[SOLVED] 3.7V to 5V with a MC34063A

[awesometastic1]

I'm kind of a EE newb and am trying to use an MC34063AP1 to boost 3.7 Volts to 5 volts even and after studying the datasheet am a little confused on how I actually do that for the values I want?

For reference for what this will be used for, I found a bulk seller that was selling a lots of 10 XBOX controller rechargeable batteries, with the charge circuit built in and charges by hooking up to USB (or obviously whatever 5V source). The lot was only $14 for 10 of them and they do something like 2400mah at 3.7 volts, if memory serves. So not a bad little power source for little projects and has the charge circuit built right in. So I spliced the USB cables and hooked one up to charge with a couple little solar cells I had for a battery backup when the sun goes down and hooked one up to a wall usb charger and the battery then functions as a power backup if the power goes out for that project. The charging part works great. My problem of course is my micrcontroller in both cases needs 5 volts and the xbox controller batteries only put out 3.7 volts.

 

[FvM]

Did you identify the step-up (boost) circuits from 34063 datasheet? Did you notice the design formula table? Also the Onsemi application notes mentioned in the datsheet should help you.

The first step is to calculate the feedback divider R1/R2 for intended 5V output.

 

[awesometastic1]

Thanks, the datasheet I originally looked at didn't have nearly as much info as the one I just looked at.

Anyways, so if I'm doing this right (modifying slightly the example step-up diagram where it goes from 12V to 28V), I'll just want to use that same schematic, but change the following values: Ct = about 2000 pF, L = about 68 uH, R1 = 1K, and R2 = 3K.

Does that look about right? I used approximate values as the xbox controller batteries go from around 2.8 volts up to 3.7 fully charged. So I just went with 3 volts to make the math easier.

 

[betwixt]

Look at the design tool at **broken link removed** it will give you a schematic and include the values. You may need to drop the input voltage to 3.5, this should be OK as the cells will drop as they discharge anyway.

Brian.

 

[awesometastic1]

wow, that's a nice little tool.

I have what I'm sure is a stupid question, but what sort of value should I be putting in the Fmin? Seems like the values that change are the caps and inductor when I adjust that value. I assume that stands for "minimum frequency". I put in about 10 kHz. Does that sound ok for my type of application?

 

[betwixt]

There isn't a right and wrong frequency, somewhere in the 10KHz to 30KHz range will work fine. If you go too low the inductor gets bigger, if you go too high the losses increase.
Brian.