Please help me understand strange VN7050AS behaviour

[fyrcyning]

At about 6.3A load the VN7050 switches off the output, which is much lower than I expected given the limit ratings in the datasheet, I suspect my thermal via's are not sufficient and that it is reaching an overtemperature condition, so have made changes to the board design to address this.

My concern is that this may not be the issue, or at least the only issue, because when it switches off the output the voltage drops to about 20mA, as expected, but the IC continues heating to about 100C and only starts to cool when the load current is reduced to below about 260mA, at which point the output switches on again and the IC rapidly cools.

Any suggestions on what may be happening here would be appreciated. 

[MBONA.1]

Hi, the current limit reported in the datasheet is linked to inrush (transient) condition, not steady state.

The VN7050AS is housed in a SO-8 package and the maximum DC current depends on PCB copper area on which the device is soldered. Looking at the datasheet, section 5.2 pag. 29, the best case is to have 4-Layer PCB with RTHj_amb = 45.8°C/W. In that case, considering 25°C ambient temperature, the maximum current the device is able to withstand without reaching thermal shutdown is in the range of 5A. In case of 2-Layer PCB, the performances will be worse and the related current lower.

Considering 6.3A, the device will surely trigger the thermal protection (thermal shutdown) and, according to its protection strategy, the output is automatically restarted as soon as the junction temperature falls down below thermal reset.

Feel free to ask in case of any further information is needed.

Best regards