Powering Digital Circuitry From SMPS

[fobos3]

I was thinking of powering a microcontroller directly from a buck-boost converter, but after doing a few calculations I ran into the following problems:

• Ringing when switching on/off
• Current rise time

You can get the ringing down with a snubber. The current rise time however is a big problem. Imagine that the micro is in sleep mode and it is woken up. All of a sudden a higher current is required. The control system disconnects the load from the power supply and starts charging the inductor. The inductor needs to be charged to the higher current before the capacitor drops below the permissible microcontroller voltage (say 5%).

I ran some calculations for a current of 100mA and I ended up with huge capacitor values (mF for a rise time of a ms).

So I was wondering is it at all possible to power a microcontroller directly from an SMPS? Obviously you can increase the current slew rate but then your switching frequency goes through the roof.

 

[Baluncore]

In reality the relative size of the inductor and capacitor will decide the regulation or supply noise. For a huge variation of current requirement between sleep and active mode you will need a large capacitor. I have built SMPS that tick away in sleep mode at 1Hz but then rise to 50kHz when active. The frequency ratio will be proportional to the current ratio.

It will take time to start a SMPS from cold. If that delay is a problem then you should keep a capacitor charged, or a rechargeable battery ready for the instant start.