ORing low-voltage sources simply and cheaply on 3.3V microcontroller boards

Linear Technology LT4351

Most of today’s microcontrollers are powered with a DC voltage of 3.3 V or less. Batteries are involved for low-power embedded systems that have intermittent uses. For permanent usage, designs usually include a mains supply (with transformer and AC/DC circuits), and use ORing diodes to connect the power sources together (references 1 and 2). The well-known forward drop (0.6 V) of diodes was not an issue for earlier designs, often powered with battery sources of 9 V or more. But in modern circuits it is not a recommended solution, even if we choose Schottky diodes (0.3 V).

A better alternative is to use a dedicated IC controller for combining battery- and mains-sourced power. Devices such as an LT4351, create a forward drop measured in just tens of millivolts (mV), thanks to the ballast MOSFET transistor’s very low RDS(ON). However, these specialized ICs are often expensive and somewhat difficult to find, versus the following discrete and simple solution.

The circuit in Figure 1 turned out to be crucial when I wanted to increase the overall efficiency in my design of a very low-power portable datalogger for long term use.

This simplified discrete circuit for ORing power supplies offered improved efficiency over a diode-based approach.
Figure 1. This simplified discrete circuit for ORing power supplies offered improved
efficiency over a diode-based approach.

Here is a brief description: If the primary power supply (VIN1) is present, the N-channel MOSFET transistor T3 is ON, which pulls down the gate of the P-channel MOSFET T2 and turns T2 ON. Transistor T1 sees as its gate-to-source voltage (VGS) the drain-to-source voltage (VDS) of T2, which is only tens of mV. So, T1 is OFF and the external supply path (VIN2) is open.

Now, in the case of an intermittent VIN1 blackout, T3 turns OFF because its gate is pulled down through R1, which turns T1 ON. The transistor T2 is turned OFF as its gate is pulled up thru R2 (VGS of T2 is almost null).

The MOSFET transistors T1 and T2 should be chosen with low-level gate and very low on-resistance features (for instance: T1 = T2 = PMN50XP, which have RDS(ON) of 60 mΩ at VGS = 3.3 V). Transistor T3 can be a popular 2N7000 (or 2N7002 for surface-mount parts).

The circuit’s quiescent current is about 20 μA when the primary supply is present, and almost null otherwise. So, a battery is suitable as the external supply.

The values of R1 and R2 are not critical. They can be hundreds of kΩ if we prefer to obtain a very low quiescent current, or tens of kΩ if we prefer to decrease the swap time commutation between input supplies.

References

  1. Fundamentals of power system ORing, Martin Patoka, EDN, March 21, 2007
  2. Use op amps to make automatic-ORing power selector, Bob Zwicker, EDN, August 11, 2011

Materials on the topic

  1. Datasheet Linear Technology LT4351
  2. Datasheet NXP PMN50XP
  3. Datasheet Microchip 2N7000
  4. Datasheet Microchip 2N7002

EDN

1-4 Layer PCBs $2

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