Voltage regulation advice!!

[ayeung]

Hello-just joined this fine site, anyhow im a graduate engineer and need some advice.

The problem: Need to generate multiple voltages for a portable device[video interface demo].

However I plan on using a 3.6V lithium battery and need to generate the following voltages...1.8, 2.5,-2.5, 3.3 and 5V--from current considerations the max any one of the regulators would source could be 100mA.

Efficency is a concern but board space is also a priorty.

The device will roughly source 200mA to 400mA current.

Should I step up with a high efficeny step-up converter to 5V...then I could

(1) step down with high efficent step-down converters to the required voltages....use a voltage inverter for the -2.5V??

(2) Just use LDO's and step down to the required voltages-- saving space??

Which of the above would be better, or is should I consider some other means.....

Any advice would be great!!!!

Thanks

Alan

 

[hjames]

Is this an exercise in estimating things, or do you actually need to build something?

First thing you need to completely specify is the battery - is it a rechargable Li, or some primary one - what's the peak voltage and what does it decay to at it's end of life?

Other than that, keep in mind that buck and boost converters tend to be 80%-95% efficient and LDO's efficiency can vary from very close to 100% (in dropout), all the way down into pathetically inefficient. Also, switching regulators do tend to burn more power (they'll eat up ~10mA or so of current just running, so be sure to factor that in).

 

[Hero999]

**broken link removed**

 

[ayeung]

Thnaks Hjames, no i must build it.

More Info:---The battery is a 3.6V SAFT Lithium cell:non-rechargeable so it will work at approx 3.6V for most its life -from the datasheet anyway it looks pretty flat upto the point where it drops off at a fast rate.

Any other ideas??

 

[Hero999]

Then it's probably better to use a LDO for 3.3V and switchers for the other voltages.

Are you allowed to use ICs from somewhere like Maxim or Linear, or is that cheating? Because I strongly recommend using them if you can.

 

[ayeung]

thnks Hero999.

Yes I can use any I.C to generate these voltages, cost is not really an issue, but space on PCB and also efficency is also very important. So design trade offs are doing my head in- so I reckon that the best way might be to...

Step up to 5V- then use charge pump switchers to drop down to 1.8 -2.5-3.3 then invert the 2.5 to -2.5 and use good layout to minimise noise....However should I be worried about running 4 regulators off this 5V reg???

[I need obviously to make sure that the 5V reg can handle the current to the rest of the regs]

or

Step up/down from the Lithium battery 3.6V to required voltages, in effect run all the reg's off the battery supply >>>aleviating any problems that might happen above ??

So guys is my thinking crap???

Any input is well apreciated as I must get it right!!!

 

[hjames]

Don't use charge pump switchers unless your current reqirements are really low - their efficiency isn't exactly great.

Use a LDO for the 3.3V - you'll get >90% efficiency and it'll take up very little space - 1 sot-23 package and some bypass caps are all.
The easy way is to use a seperate switcher for each additional power supply coming directly from the battery - each one could be ~1cm^2 for these currents. If you want to start playing tricks using transformers, you might be able to do some combined +-2.5V, 5V regulator in exchange for regulation/accuracy.

Chaining regulators isn't very efficient in terms of power or size, so it's probably better not to do that. The only reason to do that is to put a cheap LDO after a switcher, but you don't have any real reason to do that.

 

[tunedwolf]

given the number of different supplies involved, I would be considering designing a small switching supply to handle it all in one go...you might save a little board area, efficiency might be about the same as the other methods, perhaps a little better because you won't have the added losses involved with multiple dc-dc switchers etc...