ADM696/ADM697
REV. 0
–7–
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
which senses either the unregulated dc input to the system’s 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.
ADM69x
POWER
FAIL
INPUT
R2
INPUT
POWER
1.3V
PFO
POWER
FAIL
OUTPUT
R1
Figure 7. Power Fail Comparator
Table II. Input and Output Status In Battery Backup Mode
Signal Status
V
OUT
(ADM696) 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 (ADM696) Logic high. The open circuit voltage
is equal to V
OUT
.
WDI WDI is ignored. It is internally disconnected
from the internal pullup 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 pullup and does not source or
sink current as long as its input voltage is be-
tween 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.
CE Gating and RAM Write Protection (ADM697)
The ADM697 contains memory protection circuitry which
ensures the integrity of data in memory by preventing write
operations when LL
IN
is below the threshold voltage. When
LL
IN
is greater than 1.3 V, CE
OUT
is a buffered replica of CE
IN
,
with a 5 ns propagation delay. When LL
IN
falls below the 1.3 V
threshold, an internal gate forces
CE
OUT
high, independent of
CE
IN
.
CE
OUT
typically drives the CE, CS, or Write input of battery
backed up CMOS RAM. This ensures the integrity of the data
in memory by preventing write operations when V
CC
is at an in-
valid level.
If the 5 ns typical propagation delay of
CE
OUT
is excessive, con-
nect
CE
IN
to GND and use the resulting CE
OUT
to control a
high speed external logic gate.
ADM697
CE
OUT
CE
IN
LL
IN
LOW = 0
LL
IN
OK = 1
Figure 5. Chip Enable Gating
t
1
t
1
= RESET TIME
V1 = RESET VOLTAGE THRESHOLD LOW
V2 = RESET VOLTAGE THRESHOLD HIGH
HYSTERESIS = V2–V1
V2 V2
V1 V1
t
1
LL
IN
LOW LINE
RESET
CE
IN
CE
OUT
Figure 6. Chip Enable Timing
Power Fail Warning Comparator
An additional comparator is provided for early warning of fail-
ure in the microprocessor’s power supply. The Power Fail Input
(PFI) is compared to an internal +1.3 V reference. The Power
