PCF8883USSLASH7EASLASH1 datasheet(8/35 Pages) NXP

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PCF8883

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Product data sheet

Rev. 4.1 — 14 September 2016

8 of 35

NXP Semiconductors

PCF8883

Capacitive touch/proximity switch with auto-calibration

The counter, following the sensor logic depicted in Figure 4, counts the pulses of CUP or

CDN respectively. The counter is reset every time the pulse sequence changes from CUP

to CDN or the other way around. Pin OUT will only be activated when enough consecutive

CUP or CDN pulses occur. Low-level interference or slow changes in the input

capacitance do not cause the output to switch.

Various measures, such as asymmetrical charge and discharge steps, are taken to

ensure that the output switches off correctly. A special start-up circuit ensures that the

device reaches equilibrium quickly when the supply is attached.

Pin OUT is an open-drain output capable of pulling an external load Rext (at maximum

current of 20 mA) up to VDD. The load resistor must be dimensioned appropriately, taking

the maximum expected VDD voltage into account. The output is automatically deactivated

(short circuit protection) for loads in excess of 30 mA. Pin OUT can also drive a CMOS

input without connection of the external load.

A small internal 150 nA current sink Isink enables a full voltage swing to take place on pin

OUT, even if no load resistor is connected. This is useful for driving purely capacitive

CMOS inputs. The falling slope can be fairly slow in this mode, depending on load

capacitance.

The sampling rate (fs) corresponds to half of the frequency used in the RC timing circuit.

The sampling rate can be adjusted within a specified range by selecting the value of

CCLIN. The oscillator frequency is internally modulated by 4 % using a pseudo random

signal. This prevents interference caused by local AC-fields.

8.1 Output switching modes

The output switching behavior can be selected using pin TYPE (see Figure 6).

• Push-button (TYPE connected to VSS): The output OUT is active as long as the

capacitive event2 lasts.

• Toggle (TYPE connected to VDD(INTREGD)): The output OUT is activated by the first

capacitive event and deactivated by a following capacitive event.

• Pulse (CTYPE connected between TYPE and VSS): The output OUT is activated for a

defined time at each capacitive event. The pulse duration is determined by the value

of CTYPE and is approximately 2.5 ms/nF.

A typical value for CTYPE is 4.7 nF which results in an output pulse duration of about

10 ms. The maximum value of CTYPE is 470 nF which results in a pulse duration of

about 1 s. Capacitive events are ignored that occur during the time the output is active.

Figure 6 illustrates the switching behavior for the output switching modes. Additionally the

graph illustrates, that short-term disturbances on the sensor are suppressed by the circuit.

A capacitive event is a dynamic increase of capacitance at the sensor input pin IN.

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