Transistor Constant Current Source Eliminates Sensing Resistor
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Solution Overview
Problem
Existing constant current sources for LED backlighting in large displays are not scalable, inefficient at higher currents, and require significant chip area, leading to impractical sensing resistor sizes and thermal-induced accuracy issues.
Innovation Solution
A constant current source circuit using two transistors with matched drain-to-source voltages and a bias current, eliminating the need for a sensing resistor, allowing for scalable and efficient current regulation by adjusting the transistor sizing and operational amplifier feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sensing resistor is used in the constant current source circuit, then the current can be regulated, but the chip area increases significantly and thermal-induced accuracy issues occur
Solution Approach 1:
The patent extracts and eliminates the sensing resistor from the constant current source circuit by using a transistor-based current mirror configuration. The current regulation is achieved through transistor gate control and feedback mechanisms rather than resistive sensing, removing the problematic component that consumed excessive chip area and generated thermal drift issues.
Solution Approach 2:
The patent replaces the resistive sensing mechanism with a transistor-based electronic control mechanism. Instead of using Ohm's law (V=IR) to sense and regulate current through a resistor, the invention uses transistor characteristics (gate-source voltage controlling drain-source current) and operational amplifier feedback to achieve current regulation, substituting a thermal-resistive system with an electronic-field-effect system.
2Illumination intensity
If the current is increased to provide adequate backlight for large displays, then the backlight intensity increases, but the power dissipation increases significantly
Solution Approach 1:
The patent implements a feedback mechanism using an operational amplifier that continuously monitors the current through the LED string and adjusts the transistor gate voltage to maintain the desired current level. This closed-loop feedback ensures efficient current regulation, preventing excessive power dissipation while maintaining adequate backlight intensity for large displays.
Solution Approach 2:
The patent changes the operational parameters of the circuit by using variable transistor sizing and adjustable reference voltages to optimize the balance between backlight intensity and power consumption. The current mirror ratio and feedback reference levels can be tuned to achieve the required illumination while minimizing power dissipation in the control circuitry.
3Reliability
If the sensing resistor size is increased to handle higher currents, then the current handling capability improves, but the thermal-induced drift increases
Solution Approach 1:
The patent removes the sensing resistor entirely from the circuit, eliminating the source of thermal-induced drift. Current handling capability is achieved through the transistor's inherent current carrying capacity and the feedback control mechanism, rather than relying on a resistive element that would generate thermal problems at high currents.
Solution Approach 2:
The patent uses small-signal transistors and operational amplifiers that operate at low power and generate minimal heat, replacing the need for large, heat-generating sensing resistors. The low-power control circuitry can continuously regulate current without accumulating significant thermal energy that would cause drift.
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
The solution provides a scalable, efficient, and thermally stable constant current source that maintains load current proportional to the bias current, reducing power dissipation and chip area requirements, while minimizing thermal-induced drift.
Implementation Method 1
the amplifier increases the output voltage until the voltage at the inverting input matches the voltage at the non-inverting input
Implementation Method 2
the voltage drop across the sensing resistor Rs 46 increases according to Ohm's law: voltage drop (V)=current (i)*resistance (R)
Implementation Method 3
the output of the amplifier 40 may be connected to the gate of a transistor 44
Data Source
AI summary
The present invention uses two transistors instead of a sensing resistor to provide a constant current source for a load such as an array of light emitting diodes (“LEDs”). In the present invention, a bias current is applied to a branch of the circuit. The drain-to-source voltages of two transistors are matched. The voltage at the gate of both transistors is controlled based on the bias current and the drain-to-source current of the first of the two transistors. The second of the two transistors is sized such that source current of the second transistor is a multiple of the source current of the first transistor for a given gate voltage. By the techniques of this invention, the load current in a circuit is efficiently kept constant at a multiple of the input bias current.