Electronic – Recommendations for converting 2~7V to 5V


I'm looking for a switching regulator chip (buck-boost, sepic, or other topology) to convert 2~7V input to 5V. I realize it's easier to have an input voltage range that's consistently higher or lower than the output, but this is what I need. It also needs to support 10W output (2A@5V), and I'm looking for a relatively low-cost solution with minimal external component count. Any suggestions?

Best Answer

What volumes? 1 / 10/ 100 / 1000 / 10,000 ...?
What would be a "good" cost?

Note that the 2V input is lower than almost anything will start from. If you can use the 5V output to power the IC for starting then 2V is not a problem. If not, then 2V can still be handled but it adds slight complexity.

Easiest "roll your own" solution using low cost controller ICs is probably SEPIC because you don't need synchronous switches and almost anything can handle the control function.

It should be possible to implement a SEPIC buck boost with an MC34063 (my favourite olde workhorse smps controller) with an IC cost of $US0.31/100, $US0.17/1000 and under 10 cents US in volume in Asia. Needs two inductors but has various advantages.

Here is a TI inverse SEPIC "Zeta converter buck-boost application note.

LM5085 - Designing Non-Inverting Buck-Boost (Zeta) Converters with a Buck P-FET Controller

There are numerous SEPIC converter app notes around.

Basic SEPIC converter - buck boost from here

enter image description here

You can also do seamless buck boost if you don't mind voltage inversion using a single switch and a single inductor from here

enter image description here

IF inversion is allowable then this is potentially an extremely low cost solution. A 74Cxx14 (hex CMOS Schmitt trigger) and a MOSFET and some glue parts and an inductor would probably suffice. Parts cost under $US1 in modest volume. Under $US0.50 maybe in volume in Asia. PCB etc extra.

OR buck boost without inversion using a single switch and a 2 coil inductor. The inductor can be 1:1 and may be able tp be a COTS part such as a two winding line filter on a ferrite "bead".

If you don't mind paying a few dollars for the IC and MOSFETs you can get buck-boost ICs that will swap between buck or boost as required using a two MOSFET switch pair in the input. This is easiest if price is not totally crucial.

  • Fully integrated switch:

The LTC3440 is a superb device if its ratings meet your needs.
In your case it doesn't - but this is a guide as to what can be achieved. datasheet

enter image description here Pricing - $2.91/100

The ST L6910 costs $1/100 from Digikey and uses a quad of external high/low MOSFETs so power can be whatever you wish.
L6910 datasheet

You could probably replace Q2 & Q4 with Schottky diodes at some loss of efficiency if desired (and in fact Q2 is already paralleled with a Schottky to handle the transition region.

enter image description here

Excellent You Tube tutorial here
By LT and selling their parts but well worth watching.