L6981(C50DR) dies on hard switching of input power

[HMüll.4]

Hello everybody,

I'm using an L6981C50DR for supplying a small board with 5V. The schematics is attached and should be perfectly conform to what is described in the specification from https://www.st.com/resource/en/datasheet/l6981.pdf as "Figure 4. Basic application (fixed VOUT versions)" on page 3. I'm supplying 24V to it. Everything works well when I turn on and off the power via the switching power supply that provides these 24V. But when I reboot my board the "easy way" by pulling out and reconnecting the +24V wire, the L6981 dies immediately. After this, pin 4/Vout is permanently at GND letting me assume something has melted within the IC.

When measuring the switching power supply with an oscilloscope, it provides a small peak in such a scenario but this peak does not go beyond of 28V - which is far below the 40V the L6981 should be able  to deal with.

So...any ideas what the reason for this problem is and how to solve it?

Thanks!

[Peter BENSCH]

With switching regulators, it is always helpful to include the layout as an image.

However, in this case, I am also certain that your problem is due to an unsuitable coil L3. The keyword here is saturation current, which is 1.3A for your NRS6028T 330MMGJ.

I quickly simulated your design in the eDesignSuite and came up with peak currents of around 1.66A, which are well above the limit. Even if you may not need the 1.5A permanently, the output capacitor C38 must be charged, which the controller wants to do enthusiastically and at full current – until the magnetic field collapses when the saturation current is reached and only the DC resistance is effective.

So you will have no choice but to dimension the coil accordingly (recommendation: saturation current > 2A, better 2.5A).

Hope that helps?

Regards
/Peter

[HMüll.4]

OK...I'll replace the inductivity, try again and let you know!