SP690A Datasheet, PDF (10/16 Pages) Sipex Corporation – Low Power Microprocessor Supervisory with Battery Switch-Over

The SP805L/M active-high RESET output is
the inverse of the SP690A/SP692A/SP802 RE-
SET output, and is valid with V_CC down to 1V.
Some µP's, such as Intel's 80C51, require an
active-high reset pulse.
Watchdog Input
The watchdog circuit monitors the µP's activity.
If the µP does not toggle the watchdog input
(WDI) within 1.6sec, a reset pulse is triggered.
The internal 1.6sec timer is cleared by either a
reset pulse or by floating the WDI input. As long
as RESET is asserted or the WDI input is
floating, the timer remains cleared and does not
count. As soon as RESET is released and WDI
is driven high or low, the timer starts counting.
It can detect pulses as short as 50ns.
VBATT
VCC
SW1 ^{D1 D2}
SW2
VOUT
D3
GND
CONDITION
SW1
SW2
V_CC > Reset Threshold
Open
Closed
V_CC < Reset Threshold and
V_CC > V_BATT
V_CC < Reset Threshold and
V_CC < V_BATT
Open
Closed
Closed
Open
Reset Threshold = 4.65V in SP690A/802L/805L
Reset Threshold = 4.40V in SP692A/802M/805M
Figure 13. BACKUP-BATTERY Switchover Block Diagram
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
unregulated DC input to the +5V regulator. The
voltage-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.
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 13. 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 pow-
ered from the battery Figure 14. 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.
Date: 11/29/04
SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over
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