Different inpult voltages lead to destruction???

[nelson_scheja]

Hello,

In my current project I'm using the PF8100 as PMIC. Please see attached schematics. The internal logic, SW1-4 and LDO1-4 are supplied by 3V3, while SW5-7 are supplied by 5V.

When testing the first PCB prototypes, the PF8100 always gets destroyed somehow, drawing a lot of current from 5V. Seems like a shortcircuit. This even happens when all output voltages are inactive (no event on PWRON pin).

Further testing showed that no shortcircuit occurs when I reduce the 5V to about 4.5V. Somewhere around 4.7V, drawn current increases drastically, leading to some fault.

I have searched through the datasheet several times and could not find any single mention that forbids using different voltages for the DC-DCs. 3.3V and 5V are both well within the specified limits, and the block diagram suggests that each DC-DC is an independent unit receiving its own independent supply via SWxIN.

Can you please explain why the IC burns with a 5V supply?

[nelson_scheja]

Hello Jose,

Thank you for this clarification! 

I wonder why such an important information is nowhere to be found in the datasheet or application note? This restriction must at least be noted in the absolute maximum ratings. A further comment somewhere in the operational info section would be even better.

This would have prevented that we now need a fiddly manual rework on dozens of PCBs...

Regards,

Nelson

[reyes]

Hi Nelson,

I sent your feedback to our Applications team and they told me that a note will be added to our documentation regarding this topic.

I apologize for the problems it may caused.

Thanks,

Jose

[nelson_scheja]

Hello Jose,

OK, good. Thanks.

Can you please confirm that using multiple different voltages on SWxIN pins is possible without longterm issues as long as VIN is not lower than the highest voltage on any SWxIN?

Regards,

Nelson

[reyes]

Hi Nelson,

According to our applications engineers, the SWXIN MUST be at the same voltage as the VIN.

It is not recommended to use multiple different voltages on SWxIN pins.

Regards,

Jose

[nelson_scheja]

Hi Jose,

Ehm, OK. So I will need a complete power stage redesign and several routing changes. We're losing several weeks due to that.

Please, please, please do your customers a favor and write such mandatory limitations into the datasheet! This should actually be needless to say. 

Regards,

Nelson

[reyes · Accepted solution]

Hi,

I checked your case with the applications engineers and they told me that you need to connect the VIN and all the SWxIN together.

When SWxIN is higher than VIN, the device will be damage as there is a clamp circuit in chip as below.

Regards,

Jose