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LMR23630-Q1: Placing the DC/DC on the Bottom Layer instead of Component Layer

Pradeep Mohan
Intellectual 725 points
Part Number: LMR23630-Q1
Other Parts Discussed in Thread: LMR23630

Hello TI,

we are developing a telematics device. In order to save space, I was wondering if we could place the LMR23630-Q1 and the related external components on the BOTTOM layer instead of the TOP component layer. 

We are considering either of the following stack ups shown at the bottom of this post, depending on our signal density either a 4 or 6  layer board.

The idea is to have the RF components on the top layer and the DC DC convertors and other non RF components on the bottom layer, hopefully also reducing board area in the process. 

1. What does it mean to the switching signals that would have the VCC plane as reference plane instead of a GND?

2. Should additional capacitors be placed to enable the return current to follow from the VCC plane to GND?

3. Will I be able to use the TOP side for other non RF components?

Is this even worth the effort to save space on the board?

  • 0
    TI__Expert 6245 points
    Hello Pradeep,

    Since the layout for the LMR23630 has all high current and high frequency traces on a single layer, placing the converter on the back side of your board will work. You should pay attention to two things while laying out your board:

    1) Even though the converter is over its input rather than its ground plane, it is important that the copper under the LMR23630’s input loop, ground to the closest input capacitor to the VIN pin, be solid. Even if the voltage found on the plane below the input loop is not VIN or GND, it should be solid, which will reduce the inductance of the input loop.
    2) Also, adequate connection between the ground side of the output capacitors and the ground plane should be provided to limit output ripple.

    Regards, Robert