Datasheet
8
FN3182.8
April 5, 2006
Either detector may be used alone, as well as both together,
in any of the circuits shown here.
When V
IN
is very close to one of the trip voltage, normal
variations and noise may cause it to wander back and forth
across this level, leading to erratic output ON and OFF
conditions. The addition of hysteresis, making the trip points
slightly different for rising and falling inputs, will avoid this
condition.
Threshold Detector with Hysteresis
Figure 10A shows how to set up such hysteresis, while
Figure 10B shows how the hysteresis around each trip point
produces switching action at different points depending on
whether V
IN
is rising or falling (the arrows indicated direction
of change. The HYST outputs are basically switches which
short out R
31
or R
32
when V
IN
is above the respective trip
point. Thus if the input voltage rises from a low value, the trip
point will be controlled by R
1N
, R
2N
, and R
3N
, until the trip
point is reached. As this value is passed, the detector
changes state, R
3N
is shorted out, and the trip point
becomes controlled by only R
1N
and R
2N
, a lower value.
The input will then have to fall to this new point to restore the
initial comparator state, but as soon as this occurs, the trip
point will be raised again.
An alternative circuit for obtaining hysteresis is shown in
Figure 11. In this configuration, the HYST pins put the extra
resistor in parallel with the upper setting resistor. The values
of the resistors differ, but the action is essentially the same.
The governing equations are given in Table 1. These ignore
the effects of the resistance of the HYST outputs, but these
can normally be neglected if the resistor values are above
about 100kΩ.
FIGURE 9A. CIRCUIT CONFIGURATION
FIGURE 9B. TRANSFER CHARACTERISTICS
FIGURE 9. SIMPLE THRESHOLD DETECTOR
OUT1
SET1 SET2
OUT2
V+
R
21
R
P2
V
IN
R
P1
R
22
R
11
R
12
OFF
V
OUT
ON
V
TR2
V
NOM
V
TR1
DETECTOR 2 DETECTOR 1
FIGURE 10A. CIRCUIT CONFIGURATION
FIGURE 10B. TRANSFER CHARACTERISTICS
FIGURE 10. THRESHOLD DETECTOR WITH HYSTERESIS
HYST1
SET1 SET2
HYST2
V+
V
IN
OUT1 OUT2
R
31
R
32
R
12
R
11
R
21
R
22
OVERVOLTAGE OVERVOLTAGE
V
L2
V
U2
V
L1
V
U1
ON
OUT
OFF
V
NOM
DETECTOR 2 DETECTOR 1
V
IN
V
TR2
V
SET2
R
12
R
22
+()
R
12
---------------------------------- 1.3
R
12
R
22
+()
R
12
----------------------------------
for detector 2==
ICL7665S