Component selection for MC34063-based AA battery mobile charger

[GiantGnome]

Hi all

I am trying to build a MintyBoost-like device from the parts in my junk bin. I have some MC34063's and some schottkys and I hope to scrounge the rest - I probably need to buy the inductor from my local electronics shop.

The charger for my current phone says 5V / 400mA on it, but to keep the switching current below 1.5A, I have opted to accept 375mA

I have used http://dics.voicecontrol.ro/tutorials/mc34063/ to calculate some values - I need about a 10uH inductor, 0.2 ohm sense resistor and a schottky diode (and some other parts)

But what does the power/current rating of the components need to be?

For the sense resistor, is it fine to say 0.2 ohm * 1.5A^2 = 0,45W? I was thinking of paralleling up five 1 ohm resistors.

The inductor needs to be able to handle peak current, so I was thinking 2 or more amps

The schottkys I have (SS14) says Maximum Average Forward Current: 1.0 A - this is less than 1.5A, but the 1.5A is peak, not average? As the diode is after the switcher, wouldn't the maximum average be 375mA? Can I use the SS14 diode, or will I need to buy one with a larger current rating?

[mariush]

I would seriously recommend ditching the MC34063 and going with a more modern boost regulator.

MC34063 needs at least 3v to operate in the first place and the efficiency is LOUSY.  You'll get maybe 60-65% efficiency, which is bad.

If you still insist on doing a mc34063 boost regulator, there's a more advanced calculator (software) here: http://sourceforge.net/projects/mc34063uc/  which should tell you those peak currents and more details about parts you should get.

[GiantGnome]

I will have to insist, as it is the only regulator I have. Next year, I will probably look in to making something more efficient (or just buying a mintyboost), but I think it is a good exercise (when doing one-off projects) to use what you have and not have.

The advanced calculator looks very interesting - but only runs on Windows, which I sadly do not have access to. I will have to look at it at work (in the lunch break, of course ).

[Mr Smiley]

If you've got it ,,, use it 

http://dics.voicecontrol.ro/tutorials/mc34063/

I've got a bag full and can see no reason at all to bin them for an extra 5% efficiency  and an extra 3-4 dollars in cost.

Most switchers are only highly efficient under strict working conditions, go outside those and your 34063 is just as good.