Datasheet

STM690, STM704, STM795, STM802, STM804, STM805, STM806 Operation
Doc ID 10519 Rev 9 15/42
2.4 Backup battery switchover
In the event of a power failure, it may be necessary to preserve the contents of external
SRAM through V
OUT
. With a backup battery installed with voltage V
BAT
, the devices
automatically switch the SRAM to the backup supply when V
CC
falls.
Note: When the battery is first connected without V
CC
power applied, the device does not
immediately provide battery backup voltage on V
OUT
. Only after V
CC
exceeds V
RST
will the
switchover operate as described below. This mode allows a battery to be attached during
manufacturing but not used until after the system has been activated for the first time. As a
result, no battery power is consumed by the device during storage and shipment. If the
backup battery is not used, connect both V
BAT
and V
OUT
to V
CC
.
This family of supervisors does not always connect V
BAT
to V
OUT
when V
BAT
is greater than
V
CC
. V
BAT
connects to V
OUT
(through a 100 Ω switch) when V
CC
is below V
SW
(2.4 V) or
V
BAT
(whichever is lower). This is done to allow the backup battery (e.g., a 3.6 V lithium cell)
to have a higher voltage than V
CC
.
Assuming that V
BAT
> 2.0 V, switchover at V
SO
ensures that battery backup mode is entered
before V
OUT
gets too close to the 2.0 V minimum required to reliably retain data in most
external SRAMs. When V
CC
recovers, hysteresis is used to avoid oscillation around the V
SO
point. V
OUT
is connected to V
CC
through a 3 Ω PMOS power switch.
Note: The backup battery may be removed while V
CC
is valid, assuming V
BAT
is adequately
decoupled (0.1 µF typ), without danger of triggering a reset.
Table 4. I/O status in battery backup
Pin Status
V
OUT
Connected to V
BAT
through internal switch
V
CC
Disconnected from V
OUT
PFI Disabled
PFO Logic low
E
High impedance
E
CON
Logic high
WDI Watchdog timer is disabled
MR
Disabled
RST
Logic low
RST Logic high
V
BAT
Connected to V
OUT
Vccsw
Logic high (STM795)