AN2869 datasheet(16/43 Pages) STMICROELECTRONICS

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Main capacitive sensing guidelines

AN2869

16/43

Doc ID 15298 Rev 6

2.4

Power supply

In order to reduce system costs, a regulator, which is fully dedicated to touch sensing, is

already embedded in the devices of the STM8T family. For other devices without a touch

sensing dedicated regulator, it is strongly recommended to use an external voltage regulator

to power the device only.

The voltage regulator must be chosen to provide a stable voltage without any ripple. The

actual precision of the voltage is not important, but the noise rejection feature is critical. This

voltage is used to drive CX and is also used as a reference when measuring the sampling

capacitor (CS). Any variation of this voltage may induce measurement variations which

could generate a false touch or a missed touch. For instance, a ±10 mV peak to peak

variation on VDD limits the resolution of linear sensor or rotary sensor to 4 or 5 bits.

The voltage regulator should be placed as far as possible from the sensors and their tracks.

The voltage regulator also acts as a filter against noise coming from the power supply. So, it

is recommended to power any switching components, such as LEDs, directly from VDD and

not from the regulated voltage (see Figure 9).

Figure 9.

Typical power supply schematic

Typical voltage regulator LD2980 can be used.

10 nF

GPIO

VDD LEDs

Supply input

LDO regulator

(note 1)

VDD

Touch sensing

device (MCU)

LED

ai15082

VSS

VDD

Vin

Vout

VSS

1 μF

100 nF

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