Figure 13. External Threshold Voltage Level Selection.
Electrical Considerations
The HCPL-3700/3760 optocoup-
lers have internal temperature
compensated, predictable voltage
and current threshold points
which allow selection of an
external resistor, RX, to determine
larger external threshold voltage
levels. For a desired external
threshold voltage, V± , a corre-
sponding typical value of RX can
be obtained from Figure 12.
The low clamp condition in
For one specifically selected
external threshold voltage level
conjunction with the low input
current feature will ensure
extremely low input power
dissipation.
V+ or V, RX can be determined
-
without use of RP via
V - V
+ (-) TH+(-)
In applications where dVCM/dt
may be extremely large (such as
static discharge), a series resistor,
RCC, should be connected in
series with VCC and Pin 8 to pro-
tect the detector IC from destruc-
tively high surge currents. See
Note 13 for determination of RCC
In addition, it is recommended
that a ceramic disc bypass
capacitor of 0.01 µF be placed
between Pins 8 and 5 to reduce
the effect of power supply noise.
RX =
(1)
ITH+
(-)
For two specifically selected
external threshold voltage levels,
V+ and V-, the use of RX and RP
will permit this selection via
equations (2), (3) provided the
following conditions are met. If
the denominator of equation (2)
is positive, then
Specific calculation of RX can be
obtained from Equation (1).
.
Specification of both V+ and V
-
voltage threshold levels simul-
taneously can be obtained by the
use of RX and RP as shown in
Figure 13 and determined by
Equations (2) and (3).
V+
V-
VTH+
VTH-
V+ - VTH+ ITH+
and <
≥
For interfacing ac signals to TTL
V- - VTH-
ITH-
systems, output low pass filtering
can be performed with a pullup
resistor of 1.5 kΩ and 20 µF
capacitor. This application
requires a Schmitt trigger gate to
avoid slow rise time chatter
problems. For ac input applica-
tions, a filter capacitor can be
placed across the dc input
terminals for either signal or
transient filtering.
RX 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-3700/3760 in combination
with RX and RP can be used to
allow a specific current to be
conducted through the contacts
for cleaning purposes (wetting
current).
Conversely, if the denominator of
equation (2) is negative, then
V+
V-
VTH+
VTH-
V+ - VTH+ ITH+
and >
≤
V- - VTH-
ITH-
VTH- (V+) - VTH+ (V-)
RX =
(2)
ITH+ (VTH-) - ITH- (VTH+
)
Either ac (Pins 1, 4) or dc
(Pins 2, 3) input can be used to
determine external threshold
levels.
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
VTH- (V+) - VTH+ (V-)
RP =
(3)
ITH+(V--VTH-)+ITH-(VTH+-V+)
condition be used when possible.
1-360
HP [ HEWLETT-PACKARD ]
