Backup-Battery Switchover
Power-Fail Comparator
In the event of a brownout or power failure, it
may be necessary to preserve the contents of
The Power-Fail Comparator can be used as an
under-voltage detector to signal the failing of a
power supply (it is completely separate from the
rest of the circuitry and does not need to be
dedicated to this function). The PFI input is
compared to an internal 1.25V reference. If PFI
is less than 1.25V, PFO goes low. The external
voltage divider drives PFI to sense the unregu-
lated DC input to the +5V regulator. The volt-
age-divider ratio can be chosen such that the
voltage at PFI falls below 1.25V just before the
+5V regulator drops out. PFO then triggers an
interrupt which signals the µP to prepare for
power-down.
RAM. WithabackupbatteryinstalledatVBATT
,
the RAM is assured to have power if VCC fails.
As long as VCC exceeds the reset threshold,
VOUT connects to VCC through a 0.6Ω PMOS
power switch. Once VCC falls below the reset
threshold, VCC or VBATT, whichever is higher,
switches to VOUT
througha 5Ω switchonlywhenVCC isbelowthe
reset threshold and VBATT is greater than VCC
. VBATT connects to VOUT
.
When VCC exceeds the reset threshold, it is
connected to VOUT, regardless of the voltage
applied to VBATT Figure 9. During this time,
the diode (D1) between VBATT and VOUT will
conduct current from VBATT to VOUT if VBATT is
When VBATT connects to VOUT, the power-fail
comparator is turned off and PFO is forced low
to conserve backup-battery power.
more than .6V above VOUT
.
When VBATT connects to VOUT, backup mode is
activated and the internal circuitry will be
powered from the battery Figure 10. When VCC
is just below VBATT, in the backup mode the
current drawn from VBATT will be typically
30µA. When VCC drops to more than 1V below
VBATT,theinternalswitchovercomparatorshuts
off and the supply current falls to less than 0.6µA.
V
BATT
V
CC
D2
D1
SW2
SW1
D3
V
OUT
GND
CONDITION
SW1
Open
SW2
Closed
Closed
Open
VCC > Reset Threshold
VCC < Reset Threshold and
VCC > VBATT
Open
VCC < Reset Threshold and
VCC < VBATT
Closed
Reset Threshold = 4.65V in SP703
Reset Threshold = 4.40V in SP704
Figure 9. BACKUP-BATTERY Switchover Block Diagram
SP703/704DS/07
SP703/704 Low Power Microprocessor Supervisory
© Copyright 2000 Sipex Corporation
10
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