Digital Isolator Design Considerations

Many people have heard of digital isolators, so do you know its design considerations? Are you searching for more information about digital isolators? We are here to help. Based on feedback from the TI E2E™ community, we've compiled a list of the most frequently asked questions about digital isolator design challenges. Hopefully this list provides you with useful insights into isolating signals and power.

1. What is the simple difference between basic and enhanced digital isolators?

Basic digital isolators must pass a set of tests according to component-level standards, such as Deutsches Institut für Normung (DIN) V Verband der Elektrotechnik, Elektronik und Informationstechnik (VDE) V 0884-11. DIN V VDE V 0884-11 defines the voltage levels that an isolator can withstand, such as the maximum surge isolation voltage, VIOSM; the maximum transient isolation voltage, VOITM; and the maximum repetitive peak isolation voltage, VIORM (see the white paper "High Voltage Enhanced Isolation: Definition and Test Methods"). Reinforced digital isolators, in addition to passing these tests, must also pass a minimum surge voltage test level of 10,000 VPK.

2. Can different voltages be applied to both ends of the digital isolator?

Can. Digital isolators can supply power to both ends of the device under recommended operating conditions. Since the isolation barrier isolates the two ends, each end can be independently applied with any voltage value under the recommended operating conditions. For example, the ISO7721 can be supplied with 3.3 V VCC1 (between 2.25 V - 5.5 V) and 5 V VCC2 (also between 2.25 V - 5.5 V). In addition to building isolation, this approach allows you to use the digital isolator as a logic level converter. The two ends of the isolator are independent of each other.

3. Can a digital isolator signal voltage be different from its supply voltage?

cannot. The input/output signal voltage of a digital isolator depends on its supply voltage. Therefore, if you want to make a digital isolator compatible with the device it is connected to, it is best to keep the signal voltage the same as the isolator supply voltage. For example, if the supply voltage of the ISO7721 is 5 V and it is interfaced to a microcontroller (MCU), it is important that the MCU signals also operate at 5V logic levels.

4.What is the logic state of a digital isolator with no input signal?

If the input channel of a digital isolator has no voltage or the pin is left floating, its corresponding output pin is in a predefined state (called the default state or fault-safe state), which may be low or high, depending on for the selected device. The suffix "F" in the device part number indicates the default state of the output channels of this isolator. For example, if there is no F in ISO7721DWR, it means that the default state of the device is high. Similarly, if there is F in ISO7721FDWR, it means that the default state of the device is low.

5. Can the unused channel pins of the digital isolator be left floating?

cannot. Input pins of unused channels of a digital isolator can be left floating for testing purposes, but in an application, floating unused pins will result in reduced noise immunity of the product. Floating pins are more likely to pick up noise, especially when the system is undergoing electromagnetic compatibility (EMC)/immunity testing. To make the system immune to this noise, a best practice is to lock the channel inputs in their respective default logic states.

For example, for the ISO7721DWR, best practice is to connect unused signal input pins to its VCC through pull-up resistors (4.7-kΩ resistors are preferred). For the TI ISO7721FDWR, it is best to connect the unused signal input pins to its ground pin. For both devices, it is best to leave all unused channel output pins unconnected.

6. How to determine the power consumption of a digital isolator?

You can calculate the power consumption of a digital isolator based on the specifications listed on its data sheet. Find the supply current characteristics table corresponding to the input voltages (2.5 V, 3.3 V, and 5 V). In this table, find the data rate that is closest to your application's signal speed. The current consumption for this specific data rate will be listed in the datasheet as the current at each end of the isolation barrier (ICC1 and ICC2). Add these two current values ​​and you will get the total current draw of the device under operating conditions. Divide this total current consumption by the number of channels of the digital isolator to get the current consumption per channel. Some datasheets also provide the total supply current for each channel separately. For example, the ISO7041 datasheet shows a typical current consumption of 4.2 µA at total supply current per channel parameter, which is the sum of the ICC1(ch) and ICC2(ch) currents.

7. How to construct an isolated power supply for a digital isolator?

There are several options for constructing an isolated power supply for a digital isolator; the best solution will depend on the specific application needs.

One option is to use a transformer driver like TI's SN6501, which is available in a push-pull configuration with a secondary-side transformer and an optional rectifying low-dropout regulator. The SN6501 operates up to 1.5 W and can be used as an isolated power supply. This device is highly flexible and can be used in almost all applications. This is because the transformer and turns ratio provide the necessary isolation level and output voltage for the power supply. If you need to provide isolated power to other devices, you can use the SN6505x instead of the SN6501 for up to 5 W of output power. The SN6505 has additional protection features such as overload and short circuit, thermal shutdown, soft-start and slew rate control, allowing designers to build robust solutions.

Another option for space-constrained applications is the ISOW78xx family of devices, including the ISOW7841, which provide signal and power isolation in a small 16-pin plastic integrated circuit package. This combination takes up little space; requires no transformers and is easy to pass certification. When technology develops more rapidly, future isolators must be more user-friendly.