Datasheet
9
FN3182.8
April 5, 2006
Applications
Single Supply Fault Monitor
Figure 12 shows an over/under voltage fault monitor for a
single supply. The overvoltage trip point is centered around
5.5V and the undervoltage trip point is centered around 4.5V.
Both have some hysteresis to prevent erratic output ON and
OFF conditions. The two outputs are connected in a wired
OR configuration with a pullup resistor to generate a power
OK signal.
Multiple Supply Fault Monitor
The ICL7665S can simultaneously monitor several supplies
when connected as shown in Figure 13. The resistors are
chosen such that the sum of the currents through R
21A
,
R
21B
, and R
31
is equal to the current through R
11
when the
two input voltage are at the desired low voltage detection
point. The current through R
11
at this point is equal to
1.3V/R
11
. The voltage at the V
SET
input depends on the
voltage of both supplies being monitored. The trip voltage of
one supply while the other supply is at the nominal voltage
will be different that the trip voltage when both supplies are
below their nominal voltages.
The other side of the ICL7665S can be used to detect the
absence of negative supplies. The trip points for OUT1
depend on both the negative supply voltages and the actual
voltage of the +5V supply.
TABLE 1. SET-POINT EQUATIONS
NO HYSTERESIS
Overvoltage V
TRIP
=
R
11
+ R
21
R
11
x V
SET1
Overvoltage V
TRIP
=
R
12
+ R
22
R
12
x V
SET2
HYSTERESIS PER FIGURE 10A
V
U1
=
R
11
+ R
21
+ R31
R
11
x V
SET1
Overvoltage V
TRIP
V
L1
=
R
11
+ R
21
R
11
x V
SET1
V
U2
=
R
12
+ R
22
+ R
32
R
12
x V
SET2
Undervoltage V
TRIP
V
L2
=
R
12
+ R
22
R
12
x V
SET2
HYSTERESIS PER FIGURE 11
V
U1
=
R
11
+ R
21
R
11
x V
SET1
Overvoltage V
TRIP
V
L1
=
R
11
+
R
21
R
31
R
21
+ R
31
x V
SET1
R
11
V
U2
=
R
12
+ R
22
R
12
x V
SET2
Overvoltage V
TRIP
V
L2
=
R
12
+
R
22
R
32
R
22
+ R
32
x V
SET2
R
12
OUT1
SET1 SET2
OUT2
V+
R
21
R
P
V
IN
R
P
R
22
R
11
R
12
HYST1 HYST2
R
31
R
32
FIGURE 11. AN ALTERNATIVE HYSTERESIS CIRCUIT
HYST1
V
SET1
V
SET2
HYST2
V+
OUT1 OUT2
324kΩ
13MΩ
5%
100kΩ
249kΩ
7.5MΩ
5%
+5V SUPPLY
POWER
OK
100kΩ
1MΩ
V+
OPEN VOLTAGE
DETECTOR
V
U
= 5.55V
V
L
= 5.45V
OPEN VOLTAGE
DETECTOR
V
U
= 4.55V
V
L
= 4.45V
R
22
R
32
R
12
R
11
R
31
R
21
FIGURE 12. FAULT MONITOR FOR A SINGLE SUPPLY
ICL7665S