MAX791 Maxim, MAX791 Datasheet - Page 12

• Manufacturer Part Number: MAX791
• Description: Microprocessor Supervisory Circuit
• Manufacturer: Maxim

The power-fail comparator is an uncommitted compara-
tor that has no effect on the other functions of the IC.
Common uses include monitoring supplies other than
5V (see the Typical Operating Circuit and the
Monitoring a Negative Voltage section) and early
power-fail detection when the unregulated power is
easily accessible (Figure 9b).
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
V
being restored). If unused, connect this input to
ground.
Microprocessor Supervisory Circuit
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.
12
OL
REGULATED
a)
b)
SUPPLY
FROM
(power failing), and 55µs from V
______________________________________________________________________________________
REGULATOR
0.1 F
VOLTAGE
3
V
CC
0.1
3
7
F
MAX791
4
GND
V
PFI
LOWLINE
CC
VBATT
RESET
MAX791
V
4
WDI
GND
OUT
Power-Fail Comparator
VBATT
RESET
2
1
15
10
11
V
PFO
WDI
OUT
2
1
15
6
11
3.0V
0.1 F
3.0V
0.1 F
IH
Power-Fail Input
RESET
NMI
I/O LINE
POWER TO
CMOS RAM
to V
P POWER
P
RESET
NMI
I/O LINE
POWER TO
CMOS RAM
OH
P POWER
P
(power
IL
to
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
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).
Table 1. Input and Output States in
Battery-Backup Mode
* V
backup mode.
PIN
10
11
12
13
14
15
16
CC
1
2
3
4
5
6
7
8
9
must be below the reset threshold to enter battery-
–
—
L
BATT ON
–
C
O
—
VBATT
–
R
NAME
–
W
—
—
–
C
V
–
W
GND
E
—
—
–
P
SWT
–
OUT
V
WDI
—
W
E
–
M
—
—
PFI
D
OUT
E
—
–
F
—
—
CC
—
D
OUT
S
—
—
L
R
—
P
O
–
—
IN
—
O
E
–
—
I
–
O
N
–
T
–
—
. Connecting PFI through a voltage
E
–
Supply current is 1µA maximum.
V
an internal PMOS switch.
Battery-switchover comparator
monitors V
GND—0V reference for all signals.
Logic high. The open-circuit output is
equal to V
The power-fail comparator remains
active in the battery-backup mode for
V
voltage,
The power-fail comparator remains
active in the battery-backup mode for
V
SWT is ignored.
– –
M
Logic low*
WDI is ignored, and goes high
impedance.
Logic high. The open-circuit output
voltage is equal to V
High impedance
Logic high. The open-circuit output
voltage is equal to V
Logic low*
Logic high. The open-circuit output
voltage is equal to V
—
OUT
CC
CC
R
–
is ignored.
≥ VBATT - 1.2V typ. Below this
≥ VBATT - 1.2V typ.
is connected to VBATT through
–
P
—
OUT
—
F
CC
O
–
is forced low.
for active switchover.
.
STATUS
Power-Fail Output
OUT
OUT
OUT
.
.
.