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STM802S

3V Supervisor with Battery Switchover

STMicroelectronics 

STM690/704/795/802/804/805/806

Power-fail Input/Output (NOT available on STM795)

The Power-fail Input (PFI) is compared to an inter-

nal reference voltage (independent from the VRST

comparator). If PFI is less than the power-fail

threshold (VPFI), the Power-Fail Output (PFO) will

go low. This function is intended for use as an un-

dervoltage detector to signal a failing power sup-

ply. Typically PFI is connected through an external

voltage divider (see Figure 11., page 8) to either

the unregulated DC input (if it is available) or the

regulated output of the VCC regulator. The voltage

divider can be set up such that the voltage at PFI

falls below VPFI several milliseconds before the

regulated VCC input to the STM690/704/795/802/

804/805/806 or the microprocessor drops below

the minimum operating voltage.

During battery back-up, the power-fail comparator

is turned off and PFO goes (or remains) low (see

Figure 14., page 11). This occurs after VCC drops

below VSW (2.4V). When power returns, the pow-

er-fail comparator is enabled and PFO follows PFI.

If the comparator is unused, PFI should be con-

nected to VSS and PFO left unconnected. PFO

may be connected to MR on the STM704/806 so

that a low voltage on PFI will generate a reset out-

put.

Applications Information

These Supervisor circuits are not short-circuit pro-

tected. Shorting VOUT to ground - excluding pow-

er-up transients such as charging a decoupling

capacitor - destroys the device. Decouple both

VCC and VBAT pins to ground by placing 0.1µF ca-

pacitors as close to the device as possible.

Figure 14. Power-fail Comparator Waveform (STM690/704/802/804/805/806)

VCC

VRST

VSW (2.4V)

PFO

PFO follows PFI

trec

PFO follows PFI

RST

AI08861a

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