A question about switch mode controllers, mc34063 particularly

[crispytato]

Hi there everyone, I have a quick question regarding using an mc34063 and external pass transistor in a design of mine.

Basically, the design is a portable unit, running off of a single 3.7v lithium cell. My intention is to have the ability to vary voltage on the fly, however I've run into a problem, regarding how I can get it to regulate both above and below the battery voltage using the same circuit.

I only need around 2-6 volts out of the product, at 1-1.75 amps (it will be closer to 1 most of the time)

I am aware of the necessity to up-rate components in the circuit. The accuracy of regulation is not hugely important to this application, nor is the smoothness of the supply

My thought was, perhaps I could run the mc34063 in it's inverting configuration, to avoid needing to both step up and down. is there anything I need to consider in using this config? Would any particular configuration be more or less efficient?

Perhaps someone could point me in the direction of a different regulator that'd suit my purpose better? My primary considerations are:

1) BOM cost (including the costs of additional components, I'd be happy to use a more expensive part if it saved on other BOM costs)

2) battery efficiency. I really don't want to get any worse than the 34063, better would make me a happy man.

I hope someone can provide me with some useful info, thank you very much in advance

Chris

[dannyf]

Would any particular configuration be more or less efficient?

That particular part does one thing for you and another against you: not terrible efficient but terrifically cheap.

[Harvs]

The "typical" modern configuration for this is to use a SEPIC converter, because your output can be above or below the input voltage, and positive.

These were once a PITA because they typically require a coupled inductor.  However, they have become a very common these days and there's a huge range available on the likes of Digikey for quite cheap.

If you're interested the best bet would be to read one of the numerous app notes out there from the likes of LT or TI.