# Hot-plug protection: series resistor thermal considerations

I have a circuit in place for the Linear Technology LT3502A switching regulator (datasheet here). To prevent blowing up the whole thing during hot-plugging, I have chosen to add a small parallel ceramic cap and series resistance instead of an unnecessarily large electrolytic. As so:

The LT3502A is the 3V3 output version.

I shall be using 100V rated ceramics (1206 size) (or perhaps 50V is also okay, to meet the maximum input voltage of 40V for the chip).

I was just wondering what size resistor I should slap on there. I suppose I should be fine with an 0603 (0.125W)?

My logic being that the maximum output power of my chip at full load (which it will never reach) is 3.3V*0.5A = 1.65W. Multiply that by say, 2, to account for power losses and an additional safety multiplier (~ 3W). Presuming VBUS >= 12V (not going to put a 1W resistor there), the worst-case current through R7 is 250 mA, implying 0.0625W dissipation. An 0603 rated for 0.125W should be okay, I think? Just making sure not to release any magic smoke :)

• I'd look at direct connection for VIN, and have an RC network on the ~SHDN pin so that startup is delayed, but perhaps I misunderstand the problem you are trying to solve. – Jasen Mar 25 '18 at 0:58
• @Jasen the problem is a resonant circuit when hot-plugged, which can cause the input voltage across the ceramic cap to even double - destroying both the cap and the regulator. The solution is either an electrolytic, or this. – Shreyas Mar 25 '18 at 9:46