Robust Power Supplies with Ideal Diodes

In many applications, several different voltage sources can be used for power supply. For battery-powered devices, it is common to use a plug-in power supply as a back-up solution in addition to the main power supply. It is also common practice to use a wall-plug AC-DC converter as an alternative power supply to the mains, via a USB cable.

The variety of ways to power the device is not only beneficial to the user, but also improves robustness by adding redundancy.

If several different voltage sources are to be used, more effort is required in designing the circuit. Often, it is necessary to ensure that one source does not reverse flow into another, or damage may result. The simple setup shown in Figure 1 protects unused voltage sources. Diodes are used in the power path. This setup works reliably, but has the major limitation of always using the higher voltage source to power the load. The diode also has a voltage drop of 150mV to 450mV in the power path, thus creating high power consumption, especially at low voltages. For battery-powered devices, the increased power consumption can be detrimental.

Figure 1. There are two power supplies with diodes in the power path to power the application

To circumvent these disadvantages, an ideal diode can be used. An ideal diode is a component that uses a switch (usually a MOSFET) instead of a diode. In the on-state, an ideal diode has a much lower voltage drop. The voltage drop is based on the actual current flowing through the switch and depends on the on-resistance of the switch (RDS(ON)).

The circuit shown in Figure 2 has two ideal diodes, with two LT4422 devices performing the function of the ideal diodes. In these ICs, the voltage drop is low due to the low resistance in the power path of only 50 mΩ. the IC itself consumes only 10 µA, further reducing the total power consumption. Figure 2 also shows an additional feature. An LED can be added as an indicator to point out which voltage source is powering the load at any given time.

The circuit of FIG. 2 is therefore an alternative to the circuit of FIG. 1, with lower power consumption and additional features such as an LED indicator.

Figure 2. Two ideal diodes for higher energy efficiency and additional functionality

However, in the circuit of Fig. 2, one characteristic remains the same, still the device is powered by a voltage source with a higher voltage. The ideal diode (LT4422) has an enable pin ((SHDN) ̅), but when the IN voltage is higher than the OUT voltage, the body diode with the built-in MOSFET conducts. To prevent this, the LT4423, a derivative of the LT4422, was created, using two MOSFETs connected back-to-back in the power path with a specific arrangement that ensures that if the other MOSFET does not conduct at the same time, the corresponding body diode will not allow current to flow.

Figure 3. two ideal diodes, the choice of the power supply is independent of the corresponding value of the voltage source

In the circuit design shown in Figure 3, the supply voltage can be freely determined to power the load. Therefore, the power supply is selected independently of the corresponding level of the supply voltage. However, since two integrated MOSFETs are required, the resistance in the power supply path increases from 50 mΩ (LT4422) to 200 mΩ (LT4423) during conduction.

In addition, the version with two MOSFETs (LT4423) offers integrated thermal shutdown. Unlike conventional diodes, this ideal diode switches off when the component temperature exceeds 160°C (typical). This feature helps create more robust systems.

Not only does the ideal diode help provide different power supply options for the device, but it also improves robustness by adding redundancy. In addition, ideal diodes can provide other features such as detecting the power supply status via LEDs and switching off when the temperature gets too high.