ADM8690_15 datasheet(13/20 Pages) AD

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ADM8690/ADM8691/ADM8692/ADM8693/ADM8695

Rev. B | Page 13 of 20

POWER-FAIL WARNING COMPARATOR

An additional comparator is provided for early warning of

failure in the microprocessor power supply. The power-fail

input (PFI) is compared to an internal 1.3 V reference. The

power-fail output (PFO) goes low when the voltage at PFI is less

than 1.3 V. Typically, PFI is driven by an external voltage divider

that senses either the unregulated dc input to the system 5 V

regulator or the regulated 5 V output. The voltage divider ratio

can be chosen such that the voltage at PFI falls below 1.3 V

several milliseconds before the 5 V power supply falls below the

reset threshold. PFO is normally used to interrupt the

microprocessor so that data can be stored in RAM and the shut-

down procedure executed before power is lost.

ADM869x

1.3V

PFO

POWER

FAIL

OUTPUT

POWER

FAIL

INPUT

POWER

Figure 23. Power-Fail Comparator

Table 6. Input and Output Status in Battery Backup Mode

Signal

Status

OUT

OUT is connected to VBATT via an internal PMOS

switch.

RESET

Logic low.

RESET

Logic high. The open-circuit output voltage is equal

to V

OUT.

LOW LINE

Logic low.

BATT ON

Logic high. The open-circuit voltage is equal to V

OUT.

WDI

WDI is ignored. It is internally disconnected from the

internal pull-up resistor and does not source or sink

current as long as its input voltage is between GND

and V

OUT. The input voltage does not affect supply

current.

WDO

Logic high. The open circuit voltage is equal to V

OUT.

PFI

The power-fail comparator is turned off and has no

effect on the power-fail output.

PFO

Logic low.

CEIN

CEIN is ignored. It is internally disconnected from its

internal pull-up and does not source or sink current

as long as its input voltage is between GND and

OUT. The input voltage does not affect supply

current.

CEOUT

Logic high. The open circuit voltage is equal to V

OUT.

OSC IN

OSC IN is ignored.

OSC SEL

OSC SEL is ignored.

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