DS1231/S
022698 5/9
APPLICATION – MODE PIN CONNECTED TO
GROUND
When the Mode pin is connected to ground, pin 1 is a
current source of 30 µA with a V
TP
+ of 2.5 volts. Pin 1 is
held below the trip point by a switching device like an
opto-isolator as shown in Figure 6. Determination of the
sense point has the same criteria as discussed in the
previous application. However, determining component
values is significantly different. In this mode, the maxi-
mum dynamic range of the sense point versus desired
trip voltage is primarily determined by the selection of a
zener diode. As an example, if the maximum voltage at
the sense point is 200V and the desired trip point is
150V, then a zener diode of 150V will approximately set
the trip point. This is particularly true if power consump-
tion on the high voltage side of the opto-isolator is not an
issue. However, if power consumption is a concern,
then it is desirable to make the value of R1 high. As the
value of R1 increases, the effect of the LED current in
the opto-isolator starts to affect the IN trip point. This can
be seen from the equation shown in Figure 6. R1 must
also be low enough to allow the opto-isolator to sink the
30 µA of collector current required by pin 1 and still have
enough resistance to keep the maximum current
through the opto-isolator’s LED within data sheet limits.
Figure 7 illustrates how the DS1231 can be interfaced to
the AC power line when the mode pin is grounded.
AC VOLTAGE MONITOR WITH TRANSFORMER ISOLATION Figure 5
+5V
DC
IN
MODE
TOL
GND
V
CC
NMI
RST
RST
-10% V
CC
THRESHOLD
TO µ P
DS1231
+5V
DC
VOLTAGE SENSE POINT
NOTE: RST requires a pull–up resister.
