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 Labs’ white paper “CMOS Digital
Isolators Supersede Optocouplers in Industrial Applications”. 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
Optocoupler
NC
1
2
8
7
6
5
VDD
RL
ANODE
VL
CATHODE 3
NC
VO
4
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 receiver’s 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
Copyright © 2012 by Silicon Laboratories
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