PCF8883 datasheet(6/24 Pages) NXP

6 / 24 page

PCF8883_1

Product data sheet

Rev. 01 — 16 October 2009

6 of 24

NXP Semiconductors

PCF8883

Capacitive proximity switch with auto-calibration

If the capacitance on pin IN increases, the discharge time tdch increases too. Therefore it

will take longer for the voltage on the corresponding comparator to drop below Vref. Only

once this happens, the comparator output will become LOW and this results in a pulse on

CDN discharging the external capacitor CCPC slightly. Thus most pulses will now be given

by CUP. Without further action, capacitor CCPC would then fully charge.

However, a chip-internal automatic calibration mechanism that is based on a voltage

controlled sink current (Isink) connected to pin IN attempts to equalize the discharge time

tdch with the internal reference discharge time tdch(ref). The current source is controlled by

the voltage on CCPC which causes the capacitance on pin IN to be discharged more

quickly in the case that the voltage on CCPC is rising, thereby compensating for the

increase in capacitance on input pin IN. This arrangement constitutes a closed-loop

control system that constantly attempts to equalize the discharge time tdch with tdch(ref).

This allows compensating for slow changes in capacitance on input pin IN. Fast changes

due to an approaching hand for example will not be compensated. In the equilibrium state

the discharge times are equal and the pulses alternate between CUP and CDN.

From this also follows that an increase in capacitor value CCPC results in a smaller voltage

change per pulse CUP or CDN. Thus the compensation due to internal current sink

source Isink is slower and therefore the sensitivity of the sensor will increase. Likewise a

decrease in capacitor CCPC will result in a lower sensitivity. (For further information see

Section 13.)

CSENS = sensing plate capacitance.

CCABLE = cable capacitance.

RC = external pull-down resistor.

RF = low pass ﬁlter resistor.

CF = low pass ﬁlter capacitor.

Fig 4.

Functional principle of the PCF8883

COUNTER

LOGIC

OSCILLATOR

VDD

VDD(INTREGD)

OUT

TYPE

CLIN

CPC

VSS

SENSOR

LOGIC

SENSING PLATE

COAXIAL CABLE

CSENS

CCABLE

PCF8883

VOLTAGE

REGULATOR

013aaa075

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