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LMR14050: multi rail PSU layout placement

Benjamin
Intellectual 290 points
Part Number: LMR14050
Other Parts Discussed in Thread: TPS54540

I am designing a PSU PCB with 3 different rails, but I’m unsure where to position the layouts for each of the 3 rails for best results.

Up the top are 3 black lines representing the rails that the external devices will draw their current from. I want to have the components and most circuitry on the top layer, and the bottom layer reserved for a universal ground plane.  

Rail 1: +12V @ 3A 

- LMR14050

- large output current

- Needs to be low noise

- doesn’t add a large amount of switching noise to rail

Rail 2 +5V @ 1A

- LMR14050

- low output current

- can be higher noise

- doesn’t add a large amount of switching noise to rail

Rail 3 -12V @ 1.5A (buck-boost-inverter)

- TPS54540

- large switching current 

- needs to be low noise (preferably same as +12V)

- Has a larger amount of switching noise on rail due to BBI design. 

Some restrictions:

- position of output rails, power input and board size/shape can not be changed.

- Board is limited to 2 layers.

- all components must be on the top layer

So my Question is:

Where should I position each rail for lowest noise on the +/-12V rails? (considering that the +15V power and ground inputs are on the bottom left of the PCB)

 


  • 0
    TI__Expert 8245 points

    Hi,

    Actually you just need to put every rail very close to each of their load (or external device connecting terminal), with input capacitors very close to Vin and GND pins, that should be fine. Thanks.

  • 0 in reply to Iven Su
    Intellectual 290 points

    Hi Iven,

    Thanks for the info. I have input capacitors places as suggested by you.

    But is there a preference in which layout should be closer to the 15V input, considering which one has higher current, and which is more sensitive?

    I'm just thinking of interference on the ground plane returning back to the ground input pin.

    thanks again

    - Ben

  • 0 in reply to Benjamin
    TI__Expert 8245 points

    Hi,

    If you input capacitors very close to devices, then input lines will have almost DC current flows which are of low noise. BTW, you should keep both input +15V and GND traces wide enough to ensure the current capability.