13

Figure 13. External Threshold Voltage Level Selection.

Electrical Considerations

The HCPL-0370/3700/3760

optocouplers 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 corresponding

typical value of RX can be ob-

tained from Figure 12. Specific

calculation of RX can be obtained

from Equation (1). Specification

of both V+ and V- voltage

threshold levels simultaneously

can be obtained by the use of RX

and RP as shown in Figure 13 and

determined by Equations (2) and

(3).

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-0370/3700/3760 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).

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 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.

For interfacing ac signals to TTL

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.

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-, RX can be determined

without use of RP via

V+ (--V) TH+(-)

RX =

(1)

I

TH+

(-)

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

V+ ≥ VTH+ and V+ - VTH+ < ITH+

V-

VTH-

V- - VTH- ITH-

Conversely, if the denominator of

equation (2) is negative, then

V+ ≤ VTH+ and V+ - VTH+ > ITH+

V-

VTH-

V- - VTH- ITH-

VTH- (V+) - VTH+ (V-)

RX =

(2)

ITH+ (VTH-) - ITH- (VTH+)

VTH- (V+) - VTH+ (V-)

RP =

(3)

ITH+(V--VTH-)+ITH-(VTH+-V+)

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Data subject to change.

Copyright © 2004 Agilent Technologies, Inc.

Obsoletes 5989-0785EN

December 20, 2004

5989-2101EN