ADM706SAN 데이터 시트보기 (PDF) - Analog Devices
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ADM706SAN Datasheet PDF : 16 Pages
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ADM706P/ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/ADM708T
POWER-FAIL COMPARATOR
The power-fail comparator is an independent comparator that
can be used to monitor the input power supply. The inverting
input of the comparator is internally connected to a 1.25 V
reference voltage. The noninverting input is available at the PFI
input. This input is 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 comparator is used to monitor the preregulator input simply
by choosing an appropriate resistive divider network. The PFO
output is used to interrupt the processor so that a shutdown
procedure is implemented before the power is lost.
INPUT
POWER
R1
1.25V
PFO
POWER-FAIL PFI
R2
INPUT ADM706P/ADM706R/
ADM706S/ADM706T/
ADM708R/ADM708S/
ADM708T
POWER-FAIL
OUTPUT
Figure 17. Power-Fail Comparator
ADDING HYSTERESIS TO THE POWER-FAIL
COMPARATOR
For increased noise immunity, hysteresis can be added to the
power-fail comparator. Because the comparator circuit is
noninverting, hysteresis is added simply by connecting a resistor
between the PFO output and the PFI input as shown in Figure 18.
When PFO is low, Resistor R3 sinks current from the summing
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 is achieved
by connecting a capacitor between PFI and GND.
INPUT
POWER
R1
R2
3.3V
ADM663A
VCC
1.25V –
PFO
+
PFI
ADM706P/ADM706R/
ADM706S/ADM706T/
ADM708R/ADM708S/
ADM708T
R3
TO MICROPROCESSOR NMI
3.3V
PFO
0V
0V VL
VH
VIN
Figure 18. Adding Hysteresis to the Power-Fail Comparator
VH
= 1.25
⎡
⎢1
⎢⎣
+
⎜⎛
⎜⎝
R2 +
R2 ×
R3 ⎟⎞
R3 ⎟⎠
⎤
R1⎥
⎥⎦
VL
=
1.25
+
R1⎜⎛ 1.25
⎝ R2
−
VCC
− 1.25
R3
⎟⎞
⎠
VMID
=
1.25
⎜⎛
⎝
R1 + R2
R2
⎟⎞
⎠
VALID RESET BELOW 1 V VCC
The ADM706R/ADM706S/ADM706T, ADM708R/ADM708S/
ADM708T are guaranteed to provide a valid reset level with VCC
as low as 1 V. Refer to the Typical Performance Characteristics
section. As VCC drops below 1 V, the internal transistor does not
have sufficient drive to hold it on so the voltage on RESET is no
longer held at 0 V. A pull-down resistor, as shown in Figure 19, can
be connected externally to hold the line low if it is required.
ADM706R/ADM706S/
ADM706T/ADM708R/
ADM708S/ADM708T
RESET
GND
R1
Figure 19. RESET Valid Below 1 V
Rev. C | Page 11 of 16
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