STM1404_08 datasheet(15/35 Pages) STMICROELECTRONICS

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STM1404

Operation

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The power-fail input (PFI) is compared to an internal 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 undervoltage detector to

signal a failing power supply. Typically PFI is connected through an external voltage divider

(see Figure 4 on page 9) 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

STM1404 or the microprocessor drops below the minimum operating voltage.

During battery backup, the power-fail comparator is turned off and PFO goes (or remains)

low (see Figure 9 on page 16). This occurs after VCC drops below VSW (~2.4 V). When

power returns, 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 so that a low voltage on PFI will generate a reset output.

3.4

Applications information

These supervisor circuits are not short-circuit protected. Shorting VOUT to ground -

excluding power-up transients such as charging a decoupling capacitor - destroys the

device. Decouple both VCC and VBAT pins to ground by placing 0.1 µF capacitors as close to

the device as possible.

Table 3.

I/O status in battery backup

Status

VOUT

Connected to VBAT through internal switch

VCC

Disconnected from VOUT

PFI

Disabled

PFO

Logic low

Disabled

RST

Logic low

VBAT

Connected to VOUT

Vccsw

Logic high

VREF

Pulled to VSS below 2.4 V (VSW)

BLD

Logic high

VTPU

Connected to VBAT through an internal switch

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