Powersupply

[seaton]

I'm pretty well finished on the design, but one of the things bugging me is the powersupply. Currently I just have the battery pack connecting to the board, with some basic filtering. This battery is or'ed with the USB VBUS via a LTC4411, so if the usb is plugged in power will be supplied via the USB and battery is disconnected, as soon as usb is removed, battery is re-connected. The LTC4411 also provides reverse polarity protection etc

I'm working on a supply of ~3.3v (battery) and 5v(when connected to USB), there is no onboard voltage regulation. To get 3.3v (~3.6v) with NIMH cells then it will require at least x3AA @ 1.2v per cell. Should I look at a DC-DC converter to reduce this to x2 AA cells? Which will obviously make it more compact, but will increase the component count on the board. Has anyone had experience in DC-DC converters or can you recommend something? Or should I leave it as is?

Thoughts?


Stephen...

[chp]

Thats one of my problems as well. (I'm planig a quite similar device for some time now. In the next month the first prototyp should be ready.)
In the first design I'm using 2xAA Cells to get 3V, but this is far from perfect. 3 or 4 rechargeables are to big and heavy in my opinion.

I found the MAX751 (http://datasheets.maxim-ic.com/en/ds/MAX751.pdf).
Pretty perfect but it doubles the number of the components on the board. And its not exactly cheap and hard to get here in germany.

[bilmorfoto]

What's the expected life with 2000-2500mAh NiMHs, for example? Would AAAs be an option at 800mAh or so?

[seaton]

What's the expected life with 2000-2500mAh NiMHs, for example? Would AAAs be an option at 800mAh or so?

RF modules draw ~13-15ma in Rx and ~24-26 in Tx mode, standby is ~0.3ma, not too sure of ATMEGA168 processor. Other things is the LED and opto so when lit they will draw a little. I haven't done a proper power budget.

[chp]

RF modules draw ~13-15ma in Rx and ~24-26 in Tx mode, standby is ~0.3ma, not too sure of ATMEGA168 processor. Other things is the LED and opto so
when lit they will draw a little. I haven't done a proper power budget.

Hey seaton,
the ATMEGA48 (which i'm using) draws about 4mA (@3.3V, 10Mhz) running and even less in one of the 5 sleepmodes. (I don't know if we can use one of these.)
One LED/opto uses 2 - 10 mA.
Based on this, the trigger should run a quite long time even with 2xAAA.

DC-DC:
My new fav is the MAX856 (DIP and SO) because i switched to 3.3V. I also found it here for 3 euro, but only as a surface mount.
Now I'm considering the layout of a ciruitboard against the cheap alternativ 2xAA dry cells (or 3x NiNH).

[seaton]

[quote="chp]
DC-DC:
My new fav is the MAX856 (DIP and SO) because i switched to 3.3V. I also found it here for 3 euro, but only as a surface mount.
Now I'm considering the layout of a ciruitboard against the cheap alternativ 2xAA dry cells (or 3x NiNH).[/quote]

DC-DC
Checkout the NCP1400A from onsemi.com, just found it.


Stephen...

[mobbarley]

I have been looking at maxim's MAX1551 / MAX1555 charger ICs and small lithium packs - charges from DC or USB and no external fets etc required.

However lithium packs would be harder to change 'in the field'