts its internal parameters to match the reference. This pre-calibration ensures that when all CSCs operate in parallel during normal operation, they all produce matched output currents despite being implemented in separate integrated circuits, thereby eliminating brightness non-uniformity while enabling support for large WLED arrays
Solution Approach 2:
The patent implements self-service through automatic calibration where each current source circuit independently adjusts its own output current to match the reference current. The calibration process is self-contained within each CSC, using its own internal components and control logic to achieve precise current matching without requiring external trimming or manual adjustment. This self-calibration capability enables mass production of integrated circuits with guaranteed current matching performance
2Manufacturing precision
If resistor trimming is used to improve current matching accuracy between integrated circuits, then current precision is improved, but manufacturing cost and complexity increase
Solution Approach 1:
The patent replaces mechanical/physical trimming methods (such as laser trimming of resistors or blowing fuse links) with an electronic calibration approach. Instead of physically modifying circuit components during or after manufacturing, the system uses digital control and electronic adjustment mechanisms within each current source circuit to achieve precise current matching. This substitution eliminates the need for complex trimming infrastructure, reduces manufacturing steps, and lowers overall system complexity while maintaining high current matching accuracy
Solution Approach 2:
The patent extracts the calibration function from the manufacturing process and integrates it into the operational phase of the system. Rather than requiring precision trimming during circuit fabrication, the calibration is performed automatically after the circuits are assembled and powered up. This separation allows standard manufacturing processes to be used for producing the integrated circuits, while the precise current matching is achieved through the extracted calibration routine that operates independently during system initialization
3Manufacturing precision
If external precision resistors and voltage sources are used to ensure current matching across multiple integrated circuits, then current accuracy is improved, but system cost and component count increase
Solution Approach 1:
The patent merges the reference current generation function into each individual current source circuit integrated circuit. Instead of using separate external precision resistors and voltage sources for each circuit, the reference current generator is integrated directly within each CSC block. This consolidation eliminates the need for additional external components, reduces bill of materials costs, simplifies assembly, and maintains precise current matching through on-chip calibration using the integrated reference current
Data Source
AI summary
A system involves LED strings and programmable current source circuits (CSC). An LED current flows through each LED string. Each LED current is controlled by an associated programmable CSC. In one embodiment, the CSCs form a chain. A first CSC uses a reference current for calibration, and thereafter supplies the reference current to the next CSC. When the next CSC detects the reference current, it uses the reference current for calibration. CSCs are calibrated one by one down the chain. In a second embodiment, each CSC can receive the reference current from a common conductor. If the common conductor is detected to be available, then the CSC uses the reference current for calibration. When the conductor is in use, the other CSCs detect the conductor as unavailable and do not attempt to self-calibrate. The CSCs use the reference current one by one, but in an order that changes over time.