e02ch12.pdf

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012
Triple Voltage Monitor
with ‘Power Good’ Signal
+V
In order to monitor multiple power supply voltages it is possible
to use one comparator per voltage, coupled with some logic
to combine the output signals and, for example, generate a
reset signal for a microprocessor.
Things are simpler with the LTC1727 from Linear Technology
( www.linear-tech.com/pdf/17278fa.pdf ). This IC is available in
different voltage variants. The threshold voltages are as fol-
lows:
CC, Logic
1
8
+V
IN1
COMP1
IC1
CC1
2
7
+V
IN2
COMP2
Power Good
CC2
R1
3
5
+V
Logic
Type
Vcc1
Vcc2
VccA
IN3
COMP3
CC3
LTC1727-X
R2
LTC 1727-2.5
3.08 V
2.34 V
1.0 V
6
RST
RESET
LTC 1727-5
3.08 V
4.67 V
1.0 V
4
024098 - 11
As long as one of th e thre e voltages is below the threshold
value, the open drain RST output remains at ground potential
and thereby holds the logic connected to it in a reset state. If
all three voltages are above the threshold, RST will remain low
for a further 200 ms, in order to ensure that the system is
properly reset. A clear reset signal is then generated as long as
one of the two fixed supply voltages is at least 1 V.
On the LTC1727 the individual comparator outputs are brought
out to separate pins in the form of open drain outputs COMP1,
COMP2 and COMPA. They can be connected together and
then, with the aid of an external pull-up resistor, create a
‘power good’ signal which goes high to indicate when all the
voltages are above their threshold values.
(024098-1)
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