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Data subject to change. Copyright © 2005-2009 Avago Technologies. All rights reserved. Obsoletes AV01-0542EN
AV02-2107EN - September 2, 2009
Electrical Considerations
The HCPL-0370/3700/3760 optocoup lers have internal
temperature compen sated, predictable volt age and cur-
rent threshold points which allow selection of an exter-
nal resistor, R
X
, to determine larger external threshold
voltage levels. For a desired external threshold voltage,
V
±
, a corre sponding typical value of R
X
can be ob tained
from Figure 12. Specific calculation of R
X
can be obtained
from Equation (1). Specification of both V
+
and V
-
voltage
threshold levels simul taneously can be obtained by the
use of R
X
and R
P
as shown in Figure 13 and determined by
Equations (2) and (3).
R
X
can provide over-current transient protection by
limiting input current during a transient condition.
For monitoring con tacts of a relay or switch, the HCPL-
0370/3700/3760 in combina tion with R
X
and R
P
can be
used to allow a specific current to be conducted through
the contacts for clean ing purposes (wetting current).
The choice of which input voltage clamp level to choose
depends upon the application of this device (see Figure
1). It is recom mended that the low clamp condition be
used when possible.
The low clamp condition in conjunction with the low
input current feature will ensure extremely low input
power dissipation.
In applications where dV
CM
/dt may be extremely large
(such as static discharge), a series resis tor, R
CC
, should
be con nected in series with V
CC
and Pin 8 to pro tect the
detector IC from destruc tively high surge currents. See
Note 13 for deter mination of R
CC
. In addi tion, it is recom-
mended that a ceramic disc by pass capacitor of 0.01 µF
be placed between Pins 8 and 5 to reduce the effect of
power supply noise.
V
+
- V
TH+
(-) (-)
R
X
= (1)
I
TH+
(-)
For two specifically selected external threshold voltage
levels, V
+
and V
-
, the use of R
X
and R
P
will permit this selec-
tion via equations (2), (3) provided the following condi-
tions are met. If the denominator of equation (2) is posi-
tive, then
V
+
V
TH+
V
+
- V
TH+
I
TH+
≥ and <
V
-
V
TH-
V
-
- V
TH-
I
TH-
Conversely, if the denominator of
equation (2) is negative, then
V
+
V
TH+
V
+
- V
TH+
I
TH+
≤ and >
V
-
V
TH-
V
-
- V
TH-
I
TH-
V
TH-
(V
+
) - V
TH+
(V
-
)
R
X
= (2)
I
TH+
(V
TH-
) - I
TH-
(V
TH+
)
V
TH-
(V
+
) - V
TH+
(V
-
)
R
P
= (3)
I
TH+
(V
-
-V
TH-
)+I
TH-
(V
TH+
-V
+
)
Figure 13. External threshold voltage level selection.
For interfacing ac signals to TTL systems, output low
pass filtering can be performed with a pullup resistor of
1.5 kW and 20 µF capacitor. This application requires a
Schmitt trigger gate to avoid slow rise time chatter prob-
lems. For ac input applica tions, a filter capacitor can be
placed across the dc input terminals for either signal or
transient filtering.
Either ac (Pins 1, 4) or dc (Pins 2, 3) input can be used to
determine external threshold levels.
For one specifically selected external threshold voltage
level V
+
or V
-
, R
X
can be determined without use of R
P
via
