­Ideal Diodes for Multiple Power Source Designs

Devices with more than one power source have become much more common with the rise in popularity of portable consumer goods, such as mobile phones and tablets. Those devices are designed to work from battery, but they can still operate from the mains when the charger is plugged in. There are also other designs that, for a variety of different reasons, operate from two or more power supplies to ensure that the load is always powered. It is important that those voltages are kept separate from each other to prevent damage to the system. One way to do that is to use Schottky diodes. However, these diodes have a small, but significant voltage drop of between 150mV and 450mV. In higher voltage applications the drop can be a small percentage of the overall voltage, and not matter too much apart from a small decrease in efficiency, but in lower powered applications, especially those involving batteries, it can be a larger portion of the voltage and lead to shortened battery life.

Ideal diodes are another way to isolate different voltages from each other. The name is misleading as they are neither ideal or diodes, instead, they are a circuit that blocks current flowing in a single direction and allows it in the other, similarly to a diode. In reality, the ideal diode an active switch that has a smaller voltage drop when fully turned on. It also has other benefits over a Schottky diode, including the ability to integrate thermal protection. A Schottky diode can’t really be shut down as it would lead to current loss. The ideal diode can easily be turned off if the temperature gets too high. In fact, it can be turned off at any time to close the power path.

To give a practical example of an ideal diode, Analog Devices’ recently released LTC4422 device features a 50mV voltage drop, which is around a third of the amount lost by even the best Schottky diodes. It has a wide operating voltage, which is important for applications such as USB 3.0 where the voltage can vary depending on the product attached.

Frederik Dostal, Subject Matter Expert for Power Management at Analog Devices explains, “Most systems that are powered with USB 3.0 are battery powered. The USB 3.0 type voltage source can be anywhere between 5V and 20V, depending on the power level and negotiation, so any ideal diode would need to cater for that entire voltage range. If USB 3.0 is attached and active, that we don't automatically want to charge the battery. It could be that the battery is already charged and we don't want to overcharge it. It could also be a standard battery, that you cannot recharge. Two ideal diodes can be used to safely switch over to power the load with either the USB 3.0 or with a backup battery cell”.

Dostal continues, “The ideal diode will always consume some power as soon as it's attached to the system. While current is running through the device, it consumes about 10μA, and when the device is shut down it only consume 500nA. If the output voltage is higher than the input volt voltage, it needs to react quickly, because if it takes more than a few milliseconds, then the protection mechanism wouldn't really be existent any more, so a fast turn off minimizes the reverse current, which can be a waste, or it could damage the circuit. Finally, it is very easy to implement a thermal shut off that diodes don't have typically.”

In some cases, the protection offered by the LT4422 is not sufficient. For example, when the input voltage is higher than the load voltage, there will be current flowing through the body diode of the integrated p-channel MOSFET, even if the part is turned off. A second device in the family provides protection in this scenario. Instead of a single MOSFET, the LT4423 features two MOSFETS in parallel with a diode across each MOSFET back to back. This allows the circuit to be completely turned off, with no current flow through the device in either direction, which is important for some applications.

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