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DC Step Up Chip - what is soft start?

Posted: Mon Feb 20, 2006 11:18 am
by Newz2000
These parts from Maxim all appear to do what I need. They will take a 4v - 5v supply and step it up to 15v. I only need a few ma of current, and as a bonus, these guys are cheap, need few parts and come in a usable size.

Since they're all priced the same, I'll likely use the 8575 because it provides up to 25ma and who knows if someday I need the extra current. The versatility will be nice.

My question is first, does anyone understand what is meant by "soft start?" It looks to me like that means you don't get an instant on voltage, which is something I'd kind of like to have. I'd like to go from 0 - 15 very quickly.

Now I don't really know how quickly I need the on-time to be, but it's not clear to me if this feature is something that can be enabled or disabled.

Yes, I have the datasheet, and I'm basing my analysis on page 5 and looking at the graph on the right... it looks like the on-time to go from 0 - 15v was over 800us. I'll be surprised if that's fast enough. Would someone glance at that and tell me if I'm looking at things correctly?

If anyone has personal experience with this IC I'd love to hear your opinion. I'm using it in a USB powered device where I need up to 15v and uA of current to switch on a FET.

<small>[ February 20, 2006, 11:24 AM: Message edited by: Matt Nuzum ]</small>

Re: DC Step Up Chip - what is soft start?

Posted: Mon Feb 20, 2006 11:37 am
by Chris Smith
Thats pretty much what soft start means.

Not having,... one minute in the off or Zero and then a hard bang on to full power.

The reason you need soft start in some situations is if you use a coil or inductor directly after the supply, the hard start can induce a further over shoot in the windings, possibly causing damage down the line.

<small>[ February 20, 2006, 11:38 AM: Message edited by: Chris Smith ]</small>

Re: DC Step Up Chip - what is soft start?

Posted: Mon Feb 20, 2006 1:05 pm
by Sambuchi
Hello Matt Nuzum

I have used a switching regulator in the past where soft start was used to show the ramping of the output voltage. Changing the softstart cap will change how fast you want the output to ramp to the desired output. Smaller is faster....

I remember I ran into one problem using a switching regulator.... stupid but a problem.. and it was the boost diode.. doesnt look like the chip you selected needs one :)