Buck / LDO combination

Question

I have a design question.

My design has grown in complexity because of new sensors (requires 5v).

In the old design a 3v3 output from my buck was sufficient, however now I have to create both a 5v and 3v3 rail, one for sensors and one for powering the MCU.

Now my question is how should I go about achieving this? My initial idea is to reconfigure the buck to output 5v and feed that output into an LDO to get 3v3 for my MCU (requires ~3v3).

I have omitted filtering, feedbacks and poweroring to simplify the design idea.

Question

Are there any pitfalls to this approach or perhaps a better way to achieve this? I haven't frozen the design with these two components, I suspect I will try to find components from the same manufacturer at the very least.

The current requirements are within limits of what these components can handle.

I will update this post with the real schematic once I gather some more information on how to do this.

-Thank you.

Answer 1

Some remarks:

1. the MP2331H has a max input voltage of 26V: be careful with your "24V" input, that if from battery may be above 26V during charging. In any case put an overvoltage protection.

2. You do not specify the power absorption of your MCU, but I think it will be larger than the Sensor (please, provide some detailed info). If so, the "LDO" will reduce thee overall efficiency, because you have a 1.7V voltage drop where you have the largest current. (Note: you do not need a Low Drop-Out because you have room, 1.7V: almost any regulator will work).

3. You could think of a dual output voltage regulator (I know it means two inductors and two output capacitors ...), but you would be more efficient, you avoid the LDO, and then you have plenty of current on the 3.3V and 5V buses to add other devices later. Some regulator like TPS54294 (mx 18V of input voltage), or the BD9848 (accepting up to 36V), both about 2-2.5$.

4. Regarding the USB 5V power in, it may be tied to the input of such switching regulator, because both models are wide range and accept 5V (if used often, consider efficiency at 5V input just to avoid to much heating).