Considerations for Selecting Digital Isolators
Luke Trowbridge, Product Marketing Engineer, Isolation, Interface Group

Introduction
Isolation is a means of preventing DC and uncontrolled
AC currents between two parts of a system, whileallowing signal and power transfer between those twoparts. This isolation can be required in order to protect
human operators and prevent damage to expensive
processors in high voltage systems, break groundloops in communication networks, and to communicate
to high-side devices. Digital isolators are a common
choice when looking to achieve galvanic isolation forinterfaces such as SPI, UART, I2C, RS-485, and RS232 in many different system applications, including
industrial automation systems, motor drives, medical
equipment, solar inverters, power supplies, and hybrid
electric vehicles. This application brief identifies key
considerations when selecting the right digital isolator
for a given application and provides a guide forunderstanding the different choices from Texas
Instruments (TI) broad portfolio of digital isolators.
TI Isolation TechnologyTI isolators use silicon-dioxide (SiO2) based, highvoltage capacitors to serve as the signal insulation anddielectric in digital isolators. The digital isolator productfamilies later discussed use two thick SiO2 capacitors
in series - one on each side of the isolation barrier toachieve high-voltage isolation. A graphicalrepresentation of this barrier technique can be seen inFigure 1. Compared to inductor based (polyimide)
insulators and traditional optocouplers, SiO2 provides
the highest dielectric strength, does not degrade with
exposure to ambient moisture, and can offer an
isolation barrier lifetime >100 years. For a deeper
explanation of TI's isolation technology refer toEnabling high voltage signal isolation quality andreliability.
VCCI

Isolation
Capacitors

VCCO

INx

High-voltage isolation performance of a digital isolator
is quantified at the component level by parameterssuch as maximum transient isolation voltage (VIOTM),
isolation withstand voltage (VISO), maximum surge
isolation voltage (VIOSM), maximum repetitive peakvoltage (VIORM), working voltage (VIOWM), and commonmode transient immunity (CMTI). These parameters
represent a digital isolator's capability to handle highvoltage stresses of different magnitude and transient
profiles and are key to selecting the right digitalisolator for specific system requirements.
• Maximum transient isolation voltage (VIOTM):
Defined by IEC 60747-5-5 and VDE 0884-11 as thepeak transient voltage that the isolator can handle
for up to 60 seconds without breaking down. Arcing
or load changes on a system power supply cancause disturbances where the voltage could briefly
become several times that of the line voltage. An
isolator must be able to handle these over voltages
without damage.
• Isolation withstand voltage (VISO): Similar to theVIOTM, isolation withstand voltage is defined per UL
1577 as the root mean square (rms) value ofvoltage that the isolator can handle without
breakdown for 60 seconds. The difference is thevalue is given in an rms instead of a peak voltage.
• Maximum surge isolation voltage (VIOSM):
Quantifies the ability of the isolator to withstandvery high voltage impulses of a certain transient
profile. This waveform is shown in Figure 2. Thisparameter represents direct and indirect lighting
strikes. As per IEC 60747-5-5 and VDE 0884-11,
an isolator claiming a certain VIOSM must pass thesurge test at a peak voltage of 1.3 times VIOSM forbasic isolation, and 1.6 times VIOSM for reinforced
isolation. A digital isolator can be called reinforcedat the component level, only if it passes the surge
test at a level greater than 10kV.

OUTx

GNDI

GNDO

Figure 1. Series Capacitor Isolation
Key Isolation Specifications
Before choosing the right digital isolator, it is importantfor designers to know the isolation specification
requirements for their given system application. Once
this is known, how does a designer know how muchprotection a device can provide, and what are themaximum voltages an isolator can withstand?
SLLA426 – July 2018
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Figure 2. Surge Transient Waveform

Considerations for Selecting Digital Isolators Luke Trowbridge, Product Marketing Engineer,
Copyright © 2018, Texas Instruments Incorporated

Isolation, Interface Group1

www.ti.com•••

Maximum repetitive peak voltage (VIORM):
Defined in IEC 60747-5-5 and VDE 0884-11 as themaximum repetitive peak voltage that the isolator
can withstand. This specification is intended toqualify the ability of an isolator to handle highvoltage across its barrier on a continuous, day-today basis.
Working Voltage (VIOWM): Similar to the VIORM,
working voltage is the maximum rms, or equivalent
dc voltage, that the isolator can withstand over a
specified long lifetime. Again, the difference is thevalue is given in an rms instead of a peak voltage.
CMTI: Common-mode transient immunity is theability of an isolator to tolerate high-slew-rate
voltage transients between its two grounds without
corrupting signals passing through it, which could
potentially cause bit errors. In some applications,these bit errors caused by the transients can resultin dangerous short-circuit events. Higher CMTI
indicates a more robust isolation channel.

Packages also dictate the isolation channel counts andsolution size of a digital isolator. Drawn to scale
images of TI's digital isolator package options can beseen in Figure 3 and the measurements, creepage,and channel counts of each package can be found in
Table 1.

Figure 3. Packages
Table 1. Package Properties
Package

Identifier

Length (mm)Width(mm)Creepage(mm)Channels

8-pin SOIC

D

4.90

3.91

41,216-pin SSOP

DBQ

4.90

3.90

3.7

3,4,68-pin SOIC

DWV

5.85

7.50

8.5

216-pin SOIC

DW

10.30

7.50

81,2,3,4,616-pin SOIC

DWW

10.30

14.00

14.5

1,2,3,4

Additional explanation on each of these isolation
parameters can be found in High-voltage reinforced
isolation: Definitions and test methodologies. The
isolation certifications that have been mentioned
ensure that your applications meet worldwide industry
standards. Use these tables to check which TI devices
meet each certification requirement.

Since the package of a digital isolator has a direct
effect on the isolation performance of an isolator,
packaging specifications must be considered whenselecting the right device for a given application. The
isolation capabilities of each product family andpackage can be found in Table 3.

Package Options

Conclusion

Creepage and clearance are the distance along thesurface of the package and through the air between
pins on one side of the isolator to the pins on the otherside. This distance is mandated by system levelstandards based on parameters such as isolation
voltage requirements, material group of the isolator's
package mold compound, comparative tracking index(CTI) and altitude. CTI indicates the ability of thepackage mold compound to handle steady highvoltage without surface degradation. A higher CTI
allows the use of smaller packages for the sameworking voltage. TI offers package options withcreepage distances of up to 14.5-mm and the isolator
families shown in Table 3 both guarantee a CTI of>600 V.

This application brief serves as an introduction tosome of the key considerations important to selecting
and narrowing down TI's portfolio of digital isolators foruse in industrial and automotive designs. These
considerations are also useful when evaluating TI'sadditional portfolio of isolated interface devices. For a
deeper explanation and analysis of the topics covered,consider the related technical documents in Table 2.
Video series covering both these and similar isolation
topics can be found on TI's isolation overview pageand also in the Precision Labs training center.
Table 2. Related Technical Documents
Literature Number

Document Title

SSZY028

Enabling high voltage signal isolation quality and reliability

SLYY063

High-voltage reinforced isolation: Definitions and test
methodologies

SLLA284A

Digital Isolator Design Guide

SLYT649

Pushing the envelope with high-performance, digital-isolation
technology

Table 3. Digital Isolator Families

2Device
Family

VIOTM (VPK)

VISO (VRMS)

VIOSM (VPK)

ISO77xx4242, 7071,
8000

3000, 5000

4000, 5000,
8000

ISO78xx

8000

5700

8000

VIORM (VPK)

VIOWM (VRMS)

566, 637, 1414 400, 450, 1000
2121, 2828

1500, 2000

Considerations for Selecting Digital Isolators Luke Trowbridge, Product Marketing Engineer,
Isolation, Interface GroupCopyright © 2018, Texas Instruments Incorporated

Min CMTI(kV/µs)

CTI (V)

PackageOptions

85>600

DBQ, D,DWV, DW

100>600

DW, DWW

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