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Original question:

LM25088: thermal pad connection

LM25088: thermal pad connection in inverting-buck-boost configuration

Benjamin
Intellectual 290 points
Part Number: LM25088

Hello,

I am designing an inverting PSU with the LM25088. (+15V In, -12V out).

I asked a similar questions a couple of months ago, but I am curious about where to electrically connect the thermal PAD, and how to think of the ground plane in this type of setup.

1) As I understand it the thermal PAD is NOT electrically connected to a pin, so is there any advantage to connecting it to GND as opposed to the -12V out?

2) I am new to this type of inverting-buck-boost setup, and I am wondering about how to think about grounding and planes. Should I treat -12V out as if it were main the ground plane?

This would make my electrical and thermal connections much easier, especially if I could connect the PAD to this plane. 

Thanks!

  • 0
    TI__Guru 72625 points
    You are asking good questions.

    First, the PAD should always be connected to the ground pin of the LM25088. That is true for most, if not all, regulators.
    The ground pin(s), and PAD, must always be at the most negative potential. When using a buck as an IBB that will be the negative output; -12V in your case. So, the ground pin of the LM25088 and the PAD will go to -12V.

    For IBB applications, we have found success using the plane as the negative output. This makes the layout the same as a buck, and as you mentioned, makes the connections much easier. Since this plane is connected to the PAD it will act as the heat-sink plane, just like a buck. Of course this plane will have to be isolated from the system ground, since it is your -12V.
  • 0 in reply to Frank De Stasi
    TI__Guru 72625 points

    I am closing this due to inactivity.