# LM3578 prelab help

Discussion in 'Homework Help' started by KCHARROIS, Jan 20, 2013.

1. ### KCHARROIS Thread Starter Active Member

Jun 29, 2012
302
8
Hello

So I'm trying to complete a prelab with the LM3578 switching regulator and I'm having a problem calculating C2. I'm following the example for the Buck Converter circuit and the formula for C2 is:

C2 ≥ Vo (Vin - V0) / (8 * Fosc2 * Vin * Vripple * L1

The given values in there example are:
Vin = 15
Vo = 5
Vripple = 0.01
L = 470u
Fosc = 50 KHz

C2 answer is 200uF rounded off i'm assuming

My calculation is 5 (15 - 5) / (8 * 50KHz2 * 15 * 0.01 *470uH) = C2 35.4uF

Where did I go wrong?

Thanks

2. ### KCHARROIS Thread Starter Active Member

Jun 29, 2012
302
8
Rethinking about my problem does mean that any value of 35.4uF or greater is an acceptable capacitor value and that they chose 220uF?

3. ### tshuck Well-Known Member

Oct 18, 2012
3,527
679
It would appear that that is the case, I got the same value.

We never want to design a circuit, particularly, a supply, to operate at the margins. It would seem that they chose this arbitrarily to give a large amount of leeway, though I can't be taken as an expert in this arena.

For anyone wanting to verify, the formula is listed on page 16 in the datasheet.