Datasheet

11
FN3182.8
April 5, 2006
Simple High/Low Temperature Alarm
Figure 16 illustrates a simple high/low temperature alarm
which uses the ICL7665S with an NPN transistor. The
voltage at the top of R
1
is determined by the V
BE
of the
transistor and the position of R
1
’s wiper arm. This voltage
has a negative temperature coefficient. R
1
is adjusted so
that V
SET2
equals 1.3V when the NPN transistor’s
temperature reaches the temperature selected for the high
temperature alarm. When this occurs, OUT2 goes low. R
2
is
adjusted so that V
SET1
equals 1.3V when the NPN
transistor’s temperature reaches the temperature selected
for the low temperature alarm. When the temperature drops
below this limit, OUT1 goes low.
AC Power Fail and Brownout Detector
Figure 17 shows a circuit that detects AC undervoltage by
monitoring the secondary side of the transformer. The
capacitor, C
1
, is charged through R
1
when OUT1 is OFF.
With a normal 100 VAC input to the transformer, OUT1 will
discharge C
1
once every cycle, approximately every 16.7ms.
When the AC input voltage is reduced, OUT1 will stay OFF,
so that C
1
does not discharge. When the voltage on C
1
reaches 1.3V, OUT2 turns OFF and the power fail warning
goes high. The time constant, R
1
C
1
, is chosen such that it
takes longer than 16.7ms to charge C
1
1.3V.
HYST1
V
SET1
V
SET2
HYST2
V+
OUT1 OUT2
5.86kΩ
130kΩ
BACKUP
BATTERY
7805
5V REGULATOR
UNREGULATED
DC INPUT
1MΩ
22MΩ
2.2MΩ
1MΩ
715kΩ
1MΩ
RESET OR
WRITE
ENABLE
POWER
FAIL
WARNING
470µF
4700µF
ICL7665S
FIGURE 15. POWER FAIL WARNING AND POWERUP/POWERDOWN RESET
ICL7665S