I have designed a step-down converter PCB (4-layer) with MCP16301. It converts voltage from 24V to 5V.
When I connect it to power source and then connect a 100ohm resistor (50mA@5V) it works normally.
But when the load is applied before I power it up MCP16301 dies instantly and shorts Vin to Vout (Vout is 0.5V lower than Vin - probably a voltage drop across internal mosfet).
No other components are damaged. When MCP16301 is replaced it works again. I've already destroyed 3 of them.
Schematics is an exact copy from datasheet:
Components:
PCB layout (gnd plane hidden):
with ground plane:
I've been troubleshooting this for a couple of days so any ideas/solutions would be greatly appreciated.
Note: PCB was powered by connecting an alligator clip (not a proper switch) and there was a small spark. Power source is a Chinese lab power supply.
Edit:
I followed Andy aka's advice and added a zener diode (30V) but the chip still got fried.
Luckily i was able to get an oscilloscope this time. Here are the photos (because of stackexchange link limitation i can't post clickable links)
(blue - inptut voltage, yellow - output voltage; 10V/div)
There is a 2us 60V spike at the beginning (despite 30V zener diode) so I guess zener diode is not fast enough?
Edit2:
I added an electrolytic capacitor (100uF, ~0.5ohm ESR) as suggested by @Peter Smith. It now works and the voltage is also a lot cleaner (ignoring some high frequency ripple):
