Touch Panel Precharge Logic for Fast Charging and Low Power
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Solution Overview
Problem
Touch panels with increasing size and decreasing thickness experience higher capacitance values, leading to slower charging times and false pen down events due to higher resistance-capacitance (RC) time constants, while reducing resistance to speed up charging increases power consumption, and eliminating the current limiting resistor results in increased noise and electromagnetic interference.
Innovation Solution
The implementation of precharge logic to precharge the touch panel capacitor through a parallel path to the current limiting resistor, allowing for faster charging and reduced power consumption, while minimizing false pen down events and electromagnetic interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the resistance value of the current limiting resistor is reduced to decrease the RC time constant, then the charging speed of the touch panel capacitor is improved, but power consumption increases during pen down events
Solution Approach 1:
The patent applies a periodic precharge pulse to charge the touch panel capacitor in advance before the pen down event. This preliminary action reduces the RC time constant effect by pre-charging the capacitor, allowing faster response when pressure is applied without requiring a low resistance current limiting resistor, thus avoiding increased power consumption during normal operation
2Speed
If the current limiting resistor is eliminated to speed up charging, then the RC time constant is reduced, but noise and electromagnetic interference increase
Solution Approach 1:
The patent introduces a periodic precharge pulse as an intermediary mechanism to achieve fast charging without eliminating the current limiting resistor. This mediator approach allows the resistor to remain in the circuit (maintaining noise and EMI protection) while the periodic pulse provides the necessary fast charging capability by pre-charging the capacitor at specific intervals
3Loss of energy
If the touch panel capacitor charges through the current limiting resistor, then power consumption is reduced, but false pen down events occur due to slow charging
Solution Approach 1:
The patent employs periodic precharge pulses to charge the touch panel capacitor at regular intervals. This periodic action ensures the capacitor is adequately charged before pen down detection, preventing false positives caused by slow charging through the high resistance current limiting resistor, while maintaining low power consumption by only charging during specific periodic intervals rather than continuously
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 approach enhances the speed and reliability of touch panel operations by reducing false pen down events and minimizing power consumption, maintaining system reliability and extending battery life in portable devices.
Implementation Method 1
these higher capacitance values result in higher resistance-capacitance (RC) time constants which can detrimentally slow the charging of the touch panel capacitor
Implementation Method 2
the touch panel capacitor charges up through the current limiting resistor
Data Source
AI summary
A touch panel detection circuit includes current limiting circuitry that has a first portion coupled between a first supply voltage terminal and a first input node and a second portion coupled between a second input node and a second supply voltage terminal. Programmable precharge circuitry connects the first input node to the first supply voltage terminal via a conductive path that is in parallel with the first portion of the current limiting circuitry and precharges the first input node to a predetermined voltage. Comparison circuitry is coupled to the programmable precharge circuitry and to the first input node. The comparison circuitry detects a change in resistance between the first input node and the second input node and provides a signal in response thereto when the comparison circuitry is enabled by the programmable precharge circuitry.