Are you searching for more information about digital isolators? We will help you. Based on feedback from the TI E2ETM community, we collected and compiled a list of the most common questions about digital isolator design. I hope this list will provide you with useful insights into isolated signals and power supplies.
1. What is the 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 the isolator can withstand, such as the maximum surge isolation voltage, VIOSM; maximum transient isolation voltage, VIOTM; and maximum repetitive peak isolation voltage,VIORM (see the white paper "High Voltage Enhanced Isolation: Definitions and Test Methods"). Enhanced digital isolators, in addition to passing these tests, must pass the minimum surge voltage test level of 10,000 VPK.
2. Can I apply different voltages to both ends of the digital isolator?
Yes. Digital isolators can supply power to both ends of the device under the recommended operating conditions. Since the isolation barrier isolates the ends, each end can independently apply any voltage value under the recommended operating conditions. For example, you can apply 3.3 V VCC1 (between 2.25 V - 5.5 V) and 5 V VCC2 (also between 2.25 V - 5.5 V) for ISO7721 . In addition to building isolation, you can also use digital isolators as logic level shifters. The isolators are independent of each other.
3. Can the digital isolator signal voltage be different from its supply voltage?
No. The input/output signal voltage of a digital isolator depends on its supply voltage. Therefore, if you want the digital isolator to be compatible with the device it is docked to, it is best to keep the signal voltage the same as the isolator supply voltage. For example, if the power supply voltage of ISO7721 is 5 V, it is docked to the microcontroller (MCU), then it is important that the MCU signal also operates at 5V logic level.
4. What is the logic state of the digital isolator without input signal?
If the input channel of the 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 protection status), which may be low or high, depending on the device selected. The suffix "F" of the device part number indicates the default state of the isolator output channel. For example, if there is no F in ISO7721DWR , the default state of the device is high. Similarly, there is F in ISO7721FDWR It means that the default state of the device is low.
5. Can I leave the channel pins that are not used by the digital isolator floating?
No. The input pins of the unused channels of the digital isolator can be left floating for testing purposes, but in applications, floating unused pins can cause the product's noise immunity to drop.
Floating pins make it easier to pick up noise especially when the system is performing electromagnetic compatibility (EMC)/immunity testing. To make the system immune to this type of noise, the best practice is to lock the channel inputs to their default logic states.
For example, for ISO7721DWR, the best practice is to connect an unused signal input pin to its VCC through a pull-up resistor (a preferred 4.7-kΩ resistor) . For the TI ISO7721FDWR, it is best to connect the unused signal input pin to its ground pin. For both devices, it is best not to connect all output pins that do not use channels.
6.How to determine the power consumption of a digital isolator?
You can calculate the power consumption of the digital isolator based on the specifications listed in its data sheet. Find the supply current characteristics table for the input voltages (2.5 V, 3.3 V, and 5 V). In this table, find the data transfer rate that is closest to your application's signal speed. The current consumption for this particular data rate is listed in the data sheet 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 consumption of the device under working conditions. By dividing this total current consumption by the number of channels of the digital isolator, the current consumption of each channel is obtained. Some data sheets also provide the total supply current for each channel. For example, the ISO7041 datasheet shows a typical current consumption of 4.2 μA per channel total supply current, which is the sum of ICC1(ch) and ICC2(ch) current.
7. How do you construct an isolated power supply for a digital isolator?
There are several alternative ways to construct an isolated power supply for a digital isolator; the best solution depends on the specific application needs.
One option is to use a transformer driver such as the TI SN6501, which can be used in push-pull with secondary side transformers and optional rectified low dropout regulators Configuration (Figure 1). With a power of up to 1.5 W, the SN6501 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 to make it easy for designers to build robust solutions.
Figure 1: Using the SN6501 Build ISO7741 isolated power supply
Another optional method for space-constrained applications is the ISOW78xx family of devices, including ISOW7841, which is available in a 16-pin small plastic package. Signal and power isolation are provided in integrated circuit packages. As shown in Figure 2, this combination takes up less space; it does not require a transformer and is easy to pass.
Figure 2: Using ISOW7841 to create digital isolators with integrated signals and power supplies
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