Fast-Settling Voltage Follower Circuit for Capacitive Touch Screens
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Solution Overview
Problem
Conventional voltage follower circuits are unable to provide fast-settling output voltages when driving large capacitive loads, such as those found in capacitive touch screen systems, while maintaining low power consumption and stability.
Innovation Solution
A fast-settling precision voltage follower circuit is designed with a differential input stage and a current gain boosted follower circuit, incorporating a slew boost circuit that shifts the input signal to rapidly drive the output voltage to a steady-state value during fast transitions, using a combination of P-channel and N-channel transistors and resistive elements to manage capacitive loads effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If compensation capacitance is used to stabilize the voltage follower amplifier, then stability is improved, but settling time increases significantly
Solution Approach 1:
The slew boost circuit performs preliminary action by rapidly charging or discharging the compensation capacitance Ccomp at the beginning of a transition, before the main feedback loop takes over. This preliminary charging/discharging action through transistors M1 and M2 gets the output voltage close to its final value quickly, so that the subsequent settling phase is much shorter and the stability requirements on the compensation capacitance are relaxed.
2Adaptability or versatility
If the voltage follower drives a large capacitive load, then load driving capability is improved, but response speed deteriorates
Solution Approach 1:
The slew boost circuit acts as an intermediary that temporarily provides additional current to the output stage during fast transitions. Transistors M1 and M2 are activated during transitions to directly charge or discharge the compensation capacitance, bypassing the limited current path through the main feedback loop. This intermediary action enables fast response while maintaining the ability to drive large capacitive loads.
3Loss of time
If transition speed is increased for fast settling, then response time is improved, but power consumption increases
Solution Approach 1:
The slew boost circuit uses periodic action by activating transistors M1 and M2 only during transition periods when fast settling is required, rather than continuously. The circuit monitors the differential input voltage and enables the slew boost path only when a transition is detected, providing fast settling on-demand while remaining inactive during steady-state operation to minimize power consumption.
Data Source
AI summary
A voltage follower circuit including an input stage for generating a difference between the input signal and the output signal. An output circuit receiving the first signal and producing the output signal. A slew boost circuit includes a first transistor having a control electrode for receiving the input signal, a first electrode coupled to a first current source, and a second electrode coupled to a first supply voltage, a second transistor having a control electrode coupled to the first electrode of the first transistor, a first electrode coupled to the first signal, and a second electrode coupled to the first supply voltage, and a third transistor having a control electrode coupled to the first electrode of the first transistor, a first electrode coupled to the first signal, and a second electrode coupled to a second supply voltage.