Datasheet

10
FN3182.8
April 5, 2006
Combination Low Battery Warning and Low
Battery Disconnect
When using rechargeable batteries in a system, it is
important to keep the batteries from being over discharged.
The circuit shown in Figure 14 provides a low battery
warning and also disconnects the low battery from the rest of
the system to prevent damage to the battery. OUT1 is used
to shutdown the ICL7663S when the battery voltage drops to
the value where the load should be disconnected. As long as
V
SET1
is greater than 1.3V, OUT1 is low, but when V
SET1
drops below 1.3V, OUT1 goes high shutting off the
ICL7663S. OUT2 is used for low battery warning. When
V
SET2
is greater than 1.3V, OUT2 is high and the low battery
warning is on. When V
SET2
drops below 1.3V, OUT2 is low
and the low battery warning goes off. The trip voltage for low
battery warning can be set higher than the trip voltage for
shutdown to give advance low battery warning before the
battery is disconnected.
Power Fail Warning and Powerup/Powerdown
Reset
Figure 15 shows a power fail warning circuit with
powerup/powerdown reset. When the unregulated DC input
is above the trip point, OUT1 is low. When the DC input
drops below the trip point, OUT1 shuts OFF and the power
fail warning goes high. The voltage on the input of the 7805
will continue to provide 5V out at 1A until V
IN
is less than
7.3V, this circuit will provide a certain amount of warning
before the 5V output begins to drop.
The ICL7665S OUT2 is used to prevent a microprocessor
from writing spurious data to a CMOS battery backup
memory by causing OUT2 to go low when the 7805 5V
output drops below the ICL7665S trip point.
FIGURE 13. MULTIPLE SUPPLY FAULT MONITOR
HYST1
V
SET1
V
SET2
HYST2
V+
OUT1 OUT2
274kΩ
R
21
100kΩ
22
POWER
OK
301
R
11
+5V
-15V-5V
MΩ
kΩ
787
kΩ
+5V
1MΩ
1.02MΩ
R
21A
R
21B
22MΩ
49.9kΩ
+5V
+15V
HYST1
SET1 SET2
HYST2
V+
OUT1 OUT2
R
31
R
32
R
12
R
11
R
21
R
22
GND
+
-
LOW BATTERY SHUTDOWN
1MΩ
V+
OUT1
SHUTDOWN V
SET
OUT2V+
GND
1MΩ
LOW BATTERY WARNING
SENSE
100Ω
+5V
1A
ICL7663S
ICL7665S
FIGURE 14. LOW BATTERY WARNING AND LOW BATTERY DISCONNECT
ICL7665S