SP703 Datasheet, PDF (10/16 Pages) Sipex Corporation – Low Power Microprocessor Supervisory with Battery Switch-Over

Power-Fail Comparator
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.
When V_BATT connects to V_OUT, the power-fail
comparator is turned off and PFO is forced low
to conserve backup-battery power.
VBATT
VCC
SW1 ^{D1 D2}
SW2
Backup-Battery Switchover
In the event of a brownout or power failure, it
may be necessary to preserve the contents of
RAM. With a backup battery installed at V_BATT,
the RAM is assured to have power if V_CC fails.
As long as V_CC exceeds the reset threshold,
V_OUT connects to V_CC through a 0.6Ω PMOS
power switch. Once V_CC falls below the reset
threshold, V_CC or V_BATT, whichever is higher,
switches to V_OUT. V_BATT connects to V_OUT
through a 5Ω switch only when V_CC is below the
reset threshold and V_BATT is greater than V_CC.
When V_CC exceeds the reset threshold, it is
connected to V_OUT, regardless of the voltage
applied to V_BATT Figure 9. During this time,
the diode (D1) between V_BATT and V_OUT will
conduct current from V_BATT to V_OUT if V_BATT is
more than .6V above V_OUT.
When V_BATT connects to V_OUT, backup mode is
activated and the internal circuitry will be
powered from the battery Figure 10. When V_CC
is just below V_BATT, in the backup mode the
current drawn from V_BATT will be typically
30µA. When V_CC drops to more than 1V below
V_BATT, the internal switchover comparator shuts
off and the supply current falls to less than 0.6µA.
VOUT
D3
GND
CONDITION
SW1
SW2
V > Reset Threshold
CC
Open
Closed
V < Reset Threshold and
CC
V_CC > V_BATT
V_CC < Reset Threshold and
V_CC < V_BATT
Open
Closed
Closed
Open
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
10
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