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Grounding a HTSSOP DAP chip

e14 Contributor

Hi,

I am interested in using a Texas Instruments led driver in a project. Eagle

has a part that I can use (HTSSOP38DAP footprint) found in the Texas

instruments library (texas.lbr). However, even after some concerted

attempts, I am uncertain how to connect the PTH on the part to my ground

plane. I would appreciate any suggestions, guidance, or pointers me to

documentation to help me out.

 

Having fun learning the ins and outs of Eagle,

Thanks in advance,

Shiva

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  • in reply to e14 Contributor

    Hi Shiva,

     

    You're welcomed.

     

    As I can see on the attached screenshot, you're bumping into a clearance limitation (also with trace for pin 35). You made a 4x10 matrix of holes, where each hole includes also a bit wider restricted area. This is the reason why EAGLE hadn't connected GND plane with the polygon - it avoids keep-out regions.

     

    The solution would be to swap these holes with vias.

     

     

    Additionally, check tStop layer if it contains a rectangle and, if needed, (according to your heat dissipation & production goals) delete it.

     

     

    Take care,  Ivan.

  • in reply to e14 Contributor

    Hello Ivan,

    Thank you very much for the detailed steps.  I appreciate it!  It all seems to work, but with a few small hiccups. If you have a few more moments perhaps you can give me your feedback on one remaining issue.

     

    I followed you instructions, but because my ground polygon in on the bottom layer:

    1. I added a bottom layer SMD to my driver chip package.

    2. I named this bottom layer EXP2 (the top one is named EXP as you thought), and

    3. After opening my driver chip as a device, I connected the THERMAL pin to EXP2 pad.

     

    I want to tie the THERMAL pin to the bottom layer ground polygon. Below is a snapshot of the the TLC on my board. My question is why is my polygon (named GND, and which I would like to merge with the EXP2 pad) prevented from extending under the chip? I can create a wire to extend from EXP2 and the ground polygon (you can just make out the ratsnest wire from pin 33), but somehow the polygon itself is kept out.

     

    Best Regards,

    Shiva

    PS I'm using a TLC5951. The datasheet suggests connecting the thermal pad to the ground plane to help dissipate more heat.

     

    image

  • Hi Shiva,

     

    I worked with one TI LED driver recently. In the chip's datasheet I didn't find any request/recommendation/etc. that the thermal pad is to be connected to the ground plane. They are specifying just the cases when the thermal pad is soldered or not to "floating" copper area.

     

    Nevertheless, If you'd like to solder it to a ground plane, you have to modify the library first. In the Library editor open the related library and then:

    1) Open your driver chip as symbol and add a new pin (name it THERMAL, for example).

    2) Open your driver chip as a device and click Connect button.

    3) Link your new THERMAL pin with (probably named) EXP pad.

    4) Save the library.

     

    In the Schematic editor open your schematics file and:

    1) Click Library -> Update All option.

    2) Connect new THERMAL pin to the ground.

     

    You are now ready to route thermal pad in the Board editor...

     

     

    Best wishes,  Ivan.