LM2585 - Guide to compensation circuit and turn IC off.

The best way to determine the comp values is to use Webench.  The LM2577 data sheet has equations to help calculate the values but they are specific to that device.  You can use those equations to get a first pass for the compensation, however Webench will give the best values.

If you need ON/OFF function you can use the LM2586; same as '85 with added features.  Tampering with FB is not a good idea.  You can pull the COMP pin low, but you will not get soft-start when it starts back up and you may get overshoot.

Hi Frank the Genius. (Impressive postings)

Thanks for the reply. I did try the workbench but I didn't get too far with the 300 volt output I required.

I found the design issues column and saw the current was too high. I reduced it a bit and I have a solution.

It came up with LM2587 and LM2588.

I'll look through the sheets and get a few more chips to play with.

I'm trying to use an off the shelf transformer. Is there a way to enter the inductance ? I can click the transformer properties but can't seem to change the inductance.

I'm would like to use a MYRRA  74087 (backwards) Inductance is around 25 uH.

Thanks for you help.

I will ask the Webench team if there is a way to force the inductance value.

However, I would stick close to the value that Webench calculated.

Trying to find and transformer with a specif value and a 8:1 to 10:1 turns ratio off the shelf is wishful thinking.
The Webench does not suggest a turns ratio for the transformer ? It can't be a 1:1 as that would produce 300 v on the primary.

The alternativly is can I get a bit more data on the chips and I'll do my own thing ?

The LM2588 looks like a great choice as it can handle 5 amps. Just ordered some.

Hi Marsheng,

To adjust the custom transformer parameters:

On the BOM page, click on “Select Assembled Off the Shelf Transformer”

 

Bottom left there is the button “Create a Custom Part”

 

 

You can try entering a new value there.

 

Where do I enter the transformer ratio ? Is that NS1toNP?

With my parameters I get 2 options - 2587 and 2588. One runs at 100kHz and the other 151kHz. Why the difference when basically they are the same regulars ? ?

What I was hoping for was, to change the inductance to what I have and then set the frequency to best work with it.

Changing the inductance does not appear to change any values on the schematic. 

Thanks Wallace

Correct, the Ns1toNp is the turns ratio.

The LM2587 is a fixed frequency part operating at 100kHz only and the LM2588 has an adjustable frequency range from 100kHz to 200kHz.

Try the LM2588 design.  If you adjust the Optimization Tuning knob, WEBENCH will create a design at a different frequency at each setting, and it will recalculate your transformer parameters.  Hopefully the transformer values are close to what you are looking for.