Backlight Unit Current Control Circuit for LED Brightness Uniformity
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Solution Overview
Problem
Backlight units face challenges in maintaining uniform brightness due to offset currents from operational amplifiers, leading to variations in luminous current flow through LED arrays, which affect the desired brightness levels.
Innovation Solution
Incorporating first and second switches in the controlling circuit unit to alternately input constant and sensing currents during different periods, effectively counterbalancing the offset current and ensuring consistent comparison between sensing and constant currents, thereby controlling the luminous current uniformly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an operational amplifier is used to maintain constant current through LEDs, then the current control function is achieved, but offset current causes brightness variation across channels
Solution Approach 1:
The patent applies periodic action by alternately switching the connections of the sensing current during different time periods. During a first time period, the sensing current is connected to one input terminal of the operational amplifier, and during a second time period, it is connected to the other input terminal. This periodic switching allows the system to compensate for offset current effects while maintaining reliable current control through the LEDs, thereby achieving both current stability and brightness uniformity across multiple channels.
2Measurement precision
If sensing current is continuously monitored, then current accuracy is improved, but offset current from operational amplifier causes measurement error
Solution Approach 1:
The patent implements preliminary action by pre-configuring the switching circuit to alternately connect the sensing current to different input terminals of the operational amplifier before actual current measurement occurs. This preliminary switching arrangement ensures that offset current effects are compensated for in advance, allowing accurate sensing current measurement without the interference of operational amplifier offset current.
3Use of energy by moving object
If multiple LEDs are arranged in clusters to decrease power consumption, then energy efficiency is improved, but brightness uniformity across channels deteriorates due to offset current variations
Solution Approach 1:
The patent applies periodic action by implementing time-division multiplexing for current sensing across multiple LED clusters. The switching circuit alternately connects sensing currents from different LED clusters to the operational amplifier inputs during different time periods. This periodic switching scheme enables accurate current monitoring for power-efficient LED cluster operation while compensating for offset current variations, thereby maintaining brightness uniformity across all channels.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures that the luminous current flowing through the LED array is consistently matched to the constant current, resulting in uniform brightness across all channels of the backlight unit, overcoming the issue of brightness variations caused by offset currents.
Implementation Method 1
The operational amplifier 10 maintains a constant current flowing through LEDs... The operational amplifier 10 compares the sensing current Isen with the constant current Iref and adjusts the thin film transistor 15
Implementation Method 2
Backlight units include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and light emitting diodes (LEDs) as a light source... LEDs include red, green and blue LEDs emitting red, green and blue light, respectively
Implementation Method 3
The thin film transistor 15 includes a gate electrode, a source electrode, and a drain electrode... The gate electrode of the thin film transistor 15 is connected to an output terminal of the operational amplifier 10
Data Source
AI summary
A backlight unit includes a first switch receiving a constant current applied from an outside circuit and a sensing current, a first comparator receiving an output of the first switch, a second switch receiving an output of the first comparator, a second comparator receiving an output of the second switch; a thin film transistor having a gate electrode receiving an output of the second comparator; a light-emitting diode array connected to a source electrode of the thin film transistor; and a current sensor connected to a drain electrode of the thin film transistor.