Datasheet

MAX791
Power-Fail Input
PFI is the input to the power-fail comparator. PFI has a
guaranteed input leakage of ±25nA max over tempera-
ture. The typical comparator delay is 15µs from V
IL
to
V
OL
(power failing), and 55µs from V
IH
to V
OH
(power
being restored). If unused, connect this input to
ground.
Power-Fail Output
The Power-Fail Output (PFO) goes low when PFI goes
below 1.25V. It typically sinks 3.2mA with a saturation
voltage of 0.1V. With PFI above 1.25V, PFO is actively
pulled to V
OUT
. Connecting PFI through a voltage-
divider to an unregulated supply allows PFO to gener-
ate an NMI as the unregulated power begins to fall
(Figure 9b). If the unregulated supply is inaccessible,
use LOWLINE to generate the NMI. The LOWLINE
threshold is typically 150mV above the reset threshold
(see LOWLINE Output section).
Battery-Backup Mode
The MAX791 requires two conditions to switch to bat-
tery-backup mode: 1) V
CC
must be below the reset
threshold; 2) V
CC
must be below VBATT. Table 1 lists
the status of the inputs and outputs in battery-backup
mode.
Microprocessor Supervisory Circuit
12 ______________________________________________________________________________________
Table 1. Input and Output States in
Battery-Backup Mode
* V
CC
must be below the reset threshold to enter battery-
backup mode.
Logic high. The open-circuit output
voltage is equal to V
OUT
.
–
W
—
D
—
P
—
O
–
16
Logic low*
–
R
—
E
—
S
—
E
—
T
–
15
Logic high. The open-circuit output
voltage is equal to V
OUT
.
–
W
—
D
—
O
–
14
High impedance
–
C
—
E
–
IN
13
Logic high. The open-circuit output
voltage is equal to V
OUT
.
–
C
—
E
–
OUT
12
WDI is ignored, and goes high
impedance.
WDI11
Logic low*
–
L
—
O
—
W
—
L
—
I
—
N
—
E
–
10
––
M
—
R
–
is ignored.
–
M
—
R
–
9
SWT is ignored.SWT8
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥ VBATT - 1.2V typ.
PFI7
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥ VBATT - 1.2V typ. Below this
voltage,
–
P
—
F
—
O
–
is forced low.
–
P
—
F
—
O
–
6
Logic high. The open-circuit output is
equal to V
OUT
.
BATT ON5
GND—0V reference for all signals.GND4
Battery-switchover comparator
monitors V
CC
for active switchover.
V
CC
3
V
OUT
is connected to VBATT through
an internal PMOS switch.
V
OUT
2
Supply current is 1µA maximum.VBATT1
STATUSNAMEPIN
MAX791
V
OUT
V
CC
FROM
REGULATED
SUPPLY
POWER TO
CMOS RAM
VBATT
RESET
LOWLINE
WDI
GND
RESET
NMI
I/O LINE
µP
µP POWER
2
0.1µF
0.1µF
3.0V
1
3
4
a)
b)
15
10
11
MAX791
V
OUT
V
CC
PFI
POWER TO
CMOS RAM
VBATT
RESET
PFO
WDI
GND
VOLTAGE
REGULATOR
RESET
NMI
I/O LINE
µP
µP POWER
2
0.1µF
0.1
µF
3.0V
1
3
7
4
15
6
11
Figure 9. a) If the unregulated supply is inaccessible,
LOWLINE generates the NMI for the µP. b) Use PFO to gener-
ate the µP NMI if the unregulated supply is inaccessible.