Switching regulator current protect

Jan 11, 2006 2 Replies
L

I am working on a board with 5 components and just as many voltage requirements (1.2V,1.5V,2.5V,3.3V and 5V). So, I really need to get the component costs down. For over-current protection, there are current sensing resistors (0.022 ohm) at the input stage. I found some 3.5A fuse with same resistance and cost 1/3 as much (economy of scale?).



Although the fuse won't protect at 1A, the switching regulator should kick in to shut down the circuit. Resistances are same after all. Would it be safe to use the fuse instead of current sensing resistors?



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W

Read the datasheet to find the amount of time it takes for the fuse to blow, not to mention the series resistance of the fuse. In general, a fuse will only blow quickly if the overcurrent is quite a lot higher than the fuse rating.

Really, about the only thing fuses are good for is preventing fires; they're not very good at preventing damage to equipment that contains semiconductors.

If you want to get the component costs down, maybe you should hunt for alternate parts that have more compatible voltages...

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P

The material used in the fuse probably has a fairly high tempco, so it will limit the current to a lower value at higher temperature. Maybe "this is a good thing"! If absolute accuracy is not critical, I see no problem, and this also provides catastrophic protection in case the regulator shorts out.

Paul E. Schoen, President P S Technology, Inc. Cockeysville, MD 21030-2824

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