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MCT8316Z-Q1: Grounding

Eugene Dolfi
Prodigy 20 points

Part Number: MCT8316Z-Q1

I am using the MCT8316ZT-Q1 in my design and unsure of the grounding as discribed in the data sheet they referance AGND and PGND and in the routing layout example can't tell if these grounds are the same or isolated. From previous usage of TI motor drive devices the grounds are the same, are these ground the same or are they seperate and if the same sould they be connected at the parts thermal pad, all the cartoons for circuit implementation use the same grounding symbol throught the data document which is confusing, can you provide the correct grounding approch for using this device.

Thanks in advance

  • 0
    TI__Mastermind 32642 points

    Hi ,

    Please apologize for the delay in my response.

    The thermal pad is connected to the PCB through an array of thermal vias. By connecting the pad to AGND, these vias become part of a 'quiet' ground system. This provides a stable, low-impedance path for heat dissipation without exposing the thermal path to the massive current spikes associated with the motor.


    If the pad were connected to PGND, those same vias would effectively become part of the high-current return path. Any parasitic inductance within these vias would generate voltage spikes (V = L di/dt) directly beneath the chip, which could couple noise into the sensitive internal silicon.


    Even on a low-cost, 2-layer board with limited heat-spreading capabilities, we still recommend connecting the thermal pad to AGND. In this scenario, please use a small cluster of vias (4–6 vias) to connect the pad to the AGND plane on the bottom layer.


    This approach ensures that the 'clean' ground also acts as a heat sink, effectively keeping thermal and electrical noise away from your sensitive signal traces

    Thanks and best regards,

    Venkatadri S

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 20 points

    Venkatadri, thanks for the response I totally understand the grounding as you have provided but I am still confused so let me detail it in a different way, the design I am implementing has one power source which is a 28VDC and 28VDC return, this main power will supply the VM and PGND pins on the device, I will be using the AVDD to select the operating parameters for the driving device as well as the BB to drive a LDO 5VDC supply, I understand that the BB GND_BK and analog ground should be reference to AGND at some point with a net-tie so that both the AVDD and 5VDC are at the same ground potential of AGND, what I am not getting is the AVDD and BB are NOT isolated supplies and both draw there power from the VM, if that is the case how is the return current developed on the AVDD and BB returned to the main source i.e. 28VDC and 28VDC return, am I missing something that in the ground schema you have describe not referencing PGND, I agree that you would not want the switching noise from driving the motor to couple into the control low level logic but you have a low voltage power and ground that has no reference back to the main source. is the return path provided in the chip for AGND. if not think this will have issue, can you please explain how you grounding support Kirchhoff's law current into the loop equals current out of the loop if AGND is not at some point connect to PGND. I don't think you can have a floating AGND without some path back to the source.

    Thanks

    Gene