ICL7673_05 Datasheet, PDF (5/7 Pages) Intersil Corporation – Automatic Battery Back-Up Switch

ICL7673
Applications for the ICL7673 include volatile semiconductor
memory storage systems, real-time clocks, timers, alarm
systems, and over/under the voltage detectors. Other
systems requiring DC power when the master AC line supply
fails can also use the ICL7673.
+5V
PRIMARY
SUPPLY
8
V_P
1
V_O
V_O
+5V OR +3V
LITHIUM
BATTERY
GND
2 _VS
Pbar ⁶
GND
+
4
-
R_I
STATUS
INDICATOR
FIGURE 7. ICL7673 BATTERY BACKUP CIRCUIT
+5V
PRIMARY
SUPPLY
R_C
RECHARGEABLE
BATTERY
GND
8
V_P
1
V_O
2 _VS
+
GND
4
-
V_O
+5V OR +3V
FIGURE 8. APPLICATION REQUIRING RECHARGEABLE
BATTERY BACKUP
A typical application, as illustrated in Figure 12, would be a
microprocessor system requiring a 5V supply. In the event of
primary supply failure, the system is powered down, and a
3V battery is employed to maintain clock or volatile memory
data. The main and backup supplies are connected to V_P
and V_S, with the circuit output V_O supplying power to the
clock or volatile memory. The ICL7673 will sense the main
supply, when energized, to be of greater potential than V_S
and connect, via its internal MOS switches, V_P to output V_O.
The backup input, V_S will be disconnected internally. In the
event of main supply failure, the circuit will sense that the
backup supply is now the greater potential, disconnect V_P
from V_O, and connect V_S.
Figure 11 illustrates the use of external PNP power
transistors to increase the power switching capability of the
circuit. In this application the output current is limited by the
beta and thermal characteristics of the power transistors.
If hysteresis is desired for a particular low power application,
positive feedback can be applied between the input V_P and
open drain output S_BAR through a resistor as illustrated in
Figure 12. For high power applications hysteresis can be
applied as shown in Figure 13.
The ICL7673 can also be used as a clipping circuit as
illustrated in Figure 14. With high impedance loads the
circuit output will be nearly equal to the greater of the two
input signals.
V_P
FUSE
8
2
8
1
V_O
120/240
VAC
BRIDGE
RECTIFIER
C₁
STEPDOWN
ICL7663
REGULATOR
R₂
46
R₁
R₃
D₁ ⁺
-
ICL7673
BATTERY
V_S BACK-UP
24
GND
BATTERY
STACK
TRANSFORMER
FIGURE 9. POWER SUPPLY FOR LOW POWER PORTABLE AC TO DC SYSTEMS
+5V
MAIN
POWER
POWER
FAIL
DETECTOR
MICROPROCESSOR
INTERRUPT SIGNAL
V_O
VOLATILE
RAM
V_P
V_S
ICL7673
BACKUP CIRCUIT
+
-
FIGURE 10. TYPICAL MICROPROCESSOR MEMORY APPLICATION
5
FN3183.4
July 22, 2005