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ADM8690ARN-REEL

Microprocessor Supervisory Circuits

Analog Devices 

ADM8690/ADM8691/ADM8695

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.

Data Sheet

PFO is normally used to interrupt the microprocessor so

that data can be stored in RAM and the shutdown procedure

executed before power is lost.

INPUT

POWER

R1

1.3V

POWER- PFI

R2

FAIL

INPUT

PFO

POWER-

FAIL

OUTPUT

Figure 23. Power-Fail Comparator

Table 6. Input and Output Status in Battery Backup Mode

Signal

Status

VOUT

VOUT is connected to VBATT via an internal PMOS switch.

RESET

Logic low.

RESET

Logic high. The open circuit output voltage is equal to VOUT.

LOW LINE

Logic low.

BATT ON

WDI

WDO

Logic high. The open circuit voltage is equal to VOUT.

WDI is ignored. It is internally disconnected from its internal pull-up resistor and does not source or sink current as long as

its input voltage is between GND and VOUT. The input voltage does not affect supply current.

Logic high. The open circuit voltage is equal to VOUT.

PFI

The power-fail comparator is turned off and has no effect on the power-fail output.

PFO

Logic low.

CEIN

CEOUT

CEIN is ignored. It is internally disconnected from its internal pull-up resistor and does not source or sink current as long as

its input voltage is between GND and VOUT. The input voltage does not affect supply current.

Logic high. The open circuit voltage is equal to VOUT.

OSC IN

OSC IN is ignored.

OSC SEL

OSC SEL is ignored.

Rev. C | Page 14 of 24

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