AN729 データシートの表示（PDF） - Silicon Laboratories

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AN729

REPLACING TRADITIONAL OPTOCOUPLERS WITH Si87XX DIGITAL ISOLATORS

Silicon Laboratories 

AN729

3. Applications

3.1. Replacing an Existing Optocoupler with the Si87xx

Use the Si87xx data sheet or online guide to select the correct, pin-compatible Si87xx product for the

application at hand.

Desolder the existing optocoupler from the board and discard it.

Solder the selected Si87xx device into the site previously occupied by the optocoupler.

Adjust the value of RF to achieve a maximum current of 3 mA for the Si87xx A-grade and Si87xx C-grade

devices or 6 mA for the B-Grade device (see Equation 1 below).

Connect power and verify proper system operation.

Note that the Si87xx is also compatible with typical external devices that improve optocoupler CMTI performance,

such as shorting switches, reverse diode clamps, and others. Components of this type can be left in place or

removed at the user's option.

3.2. Using the Si87xx in New Designs

Typically, the only calculated value is that of the current limiting resistor, RF. The first step is deciding if the

application benefits more from low anode current (A-Grade or C-Grade) or from high common-mode transient

immunity (B-Grade). Once this decision is made and the anode current threshold and optimum ON current values

are known, values for calculating RF (see Equation 1) are straightforward.

RF

=

V-----F----–-----2---.--0-

IF

Where:

RF is the value of the anode current limit resistor ( 

VF is the input-side forward voltage (V)

IF is the Anode forward input current (mA)

Equation 1.

Note that it is best to keep the values of both VF and RF low because this offers greater resistance against CMT

events. For example, a B-Grade Si87xx with VF = 5 V and RF = 833  exhibits higher CMTI than the same device

with VF = 10 V and RF = 1.6 k.

Rev. 0.1

3

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