LED Backlight Drive Circuit Stabilization via Error Amplifier Feedback
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Solution Overview
Problem
The drive current for LED strings is influenced by temperature variations and individual deviations in the current magnification of the output transistor, leading to unstable luminance, which existing technologies cannot effectively address.
Innovation Solution
A drive circuit that includes an output transistor, a current control resistor, and an error amplifier, where the error amplifier's sink current is used to equalize the current flowing through the current control resistor, reducing the impact of temperature variations and individual deviations on the drive current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a PNP bipolar transistor is used as the output transistor in the current source, then the circuit can provide drive current to the LED string, but the base current cannot be ignored due to finite current magnification, causing the drive current to be influenced by temperature variations and individual deviations
Solution Approach 1:
The patent introduces a feedback mechanism by connecting the error amplifier between the current control resistor and the base of the output transistor. The error amplifier detects voltage variations across the current control resistor caused by base current changes and adjusts the base voltage accordingly, creating a negative feedback loop that stabilizes the drive current despite temperature variations and individual deviations in transistor parameters
Solution Approach 2:
The error amplifier acts as an intermediary component that mediates between the current control resistor and the output transistor base. It processes the voltage signal from the current control resistor and generates the appropriate base drive signal, effectively decoupling the influence of base current variations from the drive current and eliminating the need to ignore base current effects
2Device complexity
If the current magnification hfe is assumed to be infinite to simplify calculations, then the base current can be considered zero, but in practice hfe is only tens to hundreds times, making the base current significant and causing drive current instability
Solution Approach 1:
The feedback mechanism through the error amplifier automatically compensates for the finite current magnification of the output transistor. By continuously monitoring the voltage across the current control resistor and adjusting the base voltage in response, the system maintains accurate drive current control without requiring the simplifying assumption of infinite hfe, thereby achieving both practical simplicity and manufacturing precision
Data Source
AI summary
A current drive circuit capable of reducing the influence of temperature variation or individual deviation is provided. An output transistor is a PNP bipolar transistor, and has an emitter connected to a cathode of an LED string. A current control resistor is disposed between a collector of the output transistor and a ground terminal. An output terminal of an error amplifier is connected to a base of the output transistor, a first input terminal of the error amplifier is connected to a connection point of the output transistor and the current control resistor, and a reference voltage is applied to a second input terminal of the error amplifier. The error amplifier enables a sink current sunk from the output terminal to flow to the current control resistor.