LM5067 Reverse Voltage Protection

Hi Subutay,

I'm glad you found the LM5069 application note for reverse voltage protection. FYI - we released an EVM based on that application note for the LM5069:

However, for negative voltage hot swaps such as the LM5067, we recommend a simple scheme which uses an NFET, 2x resistors and a zener:

The NFET will be on when correct polarity is applied, but will shut off when reverse voltage is applied. A similar scheme can be used for positive voltage controllers, but then a PFET would be required which is typically more expensive.

For the LM5067 we have not tested a diode based solution.

Thanks,

Alex

Hello Alex,

It is good to know a reverse input protection with additional NFET with 2 x resistor solution for the Negative DC input supply.

But even though, it is not too much power consumption with additional NFET. It will spend some additional power dissipation during normal On condition as below.

POWER DISSIPATION for reverse protection FET = Ron x normal current.

SO if there is additional solution with out NFET, it will be helpful. Especially, if we can know a solution with tiny diode that is applying to LM5069, it will be more helpful.

Please comments us.

Best regards,

Vince

Alex,

Thank you for the fast response. Do you have any plan to test?

Best Regards,

Vince

Hi Vincent,

In either case when using a hot swap controller, a second MOSFET is needed in order to protect reverse polarity. This is due to the body diodes of the MOSFET.

With the LM5069, if only a single MOSFET is used, then the diode from IC_GND to GND may protect the LM5069, but it will damage the downstream loads.

I drew up a diagram w/ the body diodes, reverse polarity event and path of conduction (and blocking) to help explain:

So in either case, back to back FETs are needed. For the LM5069, a PFET can be used instead, before the hot swap on the high side to perform the same function. But we understand that PFETs can be very expensive, and so replacing with an NFET + diode is the cheaper solution.

For low side -48V, all that is required is the NFET + resistors to begin with, which is already a cheap and robust solution.

Thanks!

Alex

Hi Josko,

I believe there are some alternative options for how to draw the arrows for NFETs depending on whether you show a separate pin for the body connection.

I may be wrong but I believe the symbols above are valid but perhaps not as common as the symbol you are referring to which shows a body pin (4 pins total - and the arrow points inward towards the body for an NFET).

Thanks!

Alex

Hi Alex,

yes, you have right, but  a little bit confusing since all datasheets from TI use Symbol wehre arrow shows "in channel" for NFET, actually N-MOSFET.

Also the negative controller LM5067 have been used for positive rail, and positive Controller LM5069 for negativ rail. Again a little bit confusing  for me...

What do you think about LM5069 for reverse polarity and inrush current limitatinon in automotive "cold cranking" application?

Whether it will be possible as LM5069 has the min Vin 9V?

BR,

Josko