Put the “small vest” of the level converter on the digital isolator

“iCoupler® digital isolators are widely used to transmit digital signals across isolation barriers. In some cases, digital isolators in non-isolated applications can actually put on the little vest of a level shifter to provide level shifting in the system!

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iCoupler® digital isolators are widely used to transmit digital signals across isolation barriers. In some cases, digital isolators in non-isolated applications can actually put on the little vest of a level shifter to provide level shifting in the system!

How do iCoupler® digital isolators provide level shifting in non-isolated applications? Let’s look at two cases:

Case 1: C48 V DC-DC power supply

In communication power supply applications, the standard power rail is C48 V DC, and the primary side control signal is referenced to this power rail. Secondary control signals are typically ground-referenced low-voltage I/Os (eg, +5 V, +3.3 V CMOS). The primary side and the secondary side are connected, and the whole system is non-isolated. In this type of DC-DC power application, digital isolators are suitable for level shifting the feedback signal.

Figure 1. C48 V to 5 V DC-DC Power Supply with Feedback Based on Digital Isolator

As shown in Figure 1, the high-level input voltage of the secondary side is 5 V, and the low-level voltage is 0 V; the high-level voltage of the primary side is C43 V, and the low level is C48 V. The primary ground of the digital isolator is connected to the C48 V rail, and the VDD supply is connected to C43 V. Due to the built-in isolation barrier, each side of the isolator operates in an independent voltage domain, providing the function of level shifting. Using iCoupler digital isolators also protects secondary Circuits from faults such as overvoltage or short circuits.

Case 2: Multiple Power Supply System

For systems with multiple power domains, one or more power supplies may be off while others are active. In this case, if there is any data line connection between the two power supply regions, the shutdown region may be parasitically powered by the voltage or current on the data line. Figure 2 shows this application. To avoid leakage current, digital lines must be set to low output or high impedance mode.

Figure 2. Leakage current between two power domains

Using an iCoupler digital isolator instead of a direct connection between the two power domains blocks leakage current between them (Figure 3). Since there is no need to control the output state at this time, the protection design of hardware and software can be simplified.

Figure 3. Connecting Two Power Zones Using Digital Isolators