12
Electrical Considerations
The ACPL-K370/K376 optocouplers have internally tem-
perature compensated, predictable voltage and current
threshold points. This allows a single external resistor, R
X
,
to determine larger external threshold voltage levels. For
a desired external threshold voltage, V
±
, the approximate
R
x
value is shown in Figure 12. Equation 1 can be used to
calculate Rx.
V
+
and V
–
voltage threshold levels can be simultaneously
set with two resistors, R
X
and R
P,
as shown in Figure 13 and
determined by Equations 4 and 5.
R
X
can provide over-current transient protection by
limiting input current during a transient condition. For
monitoring contacts of a relay or switch, the ACPL-K370/
K376 in combination with R
X
and R
P
can be used to allow a
specic current to be conducted through the contacts for
cleaning purposes (wetting current).
The choice of which input voltage clamp level to choose
depends upon the application of this device (see Figure 4).
It is recommended 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 with a static discharge), a series resistor, R
CC
,
should be connected in series with V
CC
and pin 8 to protect
the detector IC from destructive high surge currents. The
recommended value for R
CC
is 240 : per volt of allowable
drop in V
CC
(between Pin 8 and V
CC
) with a minimum value
of 240 :. In addition, it is recommended that a ceramic
disc bypass capacitor of 0.01 PF be placed between pins 5
and 8 to reduce the eect of power supply noise.
For interfacing ac signals to TTL systems, output low pass
ltering can be performed with a pull-up resistor of 1.5 k:
and 20 PF capacitor. This application requires a Schmitt
trigger gate to avoid slow rise time chatter problems.
For AC input applications, a lter capacitor can be placed
across the DC input terminals for either signal or transient
ltering.
Figure 13. External threshold voltage level selection.
Either AC (pins 1 and 4) or DC (pins 2 and 3) input can be
used to determine external threshold levels. For single
specically selected external threshold voltage level V
+
or
V
–
, R
X
can be determined without use of R
P
via:
V
+(–)
– V
TH+(–)
R
X
= Equation 1
I
TH+(–)
For dual specically selected external threshold voltage
levels, V
+
and V
–
, the use of R
X
and R
P
will permit this
selection. Two equations can be written:
V
TH+
V
+
= R
x
( I
TH+
+
) + V
TH+
Equation 2
R
P
V
TH–
V
–
= R
x
( I
TH–
+
) + V
TH–
Equation 3
R
P
Solving these equations for R
X
and R
P
yields the following
two expressions:
V
TH–
(V
+
) – V
TH+
(V
–
)
R
X
= Equation 4
I
TH+
(V
TH–
) – I
TH–
(V
TH+
)
V
TH–
(V
+
) – V
TH+
(V
–
)
R
P
= Equation 5
I
TH+
(V
–
– V
TH–
) + I
TH–
(V
TH+
– V
+
)
where
V
+
and V
–
are the desired external voltage threshold
levels, and values for V
TH±
and I
TH±
are found from the
data sheet.
Equations 4 and 5 are valid only if the conditions of
Equations 6 or 7 are met. With the V
TH±
and I
TH±
values,
the denominator of Equation 4 is checked to see if it is
positive or negative. If it is positive, then the following
ratios must be met:
V
+
V
TH+
V
+
– V
TH+
I
TH+
≥ and
<
Equation 6
V– V
TH–
V
–
– V
TH–
I
TH–
Conversely, if the denominator of Equation 4 is negative,
then the following ratios must hold:
V
+
V
TH+
V
+
– V
TH+
I
TH+
≤ and
>
Equation 7
V– V
TH–
V
–
– V
TH–
I
TH–
Refer to Application Note 1004 for more application infor-
mation and worked out examples.
ISOLATION
BARRIER
C
L
GND
R
X
1
2
3
4
8
7
6
5
AC1
DC+
DC–
AC2
V
CC
NC
V
O
GND
I
TH±
V
TH±
V
±
R
L
V
O
V
CC
R
P
