LM5576-based SMPS problems

[pozzugno]

I'm designing a small SMPS based on National LM5576 all-in-one (MOSFETs integrated) buck converter.

Vin is 50V and Vout is 28V (duty cycle 56%). The output current should change from 0A (no load) to 2.5A maximum.
I'm using 47uH inductor and 330uF low-ESR output capacitor. I set the maximum switching frequency (500kHz) with RT=10K.

I mounted the PCB and I found two problems.

The first is the output voltage ripple at no load condition. I can see a big ripple (1.5V peak-peak) at a low frequency (about 40Hz). I think the switching controller has some difficulties to keep constant the output voltage without the load, but I'm sure there's a solution. What do you suggest?

The second is the load transient. I have a 28R power resistor, so draining 1A from the output. When I connect the load, I can see for about ten seconds the same ripple at no load, but an increased frequency (about 1-2kHz). Only after about ten seconds this big-amplitude low-frequency ripple disappears and the output voltage remains constant (with a low-amplitude high-frequency ripple).

Do you have any hints? What should I check in my circuit? Thank you...

 

[manojdharap]

Hi,

If you can share schematic and details of O/P capacitor & inductor it will be useful.

From first sight, it looks like a problem with O/p inductor and Capacitor filter. Recommend you to add O/P capacitor of 10 to 22uf with very low ESR (2-5mOhm range) in parallel with existing one.

 

[ronv]

Here is the simulation from national. Does your circuit look like this one?

 

[stevenendlos]

Hi,

If you can share schematic and details of O/P capacitor & inductor it will be useful.

From first sight, it looks like a problem with O/p inductor and Capacitor filter. Recommend you to add O/P capacitor of 10 to 22uf with very low ESR (2-5mOhm range) in parallel with existing one.

Manoj,

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 data shet

thanks,
Steve