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thermal pad grounding for DRV8412

Other Parts Discussed in Thread: DRV8412

Hi,

in my design the digial ground and power ground are separated. my question is that should I connect the thermal pad under the IC to digial ground or power ground? I believe it is not electrically connected to any inside the package. can I connect it to whatever ground that has larger copper area and better heat dissipation?

Thanks

Best regards

Gordon

  • Hi, Gordon,

    We usually recommend using the PowerPAD(tm) as the star grounding point.

    But, it seems your design is different... This may or may not cause problems.

    I would recommend connecting the PowerPAD to the PGND in your case.

    But, I would highly recommend you follow the layout of the EVM, tying all grounds together at the PowerPAD...

    -d2

  • Hi Don,

    thanks for your reply. in my design there are more than one motor driver ICs so the two grounds cannot be joined below any single IC. can you explain a bit why you recommend PGND instead of digital ground? thanks.

    Best regards

    Gordon

  • Gordon,

    There are basically 2 different grounds on the DRV8412.  The GND_x pins are associated with the internal power FETs.  These need to be directly routed into the PowerPAD area as shown in Figure 14 of the datasheet (top layer of the EVM).  Pin 13, GND, also needs to be connected in the same manner.

    Please reference figure 13 where the schematic shows C10 and R5 connected directly to AGND (pin 12).  The ONLY ground connection for AGND should be through the 1 ohm resistor (R7) to the GND pin.  This is clearly shown in the schematic.  By having this small isolation, it avoids disturbances the internal regulator. 

    If you are using multiple devices, connect the PowerPADs of each together.  On our EVM, we only make an attempt to isolate AGND on the DRV8412.  We do NOT isolate DGND (from the C2000 micro) from the rest of the board GND.  DGND is connected to AGND (for ADCs on micro) and then connected to the board GND.  I have attached a schematic that shows a common ground node for our EVM that includes a microprocessor.  The ground is flooded on the board...top and bottom. 

    0743.DRV8412EVM_RevG-Schematic.pdf

     

  • Hi Ryan,

    thanks for your reply.

    I understand that GND_x is associate with MOSFETs and it is connected to power_ground in my design. I think GND (pin 13) is the ground for digital circuits and it is connected to digital_ground in my design. The AGND (pin 12) is handled the same way as in the datasheet, which joins to digital_ground with 1 ohm resistor. as there are multiple motor driver ICs on the board the digital and power grounds are joint together at power supply input on the board. do you have any customer design in a similar way, meaning separating digital and power grounds? does the design work as intended? or you feel that single ground will give the best performance? please advise.

    From your and Don's reply, I will use power_ground as heatsink layer and connected to the PowerPADs. thanks.

    Best regards

    Gordon

  • Gordon,

    I have always preferred (and do so on all our EVMs) a single GND.  Of course, their are different opinions on this and this which I understand.  It is just my experience that a single GND works better with power ICs.