A quick noob question.

Community PCB Design
1

Thanks for reading my post. I’ll apologized in advance for my cluelessness. I’m very much a beginner and certainly appreciate any help I can get.

I’m starting the process of designing my first PCB. Basically, I want to put together an Arduino clone that has on board H-bridges for DC motor control, voltage regulation for different devices, and Bluetooth capability to talk to my Nokia n800 tablet. I will used the board to drive a small remote controlled vehicle with a wireless video camera, controlled via the tablet.

I want to integrate a MC34063A switching IC in order to take a 3.7 v delivered by a LiPO cell and switch up to 9v for the camera. I found [this site which lays out a schematic for wiring the MC34063A based on the desired in/out power.

If I input 3.7v and output 9v, the schematic shows a resistor between pin 6 (VCC in) and 7 (Ipk sense). However, in the text it describes this resistor as “NaN Ohms”. I’m not sure what this means. My best guess is “Not Applicable”. Can anyone shed some light on this for me?

Thanks for your consideration.](MC34063A design tool)

2

Hi,

NaN stands for Not a Number. It probably shows up like this because you did not specify all the parameters used to calculate Rsc.

On the datasheet of the MC34063A, it shows Rsc being used to limit the peak current. So if you specify Iout you will get a value for Rsc.

3

Thanks for the reply. I think my camera needs about 55mA, so if I put that in the calculator tells me Rsc is .877 Ohm.

There are two other values that I have not yet added into the calculator. It has a box for Vripple (mV(pp)) and Fmin (kHz). Coincidentally there are two more components that the calculator describes as “NaN”. So I guess I need to figure out what the Vripple and Fmin are.

Can anyone help me out with this? What is Vripple? Does this have something to do with noise? And is Fmin minimum frequency?

Thanks again.

4

Ripple can be thought of as noise, it’s actually the residual AC.

Leon

5

Thanks again for the info. Residual AC? Maybe I’m completely on the wrong track. I was planning on feeding 3.7v DC into the MC34063A. Could this produce residual AC?

6

I meant the AC generated by the switcher.

Leon

7

Okay, so the MC34063A generates residual AC, which is referred to as Vripple. How does one determine what the optimal Vripple is?

8

It depends on your application. Try 10 mV and see how it behaves.

Leon

9

Thanks for the replies, Leon. Can you give me an idea how I can determine what the Fmin specs should be?

10

Try 100 kHz and see what you get. You want something that will give you a reasonably sized inductor, that can actually be constructed or purchased. Apart from the inductance value the core has to be suitable for your switching frequency and shouldn’t saturate.

You will probably have to fiddle with the inputs to get sensible values. I’d check what you get with the data sheet, to make sure that you meet the specs.

I’d be inclined to use the Nat Semi Simple Switcher chips, they have excellent design software.

Leon