Datasheet

ADM8690/ADM8691/ADM8695 Data Sheet
Rev. C | Page 14 of 24
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.
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.
1.3V
PFO
PFI
R1
R2
POWER-
FAIL
OUTPUT
POWER-
FAIL
INPUT
INPUT
POWER
00093-014
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.
Figure 23. Power-Fail Comparator
Table 6. Input and Output Status in Battery Backup Mode
Signal Status
V
OUT
V
OUT
is connected to V
BATT
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 its 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.
CE
IN
CE
IN
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 V
OUT
. The input voltage does not affect supply current.
CE
OUT
Logic high. The open circuit voltage is equal to V
OUT
.
OSC IN OSC IN is ignored.
OSC SEL OSC SEL is ignored.