SIGNAL
STATUS
+5V
V
VCC
Disconnected from VOUT
CC
Connected to VBATT through
VOUT
VBATT
PFI
an internal 8Ω PMOS switch
CONNECT TO
STATIC RAM
V
OUT
V
BATT
SP703
Connected to VOUT. Current
drawn from the battery is
less than 0.6µA, as long as
CONNECT
TO µP
RESET
0.1F
VCC < VBATT - 1V.
GND
Power-fail comparator is
disabled.
Figure 12. Backup Power Source Using High Capacity
Capacitor with SP703 and a +5V ±5% Supply
PFO
RESET
MR
Logic low
Logic low
If VCC is above the reset threshold and VBATT
Manual Reset is disabled
is 0.5V above VCC, current flows to V
and
VCC from VBATT until the voltage at VBOUATTT is
less than 0.5V above VCC.
Figure 10. Input and Output Status in Battery-Backup Mode.
To enter the Battery-Backup mode, VCC must be less than the
Reset threshold and less than VBATT
.
Leakage current through the capacitor charging
diode and the SP703/704 internal power
diode eventually discharges the capacitor to
VCC. Also, if VCC and VBATT start from 0.5V
above the reset threshold and power is lost
at VCC, the capacitor on VBATT discharges
through VCC until VBATT reaches the reset
threshold; the SP703/704 then switches to
battery-backup mode.
Using a High Capacity Capacitor
as a Backup Power Source
VBATT has the same operating voltage range as
VCC, and the battery-switchover threshold volt-
ages are typically +20mV centered at VBATT,
allowing use of a capacitor and a simple charg-
ing circuit as a backup source (see Figure 12).
+5V
MAXIMUM
PART
BACKUP-BATTERY
NUMBER
V
CC
VOLTAGE [V]
CONNECT TO
STATIC RAM
V
OUT
VBATT
SP704
SP703
SP704
4.80
4.55
0.1F
CONNECT
RESET
TO µP
100KΩ
Figure 11. Allowable BACKUP-BATTERY Voltages
GND
Figure 13. Backup Power Source Using High Capacity
Capacitor with SP704 and a +5V ±10% Supply
SP703/704DS/07
SP703/704 Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
11
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