STM802 datasheet(12/37 Pages) STMICROELECTRONICS

12 / 37 page

STM690A/692A/703/704/802/805/817/818/819

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Power-fail Input/Output (NOT available on STM818)

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 STM690A/692A/703/

704/802/805/817/818/819 Supervisor or the mi-

croprocessor drops below the minimum operating

voltage.

During battery back-up, the power-fail comparator

turns off and PFO goes (or remains) low (see Fig-

ure 14 and Figure 15., page 13). This occurs after

VCC drops below 2.4V (or VSO). When power re-

turns, PFO is forced high (STM817/819 only), irre-

spective of VPFI for the WRITE protect time (trec).

At the end of this time, the power-fail comparator

is enabled and PFO follows PFI. If the comparator

is unused, PFI should be connected to VSS and

PFO left unconnected. PFO may be connected to

MR on the STM703/704/818 so that a low voltage

on PFI will generate a reset output.

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 (STM817/818/819)

AI08804a

VCC

VRST

VSO (or 2.4V)

trec

RST to ECON Delay (STM818)

RST

ECON (STM818)

PFO

(STM817/819)

PFO follows PFI

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