Multi-String LED Driver Current Switching Techniques
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Solution Overview
Problem
Existing LED backlight systems for displays face challenges in maintaining uniform brightness and color across varying levels due to drive current mismatches between LED strings, especially at high PWM frequencies, leading to reduced image quality and increased power consumption.
Innovation Solution
A multi-string LED driver system that allows any driver to couple with any LED string and rotates the order of drivers during clock cycles to minimize current mismatches, using auxiliary transistors for settling and idle transistors to maintain low current paths when not in use, thereby reducing power consumption and enhancing linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multiple dedicated drivers are assigned to each LED string to maintain current precision, then manufacturing precision is improved, but device complexity increases
Solution Approach 1:
Multiple LED strings are merged under control of a single driver. The patent implements a driver that can sequentially control multiple LED strings by multiplexing the drive signal across different strings at different time intervals, reducing the total number of drivers required while maintaining current precision through temporal separation of control signals
Solution Approach 2:
The driver employs periodic switching to sequentially control different LED strings within each PWM period. By dividing the PWM cycle into sub-intervals and assigning different strings to different time slots, the system achieves precise current control for multiple strings using a single driver, eliminating the need for dedicated drivers for each string
2Reliability
If PWM frequency is increased to reduce visible pulsing, then image quality is improved, but drive current mismatches between LED strings increase
Solution Approach 1:
The driver circuit performs preliminary settling of the drive current before actually switching to control a new LED string. An auxiliary transistor is activated in advance to pre-charge or pre-discharge capacitive elements, ensuring the current reaches a stable state before the switching transition occurs, thereby minimizing mismatches at high PWM frequencies
Solution Approach 2:
An auxiliary transistor serves as an intermediary element between the driver output and the LED strings during switching transitions. This intermediary component provides a controlled path for current redistribution, preventing abrupt current changes and reducing mismatches between strings when switching at high PWM frequencies
3Manufacturing precision
If drivers are continuously switched between LED strings to reduce current mismatches, then manufacturing precision is improved, but power consumption increases
Solution Approach 1:
The driver maintains continuous control capability across all LED strings by implementing seamless switching with minimal off-time. The auxiliary transistor ensures continuous current path management during transitions, eliminating idle periods where power would be wasted while maintaining precise current uniformity across all strings throughout the PWM cycle
Solution Approach 2:
The system monitors the drive current status and uses feedback control to optimize switching timing. By detecting when current settling is complete, the driver can minimize unnecessary switching operations and reduce power consumption while maintaining current uniformity across LED strings
Data Source
AI summary
LED backlight circuits for a display and methods for operating the circuits are disclosed. The LED backlight circuit includes a set of drivers and a set of LED strings. A driver can be capable of coupling to any of the LED strings at a given time. Examples of the disclosure can include different configurations of drivers. In some instances, the driver may include an auxiliary transistor that allows the driver to settle before being switched to control a respective LED string. In some examples, the driver can include switches and an idle transistor that can be operated such that a low current path through the driver can exist when the driver is not coupled to a LED string.