High ripples in LNK306's output

Posted by: rainman1 on 03/21/2009

Hello.

I use the LNK306 and i have problem with ripples, here is the design parameters:

Vinput = 85V (min) - 250V (max)
BUCK, MDCM, High sides, Half wave modes.
No input stage resistance (only Fuse).
25c. degrees ambient temperature.
Voutput = 5V
Ioutput = 50mA typically, and a few times in a day it outputs 250mA for short pulses of 30msec.
targer ripple volage: 30mV.

I used the components that the Design software of PI determined:
Cin = 6.8uF
Freewheeling diode UF4005.
Lout = 470uH
Rbias = 2Kohm.
Rfb = 3.84Kohm.
Cfb = 10uF.

I have a problem that the ripple was too high, about 60mV, and i want it to drop below 30mV, What should i do in order to acheive that?

In the picture attached there is the design that i currently use for 7.5V output, but its the same for the 5V, only with different components.

Thank you.

Comments

what is the rippl efrequency...it may be due to that single diode rectifier

Thank you for your comment.
I'm currently not at work so i cant check this out (will get there tomorrow).
Could you please explain about it?
You are talking about the 1N4007?
What frequency range would require to change the diode? and to what kind of diode?

Could anyone help me please ?
It is very impoertant for me.
I'm going to have an order of components in the next few days and i'd like to know what parts should i replace.

Thank you.

hello.....can you say what is the frequecny of the ripples...and the amplitude?