Cascode Current Source Stabilizes LED Drive Circuit
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional indicator drive circuits experience fluctuations or flickering in LED brightness due to variations in power supply voltage, as the drive current through the current mirror circuit is affected by changes in the control part's power supply voltage.
Innovation Solution
An indicator drive circuit is designed with a current source composed of two cascode-connected PMOS transistors, which increases impedance and stabilizes the control current, preventing fluctuations in the LED's brightness by reducing the impact of power supply voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional current mirror circuit is used to drive the indicator, then the circuit is simple and easy to implement, but the control current varies with power supply voltage causing LED brightness fluctuation
Solution Approach 1:
The current source is divided into two separate PMOS transistors connected in cascode configuration. The first PMOS transistor (Q3) serves as the primary current source while the second PMOS transistor (Q4) acts as a cascode device to stabilize the current. This segmentation allows each transistor to perform a specific function, with Q4 providing voltage stabilization that prevents current variation due to power supply fluctuations.
Solution Approach 2:
The invention changes the configuration parameter of the current source from a single transistor to two transistors connected in cascode. This parameter change (adding a cascode transistor) fundamentally alters the electrical characteristics, increasing the output impedance and stabilizing the current against voltage variations, thereby eliminating LED brightness fluctuation.
2Device complexity
If a single PMOS transistor is used as the current source, then the device complexity is low, but the impedance is insufficient allowing power supply voltage variations to affect control current
Solution Approach 1:
Two PMOS transistors are merged into a single cascode-connected current source unit. The first PMOS (Q3) and second PMOS (Q4) are combined in series configuration where the drain of Q3 connects to the source of Q4. This merging creates a unified current source structure that achieves high impedance and voltage stability while maintaining compact integration.
Solution Approach 2:
The current source is extended from a single-transistor configuration to a two-transistor cascode structure, adding a vertical dimension in the circuit hierarchy. The cascode connection introduces an additional control dimension where Q4's gate voltage is independently controlled to stabilize the current through Q3 against power supply variations.
Data Source
AI summary
An indicator drive circuit includes a current mirror circuit including a first transistor located at a control side and a second transistor located at a controlled side, a current through the current mirror circuit being switched by an external control signal, a current source adapted to feed a control current to the first transistor of the current mirror circuit, and a switching element adapted to be driven by a current through the second transistor of the current mirror circuit and to switch on or off an indicator light source. The current source includes two transistors cascode-connected.