LED Backlight Current Regulation Using Intermediary Sense Resistor
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Solution Overview
Problem
Existing backlight drivers for transmissive LCDs struggle to reliably detect and control smaller electrical currents, such as those measured in microamps, due to the difficulty in accurately measuring smaller voltages.
Innovation Solution
An electrical system comprising a resistive load, a first and second current source, a current-sense resistor, a control processor, and a voltage sensor, where the control processor adjusts the current sources based on the voltage across the current-sense resistor to accurately control the LED array current, using a digitally-controlled reference voltage source and algorithmic current controller to achieve precise current regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current-sense resistor is used to detect current through voltage measurement, then current control is achieved, but measurement precision deteriorates for small currents measured in microamps
Solution Approach 1:
The patent introduces a second current source as an intermediary element that injects a known test current through the current-sense resistor. This allows the system to measure the voltage drop caused by this known current and calculate the resistance value, thereby improving the precision of current measurements even at microamp levels. The intermediary current source acts as a mediator to enable accurate measurement without directly measuring the small signal currents.
2Use of energy by moving object
If the current through LEDs is reduced to save energy, then energy consumption is reduced, but measurement precision deteriorates due to smaller voltages being difficult to detect
Solution Approach 1:
The patent applies preliminary action by first measuring the current-sense resistor value using the second current source before attempting to measure the actual LED current. This preliminary calibration ensures that the system has accurate resistance data stored, which can then be used to accurately calculate small LED currents even when the voltage drops are minimal. The preliminary measurement of resistance compensates for the difficulty of measuring small voltages during low-current operation.
Solution Approach 2:
The second current source serves as an intermediary that enables indirect measurement. Instead of directly measuring the small voltage drop across the current-sense resistor during low-current operation, the system uses the intermediary current to establish a known voltage reference, from which the resistance can be calculated and stored for subsequent accurate current measurements.
3Reliability
If a feedback loop with current-sense resistor is used to control LED current, then current regulation is achieved, but device complexity increases due to additional components
Solution Approach 1:
The second current source is designed with multi-functionality, serving both as a calibration tool for measuring the current-sense resistor value and as part of the overall current control system. This universal component performs multiple functions: it characterizes the sense resistor during initialization and enables accurate current measurement during operation, thereby reducing the need for separate calibration circuits and reducing overall device complexity.
Solution Approach 2:
The system implements self-service by using the second current source to automatically characterize and store the current-sense resistor value during an initialization phase. This self-calibration process eliminates the need for external manual calibration or additional complex calibration circuits, as the system performs its own characterization and stores the data for use during normal operation.
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 system effectively regulates the brightness of the LCD backlight by accurately detecting and controlling currents down to microamps, improving reliability and precision in controlling the LED array, thereby enhancing the display's visibility in low ambient light conditions.
Implementation Method 1
the voltage across the resistor is indicative of the current through it and thus through the device of interest
Implementation Method 2
an array of light emitting diodes (LEDs)
Data Source
AI summary
In one embodiment, an electrical system having an LED array, a first current source connected to provide a first current to the LED array, a current-sense resistor connected to the LED array, a second current source connected to provide a second current to the current-sense resistor, a control processor, and a voltage sensor adapted to provide a corresponding sensor signal to the control processor, wherein the control processor is adapted to control the first and second current sources based on the sensor signal. The first current source having a diode with first and second sides, an inductor connected between a first reference voltage source and a first side of a diode, a capacitor connected between a second side of the diode and a second reference voltage, and a transistor connected between the first side of the diode and the second reference voltage.