MAX807N Maxim Integrated Products, MAX807N Datasheet - Page 12

• Manufacturer Part Number: MAX807N
• Description: Full-Featured P Supervisory Circuit with 1.5eset Accuracy
• Manufacturer: Maxim Integrated Products

Full-Featured µP Supervisory Circuit with
±1.5% Reset Accuracy
Figure 10. Using the Power-Fail Comparator to Monitor an Additional Power Supply: a) V
Figure 11. a) If the preregulated supply is inaccessible, LOW
LINE generates the NMI for the µP. b) Use PFO to generate the
µP NMI if the preregulated supply is accessible.
12
REGULATED
a)
a)
b)
SUPPLY
______________________________________________________________________________________
REGULATOR
FROM
VOLTAGE
0.1 F
PFO
V
V
TRIP
CC
V
L
= R2
= R2
R1
R2
V
(V
(V
V
IN
PFT
CC
0.1 F
PFT
MAX807
)
+ V
(
GND
R1
PFH
1
LOW LINE
PFI
V
CC
+
)
RESET
MAX807
BATT
(
OUT
WDI
1
R2
GND
1
R1
)
+
–
PFI
RESET
1
R2
V
BATT
R1
OUT
PFO
WDI
V
CC
L
)
–
MAX807
2.8V
V
0.1 F
CC
R1
V
V
GND
TRIP
CC
2.8V
0.1 F
RESET
NMI
I/O LINE
POWER TO
CMOS RAM
WHERE
NOTE: V
P POWER
PFO
0V
P
TRIP,
V
V
RESET
NMI
I/O LINE
POWER TO
CMOS RAM
PFT
PFH
P POWER
V
= 2.265V
= 20mV
L
ARE NEGATIVE
P
V
IN
PFI is the noninverting input to an uncommitted com-
parator. If PFI is less than V
The power-fail comparator is intended to monitor the
preregulated input of the power supply, providing an
early power-fail warning so software can conduct an
orderly shutdown. It can also be used to monitor sup-
plies other than 5V. Set the power-fail threshold with a
resistor divider, as shown in Figure 10.
PFI is the input to the power-fail comparator. The typical
comparator delay is 14µs from V
and 32µs from V
unused, connect this input to ground.
The Power-Fail Output (PFO) goes low when PFI goes
below V
voltage of 0.1V. With PFI above V
pulled to V
to a preregulated supply allows PFO to generate an
NMI as the preregulated power begins to fall (Figure
11b). If the preregulated supply is inaccessible, use
LOW LINE to generate the NMI (Figure 11a). The LOW
LINE threshold is typically 52mV above the reset
threshold (see Low-Line Comparator section).
b)
PFO
V
CC
V
PFT
V
TRIP
H
= (V
CC
R1
R2
. It typically sinks 3.2mA with a saturation
=
PFT
. Connecting PFI through a voltage divider
V
V
PFT
IN
+ V
(
IN
PFH
R1
IH
is Negative, b) V
R2
+ R2
)
to V
(
R1
)
R2
+ R2
OH
V
Power-Fail Comparator
PFI
TRIP
)
PFT
(power being restored). If
V
(2.265V), PFO goes low.
MAX807
IL
H
V
GND
IN
to V
CC
PFT
is Positive
Power-Fail Output
OL
Power-Fail Input
, PFO is actively
(power failing),
PFO
MR
V
IN