Using Digital Isolators as Level Shifters in Non-Isolated Applications

iCoupler digital isolators are widely used to transmit digital signals across an isolation barrier. In some cases, digital isolators are also very useful in non-isolated applications.

– 48V DC-DC power supply

In telecom power applications, the standard rail is –48V DC, and the primary control signals are referenced to this rail. The secondary control signals are typically low-voltage I/Os (e.g., +5V, +3.3V CMOS) referenced to ground. The primary and secondary grounds are connected, and the entire system is non-isolated. In this type of DC-DC power application, digital isolators are suitable for providing level shifting for feedback signals. As shown in Figure 1, the high-level input voltage on the secondary side is 5V and the low-level voltage is 0V; the high-level voltage on the primary side is –43V and the low-level voltage is –48V. The primary ground of the digital isolator is connected to the –48V rail, and the VDD power supply is connected to –43V. Due to the built-in isolation barrier, each side of the isolator operates in an independent voltage domain, providing level shifting capabilities. Using iCoupler digital isolators can also protect the secondary circuit from faults such as overvoltage or short circuits.

Multi-power system

For systems with multiple power domains, one or more power supplies may be powered down while other supplies are powered up. In this case, if there is any data line connection between the two power domains, the powered down domain may be powered parasitically by the voltage or current on the data line. Figure 2 shows this application scenario.

To avoid leakage current, the digital line must be set to low-level output or high-impedance mode. When iCoupler digital isolators are used instead of direct connection between two power domains, leakage current between them can be blocked (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.