Microprocessor Supervisory Circuit
MAX791
Table 1. Input and Output States in
Battery-Backup Mode
PIN
1
3.0V
15
RESET
LOWLINE 10
11
WDI
GND
4
µP
FROM
REGULATED
SUPPLY
0.1µF
3 V
CC
V
OUT
2
0.1µF
MAX791
VBATT 1
µP
POWER
POWER TO
CMOS RAM
NAME
VBATT
V
OUT
V
CC
GND
BATT ON
STATUS
Supply current is 1µA maximum.
V
OUT
is connected to VBATT through
an internal PMOS switch.
Battery-switchover comparator
monitors V
CC
for active switchover.
GND—0V reference for all signals.
Logic high. The open-circuit output is
equal to V
OUT
.
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥
VBATT - 1.2V typ. Below this
–—
—–
voltage, PFO is forced low.
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥
VBATT - 1.2V typ.
SWT is ignored.
–—
–
MR is ignored.
Logic low*
WDI is ignored, and goes high
impedance.
Logic high. The open-circuit output
voltage is equal to V
OUT
.
High impedance
Logic high. The open-circuit output
voltage is equal to V
OUT
.
Logic low*
Logic high. The open-circuit output
voltage is equal to V
OUT
.
2
3
4
5
RESET
NMI
I/O LINE
a)
VOLTAGE
REGULATOR
3 V
CC
0.1
µF
V
OUT
2
0.1µF
µP
POWER
POWER TO
CMOS RAM
6
–—
—–
PFO
MAX791
VBATT 1
3.0V
15
RESET
PFO 6
11
WDI
GND
4
µP
7
8
PFI
SWT
–—
–
MR
– ——— –
— ——
LOWLINE
WDI
––
—
CE OUT
––
—
CE IN
–— –
—
WDO
–——
— —–
RESET
–— —
—–
WDPO
7
PFI
RESET
NMI
I/O LINE
9
10
11
12
13
b)
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.
14
15
Power-Fail Comparator
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).
16
* V
CC
must be below the reset threshold to enter battery-
backup mode.
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.
12
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).
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