LM5576 voltage regulator national instruments problems

[stevenendlos]

hii

I'm trying to use LM5576MQH vol reg from National to generate differnt power supplies of 5V, 3.3V,10.8V, -5V ....
My load current (min) is taken as 10mA. My input vol is 15-45v...based on that my inductor values are calcuated and I get very high values such as 820uh for 5v, 470uh for 3.3 etc...ramp capacitor values are 6800pf for 5v, 2700pf for 1.8v.....(wierd)....
i'm yet to test the board but not really sure if it is going to work!!...
what would you suggest....Are those values ok...except those values, rest of the components are very similar to the one inthe datasheet

thanks,
Steve

 

[mtwieg]

Those inductances seem high. How did you calculate them, and what is your intended operating frequency?

 

[stevenendlos]

I calclated them based on the formula given in the data sheet!!

l=vout*(vinmax-vout)/i-ripple*300khz*vinmax

vout=5v
vinmax=45v
i ripple=2(min load current)=2*10mamps

 

[mtwieg]

Yeah, getting such a low ripple current will require a huge inductor. You probably don't need to stay in continuous conduction at light loads, so you could tolerate much more ripple. Generally I go for Iripple of between %15 and 40% of full load current.

 

[stevenendlos]

Alright I got it...But is it going to be a huge problem?......the thing is the ramp capacitor values very very very big...
6800pf for 5v(sice it is based on inductor values) etc etc......does it

 

[mtwieg]

You should base the ramp capacitor value on the inductor value you actually use, not the inductor value recommended by the data sheet. If you allow for increased ripple, you can use a smaller inductor and your Cramp will also be lower.

I don't see anything wrong with operating in discontinuous conduction mode for this controller.

 

[stevenendlos]

Thanks..Ill let you know the test results.....probably in a day or 2...

 

[stevenendlos]

Working absolutely fine..Thanks!

 

[stevenendlos]

lm5576 problems with output voltage ripple

Hi,

I'm using LM5576QMH vol regulator and trying to get +5V,3.3,1.8 at the output. When the o/p vol was observed on the scope, I've found a very big ripple. Could you suggest me what to change in my ckt?

According to me, the problem should be either with the output capacitors or the inductor used.
I used a very big inductor in order to compensate for the load current. My load current is very small (10ma). I've attached my schematic .....please suggest me some changes.

It would make more sense to continue this discussion in the same thread instead of a new one, threads merged [alexan_e]