QT ICs derive their internal reference from the Vdd power supply. It is therefore
important to ensure that Vdd is stable and free of electrical noise. The proprietary drift
compensation algorithms of the QT IC are designed to overcome long-term drift.
Nevertheless, transient spikes of ±10 mV can cause erratic behaviour such as false
key activations, stuck keys, and loss of touch detection.
A simple and inexpensive regulator circuit eliminates these potential problems.
Power Supply
Considerations
for QT ICs
Application Note
QTAN0015
3. General Recommendations
Do:
• Use a dedicated regulator to power the QT IC.
• Place the regulator as close as possible to the QT IC.
• Use a GROUND plane under the regulator and under the Vss pins of the QT IC.
• For PCBs with a GROUND plane layer, extend the plane under communications
circuits, any oscillator or crystal, and also decoupling capacitors.
• Place a 0.1 µF decoupling capacitor in close physical proximity to each Vdd pin.
Don’t:
• Share Vdd with other logic circuitry or light-emitting diodes (LED).
• Use long power and GROUND traces between the regulator and the QT IC.
• Extend the GROUND plane beneath the sense pins of the QT IC, Cs capacitors,
Rs resistors, or any sense line traces.
4. Low Drop-out (LDO) Regulator
QT ICs are optimized for low power operation. To conserve power, the device mostly
operates in an ultra-low-power sleep mode in which minimal current (µA) is drawn.
Periodically, the QT IC wakes up to perform sampling and communication routines
that consume mA of current.
The poor load step response of some LDO regulators is well-known, often resulting in
significant output transients when the load increases from µA to mA, causing erratic
sensor operation. Care must be taken in the selection and use of LDO regulators.
Figure 4-1
shows the circuit for a Seiko S-817 series LDO regulator.
10703A–AT42–10/08
Figure 4-1.
LDO Regulator Circuit
S-817
Vunreg
C1
2.2 µF
C2
0.1 µF
VIN
VOUT
GND
Vdd
C3
C4
2.2 µF
0.1 µF
The Seiko S-817 regulator has a good load step response and reliable performance when used
with QT ICs. Low quiescent current makes the S-817 regulator especially suitable for
battery-driven applications.
C2 and C3 should be good quality ceramic capacitors, placed as close as possible to the
regulator. An additional ceramic capacitor should be placed close to the Vdd pin on the QT IC.
5. Standard Linear Regulator
Figure 5-1
shows the circuit for an LM78L05 linear regulator.
Figure 5-1.
Linear Regulator Circuit (LM78L05)
LM78L05
Vunreg
C1
2.2 µF
C2
0.1 µF
VIN
VOUT
GND
Vdd
C3
C4
2.2 µF
0.1 µF
C2 and C3 should be good quality ceramic capacitors, placed as close as possible to the
regulator. An additional ceramic capacitor should be placed close to the Vdd pin on the QT IC.
6. Direct Battery Power
The internal resistance of a battery causes the voltage to drop as the load increases, so load
variations result in voltage transients. As the battery ages and discharges, its internal resistance
increases and the transient effects worsen. To overcome these characteristics, QT ICs should
be powered from an LDO regulator connected between the QT IC and the battery.
Exceptionally, a QT IC can be safely powered directly from the battery, but this can only be done
in applications in which the battery load remains relatively constant. Particularly on older
batteries, a delta value of >10 mV can be caused merely by flashing an LED or driving a small
relay coil.
7. Associated Publications
The following document published by Atmel’s Touch Technology Division is also of interest:
• QTAN0017 – Checking the Effects of External Noise on Atmel Capacitive-touch ICs
2
Power Supply Considerations for QT ICs
10703A–AT42–10/08
Power Supply Considerations for QT ICs
Notes
3
10703A–AT42–10/08
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