Low voltage inrush current limit

[onekohm]

Hello everyone. I have a circuit which runs at 5V, ~330mA usually. At turn-on I have a spike to above 2A for a short time (on the order of miliseconds, not sure right now exactly how long). I would like to limit this to < 1A. The circuit is supplied by USB, and as it is now it doesn't work on some computers. Others have no problems.

I have looked into thermistors commonly used as inrush current limiters, but what I have found seems to be too high resistance (I can't afford the voltage drop) or rated for too high current (doesn't get hot enough to reach lower resistance).

Any thoughts on what to do? Any other info needed?


Thanks in advance for your help.

 

[IRstuff]

I don't get it. If you're going to redesign the ciruit to put in thermistors, or whatever, why can't you design out the surge?

TTFN

FAQ731-376

 

[onekohm]

Sorry, this is a fix to an existing board. In a small box with little room to add to the fun.

There could be a redesign in the future, but how would you "design out the surge"? I thought that a common way of doing this is with PTC thermistors.

 

[IRstuff]

The first thing you need to do is to determine what is causing the surge. In many cases, there are workarounds, like ramping the power, or delaying turn-on to certain components.

I could see how a thermistor could work, but then you'd need something that would bypass it, once everything is powered up, otherwise, you'd lose a substantial amount of supply voltage across the thermistor under normal operating conditions.

TTFN

FAQ731-376

 

[itsmoked]

Usually the surge is charging capacitors. Reduce them if you can.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[onekohm]

Yes, I think the problem is caps. Mostly on the supply to a switcher that puts out +/-15 Vdc from the 5 V. I could try dropping the values, what's on the board is what the datasheet for the switcher called for.

IRstuff, the voltage drop under normal conditions is exactly the problem with all the thermistors I've found.

 

[geekEE]

This app note shows a few methods

http://www.eetasia.com/ARTICLES/2000DEC/2000DEC13_AMD_AN5.PDF?SOURCES=DOWNLOAD

Glenn

 

[onekohm]

Thanks Glen. I've just skimmed the ap note for now, but I'll be reading deeper into it tonight. Has potential, I like the low part count.