Switching Regulator

[alex.ponce11]

Good Evening Everyone,

I am fairly new to tinkering with electronics and wanted to ask for some help.

I am undertaking a small project where I have approximately 20V input into a PCB and want a 5V rail as well as a 3.3V rail with a maximum of 100mA per rail. I want the ability to turn off the 5V rail when the battery supply gets low. I want to use a pure switching regulator with an enable pin where, using a microcontroller, I can use the enable to signal the regulator to shut off the 5V rail. I also want high efficiency. Any tips on how to get started?

I have been researching a lot, but don't really know where to start. Any tips or guideline would be greatly appreciated.

Thank You

 

[Harald Kapp]

Switch mode regulators are typically made for higher currents. Those for small currents come in very small , hard to solder cases (e.g. DFN).
You can buy complete modules, although tese too are made mostly for higher currents. And not all of them have an enable pin.
An easy to handle modular solution is a 7805 compatible replacement using SMPS technology, e.g. this one. The drawback ist that tis type of module generally lacks an enable pin. However, you could simply turn the 20 V feed to the input ofthe regulator on and off by using an additional switching MOSFET in series. The MOSFET in turn can be controlled by the microcontroller.


Alternatively, as the microcontroller obviously runs independently from teh 5 V supply (otherwise it weren't able to control the enable of the 5 V regulator), you could roll your own simple buck switcher as shown e.g. here.
Here's a more detailed explanation of the working principle based on PIC controllers as example.

 

[AnalogKid]

Your two output power levels are 0.5 W and 0.33 W. While switching regulators certainly are possible at these power levels, a reasonable question is why go to all that trouble? Starting with a 20 V input, two linear regulators would dissipate 1.5 W and 1.67 W. Those are enough to require small heatsinks on LM317's or 78xx parts, but not anything close to a difficult thermal problem.

ak

 

[Harald Kapp]

not anything close to a difficult thermal problem.

Not a thermal problem. The op is operating from batteries, so it's a matter of battery life.

 

[hevans1944]

I use "smart" batteries. When their output voltage gets too low, the attached circuitry automagically stops working.