TPSM84209: Layout Recommendation for Switch Node

John Griffith

Part Number: TPSM84209

Hello,

In the Pin Function table of the datasheet it recommends the following for the SW (switch node) pins: These pins are connected to the input side of the internal output inductor. Do not place any external components on these pins or tie them to a pin of another function.

But in the layout the pins are tied together with a small pour. So while the note doesn’t tell you to tie them together, the layout example is not going against the pin function table's recommendation. My customer is asking if it is recommended to tie the nets together like the layout picture (Fuigure 34) in datasheet. It looks like a good idea but I wanted to get confirmation from you all.

Thanks and happy holidays!

John

over 4 years ago

0 Ruby Li over 4 years ago

TI__Expert 7520 points

Our US team will feedback to you after holiday. Thanks.

Thanks,

Ruby

0 John Griffith over 4 years ago in reply to Ruby Li

TI__Mastermind 21800 points

Hi everyone,

I hope everyone had a great holiday! I just wanted to follow up on this so it doesn't get lost!

Thanks,

John

0 Jean Picard over 4 years ago in reply to John Griffith

TI__Intellectual 1125 points

Ruby Li

The data sheet recommendation is about NOT tying SW to other external signals.

It is OK to tie both SW pins together, they are same electrically speaking.

Regards

Jean

0 John Griffith over 4 years ago in reply to Jean Picard

TI__Mastermind 21800 points

Hi Jean,

I agree, and part of my reply back was that by connecting the pins together with a small pour on the PCB, you have not placed any external components onto this net or tied that net to something else.

What we are looking for is a recommendation. If they are doing their board layout right now, should they connect the pins together with a pour or just have floating pads connecting the pins to the PCB? My guess is that a pour adds another low inductive path between the pins in addition to whatever is internal to the device so it can only help. Do you agree? Should I tell my customer to connect these pins like in the example layout with a small pour?

Thanks again,

John

0 Jean Picard over 4 years ago in reply to John Griffith

TI__Intellectual 1125 points

John Griffith

I don't think it really matters, the low impedance path is through the "triple post" pin #4.

I suspect that pin #5 is there to give closer access to switch node for scope measurements.

I left a message earlier today to the team who designed that module to get more details about it, I'll let you know when I get an answer.

Regards

Jean

0 Jean Picard over 4 years ago in reply to Jean Picard

TI__Intellectual 1125 points

John Griffith

Latest info is that the separate SW pin was defined only to make the pins symmetrical on both half of device.

Internal layout shows that they share same copper area.

Final recommendation: for best thermal connection, the SW pins 4 and 5 should be connected on the host PCB using copper pour.

Regards

Jean

0 John Griffith over 4 years ago in reply to Jean Picard

TI__Mastermind 21800 points

Thanks for taking the extra time to chase this down!

John

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TPSM84209: Layout Recommendation for Switch Node