I am trying to design a micro-controller power supply that can use three power sources with a particular order of priority. The sources are:

  1. 5V USB - overrides all other sources when available
  2. 12V DC - only on when micro-controller provides a 3.3V enable.
  3. 3.7V LiIon battery - charged from the other two when available

The current requirements of this system are less than 100mA @ 3.3V.

My concept, shown in the schematic, is to use power a 3.3V LDO regulator from 5V (if present) or the battery. The battery has a charge controller that is powered by 5V (this charging circuit is described in Microchip AN1149). The 5V can come from either USB or from another 12V rail via a buck regulator. The USB 5V must supersede the 12V supply and the microcontroller can also turn off the 12V source. I would like the buck regulator to power off when 12V source is not used.

I have attached a sketch of my circuit concept but I am unsure about the correct use of MOSFETs and Schottky diodes in this context. With the voltage regulators and charge controller in the system I am not sure how to simulate this.

Can anyone help me determine if this MOSFET configuration makes sense and what I need to do to bias them correctly for each operating state?


  • 2
    \$\begingroup\$ Please use reference designators for the components. Then it is easier to say: The PMOS that has a gate resistor of 10k will always conduct: either by being turned on or by its body diode. \$\endgroup\$
    – Huisman
    Jun 7, 2019 at 21:26
  • 1
    \$\begingroup\$ There is quite some more to say about the circuit, but it's easier if you use reference designators (i.e. 'number' the components, mosfets as M1, M2, etc and diodes as D1, D2, etc) \$\endgroup\$
    – Huisman
    Jun 7, 2019 at 21:35
  • \$\begingroup\$ Sorry, added the designators. Good point. \$\endgroup\$
    – Mike
    Jun 7, 2019 at 21:39
  • \$\begingroup\$ If you want 12V on the input of U1, you need 12V on the source of Q2. If you drive the gate with 3.3 V, Vgs would be 3.3-12= -8.7V. But that Vgs never turns on a NMOS. \$\endgroup\$
    – Huisman
    Jun 7, 2019 at 21:49
  • 1
    \$\begingroup\$ Note that USB isn't exactly 5.0V, but can be +/-10%. So, when USB is present, but only with 4.5V, the 12V to 5V DC/DC converter will still supersede. \$\endgroup\$
    – Huisman
    Jun 7, 2019 at 21:52


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.