How to Select a Digital Isolator

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As digital isolators become more prevalent in industrial and automotive applications, designers are faced with a plethora of available options. How do they choose the right device for their system? Faced with these challenges, most digital isolators are designed with specific system requirements and applications in mind, forcing designers to sort through countless specifications and features to ensure that the device they choose can meet the system requirements. Selecting the wrong device can have a significant impact on the overall design of the system, resulting in a product that fails to meet regulatory requirements or fails to provide a reliable solution within budget.

Finding the right device doesn’t have to be difficult. This article will simplify your search by walking you through some key steps in selecting a digital isolator.

Step 1 : Understand your isolation requirements

The first step is to understand the isolation specification requirements of the system. Although the requirements can seem endless at times, there are some key factors that engineers can start with in the early stages of selection.

• Isolation Withstand Voltage ( V _ISO ): Is basic isolation and ≤3,000 V _RMS sufficient for your design? Or does the design require ≥5,000 V _RMS ? This specification is usually set by the regulatory requirements of the system and means the isolator can withstand voltage breakdown for at least 60 seconds.

• Working voltage ( V _IOWM ) : What is the constant voltage that the isolation barrier needs to withstand during the product's service life?

• Surge Isolation Level ( V _IOSM ): Is reinforced isolation required? An isolator capable of withstanding > 10 kV surge pulses is required.

• Creepage / Clearance : Is the creepage/clearance of a 4mm package sufficient, or does your system standard require an 8mm or higher specification?

• Common-mode transient immunity (CMTI): Will the isolator be used in noisy environments such as motor drives or solar inverters, where data integrity is critical and any bit error could result in a dangerous short-circuit event? If so, a high CMTI rating is critical for your digital isolator.

• Energy consumption: Is overall system power consumption a critical specification for your application (for example, a 4 to 20 mA loop-powered or battery-powered system)?

• Data Rate : What data rate do your communication interfaces require? Are you running low-speed UART speeds or a high-speed ≥100-Mbps data protocol?

Step 2 : Choose the right package

Once you have narrowed down the digital isolator specification requirements, the next step is to consider the different package options. The package can make a big difference in isolation, as the package size and features directly impact the high voltage performance of the device. Some of the same high voltage requirements discussed above (creepage, clearance, working voltage, surge voltage, isolation withstand voltage) may come into play when choosing the right package. Larger packages with greater creepage and clearance will allow for higher isolation voltage specifications. If using a smaller package option can also meet your system regulatory requirements, consider using this option to help save board space and cost. Also, consider how many isolation channels your communication interface requires, as higher channel counts will dictate which package type can be used.

Learn more about creepage and clearance and their impact on isolation in the TI Precision Labs video “1.6 TI Precision Labs – Isolation: What are creepage and clearance?”

Step 3 : Determine the number of channels and configuration

Once you have your specifications, requirements, and packaging in place, there are only a few more options to consider. First, determine how many isolation channels your signals require and in which direction each signal will be sent. This will help determine the number of channels required and the channel configuration. Next, consider the default output state (or fail-safe state) you prefer for your design. This determines which predefined state (high or low) the output pin will be in when the digital isolator's input channel is not powered or the pin is floating. Options may apply to both default high and low outputs.

Step 4 : Evaluate available equipment

If you are ready to put your digital isolator knowledge into practice, Figure 1 is a simple digital isolator selection flow chart that can help you determine which TI device is right for your design. Figure 2 helps you find the right package designator, and Figure 3 shows the scale of each package side by side.

Figure 1 : TI digital isolator selection flow chart

Use this list to learn more about the devices in the TI digital isolator selection flow chart:

• ISO78xx : Highest isolation-rated reinforced digital isolators with 8000-VPK isolation voltage in DW and DWW packages. Devices include the ISO7810.

• ISO77xx : Basic and reinforced digital isolators. Devices include the ISO7741.

• ISO70xx : Ultra-low power digital isolators. Devices include the ISO7021.

• ISO73xx : Low-power, low-jitter digital isolators. Devices include the ISO7341-Q1.

• ISO76xx : High-speed 150 Mbps isolation. Devices include the ISO7641FM.

• ISO67xx : Basic and reinforced isolation for cost-sensitive applications. Devices include ISO6741.

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Figure 2 : TI digital isolator package type selection flow chart

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Figure 3 : Comparison of available packaging options

w494143467

Isolators are used in recent projects. Let's take a look at them first. It is really a problem to select unclear components.

alan000345

w494143467 Published on 2020-10-22 14:59 Isolators have been used in recent projects. Let's take a look at them first. It is really a problem to select unclear components.

As long as it helps.