PCF88831 NXP [NXP Semiconductors], PCF88831 Datasheet - Page 6

• Manufacturer Part Number: PCF88831
• Description: Capacitive proximity switch with auto-calibration, large voltage operating
• Manufacturer: NXP [NXP Semiconductors]

NXP Semiconductors
PCF8883
Product data sheet
Fig 4.
SENSING PLATE
C
SENS
C
C
R
R
C
Functional principle of the PCF8883
SENS
CABLE
C
F
F
= low pass filter resistor.
= low pass filter capacitor.
= external discharge (pull-down) resistor.
= sensing plate capacitance.
= cable capacitance.
COAXIAL CABLE
If the capacitance on pin IN increases, the discharge time t
will take longer for the voltage on the corresponding comparator to drop below V
once this happens, the comparator output will become LOW and this results in a pulse on
CDN discharging the external capacitor C
by CUP. Without further action, capacitor C
However, a chip-internal automatic calibration mechanism that is based on a voltage
controlled sink current (I
t
the voltage on C
quickly in the case that the voltage on C
increase in capacitance on input pin IN. This arrangement constitutes a closed-loop
control system that constantly attempts to equalize the discharge time t
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 C
change per pulse CUP or CDN. Thus the compensation due to internal current sink
source I
decrease in capacitor C
Section
dch
C
with the internal reference discharge time t
CABLE
sink
13.)
is slower and therefore the sensitivity of the sensor will increase. Likewise a
R
F
All information provided in this document is subject to legal disclaimers.
CPC
R
which causes the capacitance on pin IN to be discharged more
C
CPC
Rev. 2 — 8 March 2011
sink
C
F
) connected to pin IN attempts to equalize the discharge time
will result in a lower sensitivity. (For further information see
IN
Capacitive proximity switch with auto-calibration
V
SENSOR
LOGIC
CPC
SS
CPC
CPC
is rising, thereby compensating for the
slightly. Thus most pulses will now be given
dch(ref)
would then fully charge.
CPC
. The current source is controlled by
V
DD(INTREGD)
PCF8883
OSCILLATOR
CPC
dch
COUNTER
LOGIC
VOLTAGE
REGULATOR
increases too. Therefore it
results in a smaller voltage
CLIN
PCF8883
f
s
© NXP B.V. 2011. All rights reserved.
dch
013aaa075
with t
dch(ref)
ref
. Only
V
OUT
TYPE
DD
6 of 25
.