STM690A_10 Datasheet, PDF (14/43 Pages) STMicroelectronics – 5 V supervisor with battery switchover

Operation
STM690A/692A/703/704/802/805/817/818/819
2.4
Note:
Note:
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.
When the battery is first connected without V_CC power applied, the device does not
immediately provide backup battery 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. For the
STM81x devices, the battery freshness seal can be initiated again by following the
procedure outlined in Section 2.12. If the backup battery is not used, connect both V_BAT and
V_OUT to V_CC .
Whenever V_CC falls below the switchover voltage, V_SO, V_OUT is connected to V_BAT through a
100 Ω switch. V_SO is the lesser of V_BAT and V_RST. Choosing the lesser allows the device to
be powered by V_CC for as long as possible before switching over thereby maximizing the
battery life.
Assuming 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.
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
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
WDI
Logic high
Watchdog timer is disabled
MR
Disabled
RST
Logic low
RST
Logic high
V_BAT
Connected to V_OUT
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Doc ID 10522 Rev 10