STM690 datasheet(15/42 Pages) STMICROELECTRONICS

15 / 42 page

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 VOUT. With a backup battery installed with voltage VBAT, the devices

automatically switch the SRAM to the backup supply when VCC falls.

Note:

When the battery is first connected without VCC power applied, the device does not

immediately provide battery backup voltage on VOUT. Only after VCC exceeds VRST 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 VBAT and VOUT to VCC.

This family of supervisors does not always connect VBAT to VOUT when VBAT is greater than

VCC. VBAT connects to VOUT (through a 100 Ω switch) when VCC is below VSW (2.4 V) or

VBAT (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 VCC.

Assuming that VBAT > 2.0 V, switchover at VSO ensures that battery backup mode is entered

before VOUT gets too close to the 2.0 V minimum required to reliably retain data in most

external SRAMs. When VCC recovers, hysteresis is used to avoid oscillation around the VSO

point. VOUT is connected to VCC through a 3 Ω PMOS power switch.

Note:

The backup battery may be removed while VCC is valid, assuming VBAT is adequately

decoupled (0.1 µF typ), without danger of triggering a reset.

Table 4.

I/O status in battery backup

Status

VOUT

Connected to VBAT through internal switch

VCC

Disconnected from VOUT

PFI

Disabled

PFO

Logic low

High impedance

ECON

Logic high

WDI

Watchdog timer is disabled

Disabled

RST

Logic low

RST

Logic high

VBAT

Connected to VOUT

Vccsw

Logic high (STM795)

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