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LMZ35003 power and ground planes

MADIWALESH PATTAR
Prodigy 230 points

hi

there are two GNDs:  AGND and PWRGND as per recommendations both have to be connected to each other through pins no 8, 9 GND_PT. out of these two GNDs which has to be connected to main PCB GND

1) out of these two GNDs which has to be connected to main PCB GND ?

2) PWRGND is the GND where ground bounces may happen , should the area of that be minimised? and connected to AGND through pins 8,9. then AGND is connected to main PCB GND??

3) as per datasheet page no 22 layout guidelines instructions point no 4 it says pour AGND beneath the IC. but recommended layout shows poured PWRGND.??

should AGND and PWRGND be separated in top layer only??

  • 0
    TI__Mastermind 28020 points

    Connect PGND to the main PCB ground. The PGND is the return current path for the switching current of the supply.

    The AGND is the zero ground reference for the internal analog control circuitry. The AGND should be kept as quiet as possible.

    The AGND must be connected to the PGND at one point (pins 8 & 9) to avoid ground loops and noise.

  • 0 in reply to J Arrigo
    Prodigy 230 points
    Thank you,
    below the ic i should poor a AGND plane or PWRGND (=main PCB GND) palne?
    AGND in top layer is only enough??
  • 0 in reply to MADIWALESH PATTAR
    TI__Mastermind 28020 points
    Connect pin 37 to an AGND copper island on the top layer of the PCB. Adding vias to an internal AGND layer or copper area will help with thermal performance.
    PGND should also be present on the top layer of the device for connection to the PGND pins and input and output caps. Vias are also required to connect to PGND internal layers for current carrying and thermal performance.
    AGND should only be connected to PGND at one point on the top layer and should not be connected to one another on internal layers.
  • 0 in reply to J Arrigo
    Prodigy 230 points
    thank you so much..:)
    in nut shell
    1) PWRGND can be taken as connection to main PCB GND.
    2) AGND is the noisy one.
    3) Never touch PWRGND and AGND except at pins 8,9 in top layer. not even on interrnal layers.
    4) "polygons or copper areas" of PWRGND & AGND in top layer can be repeated on internal layers for better thermal perforance taking care they do not touch eah other as in top layer.
  • 0 in reply to MADIWALESH PATTAR
    TI__Mastermind 28020 points
    All are correct except for #2. AGND is the zero ground reference for analog circuitry and must be kept quiet. PGND is the noisy, switching ground.
  • 0 in reply to J Arrigo
    Prodigy 230 points
    then its better to connect AGND to main PCB GND as its quiter one and will not disturb my main PCB GND??