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designing around the MC34063 min inductor value formula

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Simon:
Right well, inspired by daves 110 video post i thought I'd run through the formula's for making up a basic converter. The final end for the converter is a stepdown MPPT battery controller for a solar system using an external switch.

Now the datasheets bang on about BJT switches, anything against mosfets ?

I ran the formula for the minimum inductor and was suprised to find that with a 16V in and a 14.4 V out @ 5A (worse case scenario and "by the book") I got a massive... 1.2 uH - wow datasheets go on about 100 uH plus. Am I missing something. This is a 100 KHz design.

Looking at the formula I realized that if my Vin got bigger and my Vout go smaller I'd actually need a larger inductor, making 18V in requires a 3.1 uH inductor. So why is the minimum inductance based on minimum input voltage ? it seems like the higher the voltage difference the more inductance required (and that makes intuitive sense)

what value am I supposed to choose ? How much bigger than the min value can I go ?

tecman:
The numbers seem OKay by your formula.  Remember you are not doing much conversion with 16 in and 14 out.  Watch 100K on the standard chip.  I find that the chip looses efficiency at those frequencies.  I try to stay at 40-50K.  As for mosfets, same rules apply, just figure the on DS voltage at the current and use it for Vsat.  Also a bigger inductor will usually work just fine, as long as you don't go overboard.

paul

Simon:
yes i was worried about getting massive values with my "big" current of 5A, I think I will drop to 50 KHz

But see I don't get it, if the smaller voltage step calculates for the samller value of inductance why not use the maximum step scenario ? I could well have up to 20V in and 13 out would the inductor calculated for 16-14.4 still be ok then ?

Can I get away with not using the current sensing resistor ? I mean for 5A I get 3R, that's crazy, I'll have other current controlling circuitry anyhow

Simon:
hm that being one aspect I didn't think to check, thanks for the heads up

Simon:
where do I find the maximum Ton versus Toff ? or do i work it out from the max charge/discharge values, or msybe I have a real duff datasheet

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