Start up L6981(C33) problems

[Chris_SA]

I'm using the L6981C33DR bug converter to convert 24V (from an SDR-120-24 power supply) down to 3.3V. However, I've had instances where the regulator overheats and "exploaded".

The circuit only needs to supply around 100mA at startup, so the current draw isn't excessive. My suspicion is that there might be a voltage spike at startup that occasionally damages the regulator, but it happens infrequently and I haven't been able to capture it on an oscilloscope.

Has anyone encountered a similar issue or have any insights on how to troubleshoot this?

I've attached a picture of the schematic and PCB layout for reference. Any guidance or suggestions are greatly appreciated, and I'm happy to provide additional information if needed.

 

Chris Salimans

[AScha.3]

>supply around 100mA at startup

And then..? avg. out current ?

 

 

[Chris_SA]

Thanks for following up! The total maximum current is around 500mA, but when the issue happens, it’s far below that, just powering some LEDs and a microcontroller.

[Peter BENSCH]

@Chris_SA What type of inductor do you use?

 

[Chris_SA]

The Inductor used is from Wurth

33uH 744777133

https://www.we-online.com/en/components/products/WE-PD?sq=744777133#744777133

https://www.we-online.com/components/products/datasheet/744777133.pdf

[Peter BENSCH]

OK, I was afraid that you were using a magnetically unshielded coil and would inductively feed switching pulses from the coil back into the FB pin, which could lead to oscillation and thus great instability. But the 744777133 is actually magnetically shielded.

[Chris_SA]

It might have something to do with the inductor value.

I selected the predefined value for 3.3V, but it could be too high or too low for my application?

[Chris_SA]

I found the problem. the 24V was via a connector, when plugging this connector in it made a smal spark what probably made the voltage for a realy short time below or above the max voltage of the chip. Solved it with a 1uh condensator and a TVS over 24V and gnd.