Boost regulator with very low duty cycle

[Mad ID]

Hi,
is it possible to design a boost converter with 4.2V output and Li-Ion battery input?
When battery is fully charged, the input voltage is 4.2V, i.e. equal to the output voltage.

Some boost converters have Minumum Duty Cycle "0". I'm thinking that with efficiency cca 85% the duty dycle would be around 10% even when input equals output.

I'm wondering if I can avoid SEPIC in this case which is more complicated and a bit more expensive.
If important, maximum output current is 2.5A.

Thanks!

[rexxar]

I'm using the Micrel MIC2876 boost regulator. The adjustable version goes down to 3.2V, I think (I'm using fixed 5V out), and it has a passthrough mode where it will directly connect the battery to the output if the battery voltage is higher than set voltage. Here's the mouser link

[codeboy2k]

TI's WebBench designer seems to think you can do it with a LM3478 boost controller, but running their simulations show that it actually only gets to around 4.1V with 4.15V in (even though I told it V_in(max) was 4.2V).

I just put your specs into WebBench and it spits this design out:

http://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=55F1381D3139B711

The design from WebBench uses the LM3488 but you can use the LM3478 if you don't need to make it synchronous to an external clock.

In cases like these, it's best to just get a sample or 2 and try it out.

EDIT:  The Micrel part mentioned above looks useful too, it's cheaper and has a built in switch.

[kjs]

Why not just use one of these buck-boost (SEPIC) ready made modules based on the LM2577? I use several of them and they work just fine and can deliver the current you need. Can be had for <US$3 on fleabay and alislow.
Careful, there are several types (most boost only) and you have to get the one which has two pot-core inductors and only one potentiometer on the board.
If you buy the IC and capacitors and inductors you are at a higher cost.......

Edit: OOOPS: your input voltage will go below the min for the LM2577 (3V) when you battery gets empty......

[Mad ID]

I'm using the Micrel MIC2876 boost regulator. The adjustable version goes down to 3.2V, I think (I'm using fixed 5V out), and it has a passthrough mode where it will directly connect the battery to the output if the battery voltage is higher than set voltage. Here's the mouser link

Hi, thanks!
This looks very promising!

EDIT: This chip is not good for any application with load transients. I need a good response with 2.5A current step. Figure "Load Transient" shows 600mV drop at 0-->1.2A transition. That's very very low performance.

[PeterFW]

EDIT: This chip is not good for any application with load transients. I need a good response with 2.5A current step. Figure "Load Transient" shows 600mV drop at 0-->1.2A transition. That's very very low performance.

A external switch buck boost will do what you want, 2.5A is a lot of power...

[Mad ID]

A external switch buck boost will do what you want, 2.5A is a lot of power...

Yes it is.

I'm thinking of putting 1mF elco at the output + 10uF ceramic.
What I'm not sure is how this will affect stability and loop reponse.

Another issue inrush currents. What do you think?

[T3sl4co1l]

No point using huger caps, fix the problem (slow regulator)!

Is this really a problem?  I've never bothered to use any of those web design tools.  Do they make poor choices for edge cases?  Are they just fatally conservative (slow loop = unlikely to oscillate, shitty transient response)?

How important is it that the output is 4.200V?  If it's powering e.g. a mobile device, most parts would probably function down to 3V or so (e.g., 2.8-5.0V MCU, backlight controller -- but not a parallel-type LED backlight, and maybe not an LCD panel), or are happy enough with the lowest voltage (LCDs, MCUs, etc. most commonly operating in the 1.8-3.3V range; just use an LDO or buck regulator).  If a power circuit (RC, motors/servos?), why not choose motors with minimal voltage ratings and use the controllers to buck down the spare change?  (Thus giving a nominal 3V output, but a wide input compliance so regulation doesn't matter.)

I'm oddly reminded of a (RC?) contest that demands the internal supply voltage no higher than 4.20V or something like that...

Tim

[Mad ID]

No point using huger caps, fix the problem (slow regulator)!

Is this really a problem?  I've never bothered to use any of those web design tools.  Do they make poor choices for edge cases?  Are they just fatally conservative (slow loop = unlikely to oscillate, shitty transient response)?

How important is it that the output is 4.200V?  If it's powering e.g. a mobile device, most parts would probably function down to 3V or so (e.g., 2.8-5.0V MCU, backlight controller -- but not a parallel-type LED backlight, and maybe not an LCD panel), or are happy enough with the lowest voltage (LCDs, MCUs, etc. most commonly operating in the 1.8-3.3V range; just use an LDO or buck regulator).  If a power circuit (RC, motors/servos?), why not choose motors with minimal voltage ratings and use the controllers to buck down the spare change?  (Thus giving a nominal 3V output, but a wide input compliance so regulation doesn't matter.)

I'm oddly reminded of a (RC?) contest that demands the internal supply voltage no higher than 4.20V or something like that...

Tim

Hi, GPRS modem has operating range od 3.4V - 4.4V @ 2A burst. Li-Ion battery 3V-4.2V or lower due to minimum temperature of -10C (I think that low battery temperature & 2A peak load are problematic).

I definitely need a boost or SEPIC. I opted for MIC mentioned above, will set the output voltage so that it works in bypass mode when battery fully charged. I'm not sure if I will be able to fix slow response with larger output capacitor. The problem is that this will create a pole at lower frequency and slow down the regulator even more...that's my main concern. Nevertheless, I will try it for a prototype.

Any recommendations?