Polyswitch-like circuit

I'm looking at an existing design that has a polyswitch device to limit our exposure to shorts on a cable.

The existing part will hold 0.050A and trip at 0.10A which is right for us, but it's thruhole. All the SMD devices I've seen are 0.150 trip, and they tend to have larger untripped resistance. If I can find a suitable SMD PTC, then I'm all for that.

I need to emulate the original part, with low on resistance, with some minimum parts count circuit.

I looked at setting up a CCS with an LDO, but couldn't find anything that had the right parameters. Vin is Vin by a volt or two.


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How about putting a small value resistor in series with a pass FET and that in series with a Schottky diode. The diode protects for voltage of output being greater than the input voltage. The voltage drop across the resistor is monitored for current trip point via a small single opamp which is setup to turn off the FET when current gets too high. With proper component selection you should be able to realize a cutout switch with more accuracy than the PTC, faster acting behavior and voltage drop of less than half a volt in normal operation.

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Yup, thought about that, but the total cost gets up there. Components plus placement cost. It seems like such a part should exist, it's not too far removed from a linear voltage regulator, and I may end up implementing it that way. A 317 or similar set up as a constant current source, using its thermal protection to emulate the polyswitch current foldback.

Linear tech has a part that's close to what I want, but they want more than $3 for it, which is too much by far.

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dbvanhorn a écrit :

How low a resistance do you want?

Hmmm, 5V max at 50/100mA? Murata has some SMD PTCs that should fit the bill.

PRG21BB220MB1RK : 0805 size, 22R nom. trip at 90mA@25°C, 110mA@-10°C, 70mA@60°C

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Fred Bartoli

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