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TIDA-00913: Questions about Connecting AGND to PGND of LMG5200

Hui Chen
Intellectual 730 points
Part Number: TIDA-00913
Other Parts Discussed in Thread: LMG5200

In order to avoid the interference of power part to signal part, NT1 is used as the connection point form AGND to PGND.In order to make the driving signal electrically stable relative to the power ground, two resistors, R 1 and R 2, are placed in the design.

I have the following questions. I look forward to your help.

1.Are the above descriptions of NT1, R1 and R2 correct?

2.Why not add three resistors and put  under every LMG5200 to reduce the area of A2?

3.Why choose NT1 instead of zero ohmic resistance for grounding control power supply?

4.Why NT1 is not closer to 48V input to reduce the effect the fluctuation of PGND potential level by high current ? For example, short circuit current.

Thank you and look forward to your reply.

  • 0
    TI__Expert 3225 points
    Hello Tina,

    Regarding the connection between PGND and AGND, actually the ideal case is not keep them separate without connection. This is because we want to have Kelvin connection inside the FET and we do not want to create external connection to allow a ground loop. PGND and AGND are connected internally as Kelvin connection. But if you need to make them the connected due to layout constraint, I think single point connection is actually better. Either net tie or resistor are fine. Hope this helps.

    Thanks and regards,
    Lixing
  • 0 in reply to Lixing Fu
    Intellectual 730 points

    Thank you for your reply.！I don't have this chip on hand.However, as can be seen from the data sheet image, AGND is not connected to PGND inside the chip.Please help confirm whether the chip is connected internally. If there is no connection, where is the best way to Layout?

  • 0 in reply to Hui Chen
    TI__Expert 3225 points

    Hello Tina,

    The diagram that you have shown is quite simplified. Actually PGND and AGND have connection inside the IC to create a Kelvin connection. Therefore, you can choose not to have any external connection, which is the best actually. A single point connection between AGND and PGND would also be okay. But please try to avoid large portion of merge between the two since this is away from the idea that we want to separate the two grounds and create Kelvin type of connection to avoid interference from power loop noise. 

    Thanks and regards,

    Lixing