AN681 Datasheet, PDF(1/10 Page) Silicon Laboratories – USING THE Si87XX FAMILY OF DIGITAL ISOLATORS

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AN681 Datasheet, PDF (1/10 Pages) Silicon Laboratories – USING THE Si87XX FAMILY OF DIGITAL ISOLATORS

AN681

USING THE Si87XX FAMILY OF DIGITAL ISOLATORS

1. Introduction

Optocouplers provide both galvanic signal isolation and output level shifting in a single package but are notorious

for their long propagation times, poor common-mode transient immunity (CMTI), and performance degradation with

temperature and device age. Modern isolation devices fabricated in CMOS process technology offer higher

performance and reliability with none of the downside issues of optocouplers. The Silicon Labs Si87xx family of

digital isolators can directly replace optocouplers while providing substantial gains in performance and reliability.

This application note describes how to correctly apply the Si87xx. (For more information about Silicon Labs CMOS

isolation technology and comparisons with optocouplers, please see Silicon Labsâ white paper âCMOS Digital

Isolators Supersede Optocouplers in Industrial Applicationsâ. For more information on the Si87xx isolator family,

see the Si87xx product data sheet. Both publications are available for download at www.silabs.com/isolation.)

2. Device Overview

NC 1

ANODE 2

Optocoupler

8 VDD

7 VL

CATHODE 3

6 VO

NC 4

5 GND

Figure 1. Optocoupler Block Diagram

The optocoupler of Figure 1 consists of an input side LED with a transparent shield (to reduce capacitive coupling

for higher CMTI), an optical receiver, and an open-collector output. The LED emits light when sufficient current

flows from anode to cathode. Emitted light passing through the transparent shield strikes the receiverâs photo

diode, initiating bias current flow from VDD and causing the open-collector output to go low. Absence of current

through the input side LED causes the open-collector output to remain high.

As shown in Figure 2, anode-cathode voltage VF provides Si87xx input-side bias voltage. The diode emulator

circuit provides two functions: First, it mimics the input behavior of an optocoupler LED to ensure compatibility with

external drive circuits, and, second, it enables the transmitter when forward diode current (IF) is at or above its

threshold level. When enabled, the transmitter sends a continuous carrier to the receiver, which forces the output

(VO) low when sufficient in-band receiver carrier energy is detected. Conversely, the receiver drives VO high when

IF is below its minimum threshold level.

Rev. 0.3 8/12

AN681

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