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TPS7A89: AGND Routing questions

Part Number: TPS7A89

Hi there, 

I have a few questions regarding the AGND routing for the TPS7A89. Currently in routing....

 In the board layout section of the datasheet, it is called out to have an isolated AGND for the noise-reduction caps and lower feedback resistors. What I wanted to clarify was the underlined comment below: “These components must have a separate connection back to the thermal pad of the device”.

  • Does this “separate connection” need to be isolated from the power ground? In the layout example (Figure 68. of datasheet), it appears to be connected to thermal ground when connecting back to the thermal pad (I’ve drawn a magenta/red line below). Wouldn’t this layout example suggest that AGND is tied to thermal ground and not separated? Perhaps I am just misinterpreting the image.
    • Is the AGND connection to GND done “through” the thermal pad?
      • Ex. Bottom feedback resistor – AGND – Thermal Pad – GND
    • Does this “separate connection” mean there should be separate connections for the components from each other?
      • Ex. Noise-reduction cap AGND to thermal pad is not connected lower feedback resistor AGND except through thermal pad?
  • I tried looking at the eval board (TPS7A8901EVN-853) as a reference and the schematic does not call out AGND specifically. What I see are:
    • There’s a GND island for the bottom feedback resistors (jumpers J11 and J10 to be specific). This appears to be routed to thermal pad directly on layer 3.
    • The noise-reduction caps (C15 and C16) GND appear to be routed to thermal pad directly on bottom layer.
  • Is there any difference on whether AGND is done via trace(s) or a plane?

 Can you provide guidance on the AGND routing? Thanks!

 

  • Hi Kannan,

    I'm looking into this. I will get back to you in 1-2 business days.

    Regards,

    Nick

  • Hey Nick, 

    Thanks for looking into this. Just wanted to follow up and see if there were any updates. 

    Given that I have available board/layer space at the moment, I decided to route the AGNDs from Cnr/ss and R2x separately to the thermal pad, seemingly the same way the eval. board does so. 

    Of course, if you can provide some official response, that'd be great. 

    Thanks again,

    Nick

  • Hi Nick,

    I will continue support for this in email. 

    Regards,

    Nick