Datasheet

Operation STM690A/692A/703/704/802/805/817/818/819
16/43 Doc ID 10522 Rev 10
Figure 12. Chip-enable waveform
2.8 Power-fail input/output (NOT available on STM818)
The Power-fail Input (PFI) is compared to an internal reference voltage (independent from
the V
RST
comparator). If PFI is less than the power-fail threshold (V
PFI
), the Power-Fail
Output (PFO
) will go low. This function is intended for use as an undervoltage detector to
signal a failing power supply. Typically PFI is connected through an external voltage divider
(see Figure 12) to either the unregulated DC input (if it is available) or the regulated output
of the V
CC
regulator. The voltage divider can be set up such that the voltage at PFI falls
below V
PFI
several milliseconds before the regulated V
CC
input to the
STM690A/692A/703/704/802/805/817/818/819 Supervisor or before the microprocessor
drops below the minimum operating voltage. This provides several milliseconds of advanced
warning that power is about to fail.
During battery backup, the power-fail comparator turns off and PFO
goes (or remains) low
(see Figure 13 below and Figure 14). This occurs after V
CC
drops below 2.4 V (or V
SO
).
When power returns, PFO
is forced high (STM817/819 only), irrespective of V
PFI
for the
WRITE protect time (t
rec
). 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 V
SS
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.
2.9 Applications information
These supervisor circuits are not short-circuit protected. Shorting V
OUT
to ground -
excluding power-up transients such as charging a decoupling capacitor - destroys the
device. Decouple both V
CC
and V
BAT
pins to ground by placing 0.1 µF capacitors as close to
the device as possible.
AI08803b
RST
E
V
CC
V
RST
V
BAT
E
CON
t
rec
t
rec15µs
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