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PCF1252

Threshold detector and reset generator

Philips Electronics 

Philips Semiconductors

Threshold detector and reset generator

Product specification

PCF1252-X family

PINNING

SYMBOL PIN

DESCRIPTION

CT

1 connection for the external capacitor

SELECT 2 select polarity or external reset input

COMIN

3 comparator input

VSS

4 ground (0 V)

COMOUT 5 comparator output

RESET

6 reset output

POWF

7 power failure signal output

VDD

8 supply voltage

handbook, halfpage

CT 1

8 VDD

SELECT 2

7 POWF

PCF1252-X

COMIN 3

6 RESET

VSS 4

5 COMOUT

MGC916

Fig.2 Pinning diagram.

FUNCTIONAL DESCRIPTION (see Fig.1)

The PCF1252-X contains:

• A precise factory-programmed voltage reference

• Two comparators

• A delay circuit.

The PCF1252-X family is comprised of 10 versions with

different factory-programmed voltage trip-points (VTRIP),

see Chapter “Characteristics”.

Supply

The supply voltage (VDD) is internally divided before being

compared, via comparator 1, with the internal reference

voltage.

POWF (see Fig.3)

The POWF output is:

• LOW, if VDD is below VTRIP

• HIGH, if VDD is above VTRIP.

Power-on reset (SELECT = LOW)

As VDD rises past VTRIP, a positive reset pulse is generated

at RESET. The duration of the reset pulse (tR) is

determined by the value of the external capacitor (CCT;

maximum 1 µF, see Fig.8) connected to CT. With no

external capacitor connected, CCT assumes a minimum

value of 100 pF. If SELECT is HIGH, the reset pulse is

inverted.

Voltage trip-point

By selecting the voltage trip-point slightly higher than the

minimum operating voltage of the

microcontroller/microprocessor, there is sufficient time for

data storage before the power actually fails.

In order to prevent oscillations around the voltage

trip-point, a small hysteresis has been included, resulting

in a power-on switching point that is higher than the

voltage trip-point (minimum of 15 mV). The voltage

trip-point refers to the value at which power-off is signalled.

COMIN

Input to the second comparator (comparator 2).

When used in conjunction with an external voltage divider,

this allows a second point in the system to be monitored.

This input has no built-in hysteresis. When not in use

connect to VDD. COMOUT will be LOW or HIGH

depending on the voltage at COMIN:

• COMOUT = HIGH, if voltage at COMIN is above the

switch point VSP (typically 1.30 V).

• COMOUT = LOW, if voltage at COMIN is below the

switch point VSP (typically 1.30 V).

Power failure

During a power-off condition (VDD < VTRIP), POWF goes

LOW. After a time delay (tS), also determined by CCT,

RESET goes HIGH. Any POWF assertion (VDD < VTRIP)

will result in a subsequent RESET pulse.

1998 Apr 16

3

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