another current limiter problem

John Larkin · 2015-09-19T00:49:34+00:00

I spend too much of my life worrying about current limiters. This time, I have a 5 volt power supply, powering some logic that drives a customer load. The load should be 50 ohms, but someone might decide to short it, and I don't want to fry the driver or drag down the +5 rail. So I need a low dropout (under 0.1 volts maybe) roughly 120 mA current limiter. I need two, actually, for two output drivers, and I don't have much room or budget. I can't find any commercial limiter chips or LDOs that can do this. The limiters mostly work at USB levels, 1 amp roughly, and the LDOs have wild short-circuit current spans, like 3:1 or something. Looks like I'll have to do it myself. This isn't bad: It's a limiter, not a regulator, so the loop doesn't need to be stable. I think. (I think I need some new whiteboard markers, too.)

Sep 19, 2015 21 Replies

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Winfield Hill 10 years ago

If you can add a pair of resistors (in the source feedback) for hysteresis, the MOSFET will switch cleanly. You can use a sot-23 package, e.g., zxmp6a17e6, or two MOSFETs in an so-8.

Thanks, - Win

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John Larkin 10 years ago

I guess it's an artistic decision: if the customer overloads the output, should the output current limit, or should it switch off and retry periodically?

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