PCF8883 datasheet(5/24 Pages) NXP

5 / 24 page

PCF8883_1

Product data sheet

Rev. 01 — 16 October 2009

5 of 24

NXP Semiconductors

PCF8883

Capacitive proximity switch with auto-calibration

Functional description

Figure 3 and Figure 4 show the functional principle of the PCF8883. The discharge time

(tdch) of a chip-internal RC timing circuit, to which the external sensing plate is connected

via pin IN, is compared to the discharge time (tdch(ref)) of a second chip-internal reference

RC timing circuit. Both RC timing circuits are periodically charged from VDD(INTREGD) via

identical switches and then discharged via a resistor to ground (VSS). Both switches are

synchronized.

The charge-discharge cycle is governed by the sampling rate (fs). If the voltage of one of

the RC timing circuits falls below the internal reference voltage Vref, the respective

comparator output will become LOW. The logic following the comparators determines

which comparator switches ﬁrst. If the upper (reference) comparator switches then a pulse

is given on CUP. If the lower (input) comparator switches ﬁrst then a pulse is given on

CDN (see Figure 3).

The pulses control the charge on the external capacitor CCPC on pin CPC. Every time a

pulse is given on CUP, capacitor CCPC is charged from VDD(INTREGD) for a ﬁxed time

causing the voltage on CCPC to rise. Likewise when a pulse occurs on CDN, capacitor

CCPC is connected to a current sink to ground for a ﬁxed time causing the voltage on CCPC

to fall.

Fig 3.

Functional diagram of the sensor logic

013aaa093

CPC

VSS

VDD(INTREGD)

Vref

VDD(INTREGD)

CDN

CUP

Isink

COUNTER

LOGIC

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