ADM706R AD [Analog Devices], ADM706R Datasheet - Page 6

• Manufacturer Part Number: ADM706R
• Description: +3 V, Voltage Monitoring uP Supervisory Circuits
• Manufacturer: AD [Analog Devices]

ADM706P/R/S/T, ADM708R/S/T
Power-Fail Comparator
The power-fail comparator is an independent comparator which
may be used to monitor the input power supply. The compara-
tor’s inverting input is internally connected to a 1.25 V reference
voltage. The noninverting input is available at the PFI input.
This input may be used to monitor the input power supply via a
resistive divider network. When the voltage on the PFI input drops
below 1.25 V, the comparator output (PFO) goes low indicating
a power failure. For early warning of power failure the compara-
tor may be used to monitor the preregulator input simply by
choosing an appropriate resistive divider network. The PFO output
can be used to interrupt the processor so that a shutdown proce-
dure is implemented before the power is lost.
Adding Hysteresis to the Power-Fail Comparator
For increased noise immunity, hysteresis may be added to the
power-fail comparator. Since the comparator circuit is nonin-
verting, hysteresis can be added simply by connecting a resistor
between the PFO output and the PFI input as shown in Figure
6. When PFO is low, resistor R3 sinks current from the sum-
ming junction at the PFI pin. When PFO is high, resistor R3
sources current into the PFI summing junction. This results in
differing trip levels for the comparator. Further noise immunity
may be achieved by connecting a capacitor between PFI and GND.
Figure 6. Adding Hysteresis to the Power-Fail
Comparator
POWER
INPUT
V
POWER
INPUT
H
R2
Figure 5. Power-Fail Comparator
R1
1.25 1
POWER-FAIL
PFO
R1
R2
3.3V
ADM663A
0V
0V
INPUT
+3.3V
V
L
R2
R2
PFI
PFI
1.25V
1.25V
V
IN
V
ADM70x
ADM70x
H
V
R3
R3
CC
R3
R1
PFO
PFO
TO P NMI
POWER-FAIL
OUTPUT
–6–
Valid RESET Below 1 V V
The ADM70x family of products are guaranteed to provide a
valid reset level with V
cal Performance Characteristics. As V
internal transistor will not have sufficient drive to hold it ON so
the voltage on RESET will no longer be held at 0 V. A pull-
down resistor as shown in Figure 7 may be connected externally
to hold the line low if it is required.
Typical Performance Characteristics
Figure 8. ADM706/ADM708 RESET Output Voltage vs.
Supply Voltage
Figure 9. RESET Output Voltage vs. Supply Voltage
RESET
RESET
V
V
V
CC
CC
L
Figure 7. RESET Valid Below 1 V
1.25
V
MID
CC
R1
ADM70x
1.25
as low as 1 V. Please refer to the Typi-
GND
1.25
CC
RESET
R2
400ms/DIV
400ms/DIV
R1 R2
–
R2
V
CC
CC
R1
R3
– 1.25
drops below 1 V, the
REV. A