3
\$\begingroup\$

Any ideas on how to get 3.3V from 5V - 35V at a low 10mA current level? 3.3V need not be precise. It can be 3.5V or 3V, it is powering a small microcontroller.

I'm looking for a small and cost effective solution. That pretty much rules out through hole stuff.

\$\endgroup\$
5
\$\begingroup\$

Use a linear regulator that can handle 5V to 35V input. It outputs 3.3V.

It is that simple. You can use a buck regulator, if efficiency is an issue. but 10mA at a 35V drop is still only a 350mW dissipation.

I suggest the linear, keep components/complexity/noise down.

\$\endgroup\$
  • 3
    \$\begingroup\$ efficiency gain from a buck regulator at only 10mA would be questionable as well, especially at the 35V point, the duty cycle would be really low. \$\endgroup\$ – Mark Nov 10 '10 at 16:50
  • 2
    \$\begingroup\$ 350mW is a lot for small surface mount components. \$\endgroup\$ – Thomas O Nov 10 '10 at 16:51
  • \$\begingroup\$ 350mW absolutely worse case is not that bad. that is if you are at 38V in. If you get a good ground plane to conduct heat, or use through hole with a heat sink(which I can put watts into without a problem) you can handle this. The question of heat dissipation we have had other places. This will easily be handled with small surface-mount linear regulator(I have done this much power that way). \$\endgroup\$ – Kortuk Nov 10 '10 at 17:04
  • \$\begingroup\$ Yeah, but I can't spare space for through hole. It's also a waste of space in that the micro is in a tiny SO20 package but the reg is in a TO-220... How would zener diodes work? they have higher dissipation ratings, maybe it would work better? \$\endgroup\$ – Thomas O Nov 10 '10 at 17:07
  • 1
    \$\begingroup\$ something like a linear.com/pc/productDetail.jsp?navId=H0,C1,C1040,P86928 in a DFN package with the thermal pad tied to the ground plane with 3-4 vias would work fine. Based on the data sheet you would need something like 25cm^2 of copper to allow operation up to ~90C ambient. Which if you have a ground plane in the board and the board is bigger than 5cm x 5cm should be fine. If you can do a ground pour on the surface layer around the IC, even better. \$\endgroup\$ – Mark Nov 10 '10 at 17:27
3
\$\begingroup\$

You'd struggle to find a buck reg with 35v input range that didn't draw 10mA quiescent before you got any power out. 35V is a bit on the high side for many linear regs so you need to look carefully at specs.

\$\endgroup\$
  • \$\begingroup\$ tcrosley disproved you without much effort :P. LTC3631 has a maximum Iq of 0.22 mA in active state. Iq is usually proportional to the full-load capability, so while you're probably right for a 5A supply, smaller ones exist. \$\endgroup\$ – Nick T Nov 10 '10 at 18:59
  • \$\begingroup\$ The OP said "Cost effective" - LTC3631 costs maybe 3-4x what the "small micro" is likely to. \$\endgroup\$ – mikeselectricstuff Nov 10 '10 at 19:07
  • \$\begingroup\$ Micro is a PIC16F690, about $1.80 in quantity. \$\endgroup\$ – Thomas O Nov 11 '10 at 13:41
3
\$\begingroup\$

This LTC3631 handles 4.5 to 45V in, 3.3v out, and comes in either a 3x3 mm or MSOP-8 package. 81% efficiency at 36v in and 10 ma out (page 4). You should be able to get a suitable inductor in an 0805 (2 x 1.3 mm) or smaller package.

The MC34063 might also work but it is a larger package (SOIC-8), uses more quiescent current (2.5 ma) and would require more external components. But it is 1/4 the cost of the LTC3631.

\$\endgroup\$
1
\$\begingroup\$

At such a low current level I would be inclined to pre-regulate down with say a 12v zener and stick a cheap-as-chips LDO on there

\$\endgroup\$
  • \$\begingroup\$ At such a low current it wouldn't be entirely out of the question to use a voltage divider instead of an LDO if they wanted to be really cheap, but yes, I think an LDO or standard 3.3 volt regulator would be better in terms of voltage stability. \$\endgroup\$ – QuickishFM Oct 3 at 18:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.