Scramble, not trying to hijack your thread but we're both looking at the same IC so my question applies here as well.

Sarge, you mention R1 and R2 are used to determine the output voltage of the step-up converter, what formula is used to determine the resistor values?

 

You mention R1 and R2 are used to determine the output voltage of the step-up converter, what formula is used to determine the resistor values?

Vout = 1.25 (1+R2/R1)

In Q2S's schematic, R1(labeled r3) is 10k, R2(r4) is 220k.
so:
Vout = 1.25 (1+220k/10k)
Vout = 1.25 (1+22)
Vout = 1.25 x 23
Vout = 28.75

In the datasheet schematic, they used 2.2k for R1 and 47k for R2.
Vout = 1.25 (1+47k/2.2k)
Vout = 1.25 (1+21.363)
Vout = 1.25 x 22.363
Vout = 27.954

The advantage of using the higher values for R1/R2 is somewhat higher efficiency/less current drain in the comparator reference level circuit; but the datasheet schematic's divider was only using about 560uA. Q2S's schematic would use 125uA current.

 

I suppose if I just looked further down the data sheet, I could have found that myself. lol, thanks Sarge.

 

I'm fiddling around with the Eagle program. Haven't quite mastered it yet.

On another note, how am I going to find an inductor that'll work since I can't get the specs from the current one?

 

I'm fiddling around with the Eagle program. Haven't quite mastered it yet.

I assure you, I'll be back home and will be able to help you a long time before you've mastered Eagle It ain't something you can learn inside a week - or a month.

On another note, how am I going to find an inductor that'll work since I can't get the specs from the current one?

That's easy - the spec is in the datasheet. For the step-up (boost) converter, it's a 170uH inductor they hand-wound using 38 turns of AWG 22 magnet wire on a Magnetics Inc. #55117 toroid. You don't have to do it exactly that way; you can buy a pre-wound inductor that's rated for enough current (say 600mA) at about the same 170uH inductance value. Doesn't have to be exact.

If you want to verify, try turning the thing on and measuring the voltage on the 220uF cap; you should get about 28.7v.

 

OK, I'm back home.

How are you doing with the schematic capture, Quick2Scramble?