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BQ2947: PWPD pin connection in the circuit

Shubham Kumar1
Prodigy 75 points
Part Number: BQ2947
Other Parts Discussed in Thread: BQ77905, BQ77915

I was going through the datasheet of over voltage protection IC bq2947 and there I found a PWPD pin at the bottom plane of the IC.

What's the use of this pin? Is this internally connected to VSS pin of the IC?

What if I left it floating as it is without connecting it to VSS pin? Does it impair any feature or safety of the IC?

In cases of series combination of cells greater than 4, say 13S/14S, in those cases is it necessary to connect PWPD pin with VSS for all the ICs used? If not done so, what will happen to circuit performance? 

Please help me understand this. Thanks

  • 0
    TI__Expert 7035 points

    Hi Shubham,

    This pin is for grounding the thermal pad. It is recommended to connect this pin to VSS, as shown Section 6 of the datasheet (See PowerPAD row).

    I would not recommend leaving this pin floating, as it needs to provide reference for VSS for accurate cell voltage measurement. This could lead to unexpected faults. 

    If you wish to have a 13S or 14S system, I would recommend one of our higher cell count ICs such as the BQ77905 or BQ77915. Those devices have detailed diagrams in their datasheets to describe what a multi-cell circuit would look like. 

    Please let me know if you have any more questions.

  • 0 in reply to Shawn Hinkle
    Prodigy 75 points

    Hi Shawn,

    Thanks for your response on the query. However, I didn't get the answer I'm seeking to understand the functionality of the PWPD pin on the IC. As mentioned in the datasheet that PWPD pin should be connected to VSS pin, I get that but what I'm unable to understand is why?

    The primary function of that pad in my opinion, please correct me if I'm wrong, is to dissipate the heat generated in the IC. So, I'm wondering how the voltage measurement feature, to which VSS is working as reference, gets affected even if PWPD pin is floating? If PWPD is that much important for the measurement I suppose then, there must already be an internal connection in the IC. So, please help me understand, why is PWPD pin is recommended to be connected with VSS and what happens if left floating?

    Thanks 

  • 0 in reply to Shubham Kumar1
    TI__Expert 7035 points

    Hi Shubam,

    It is hard to speculate exactly what might go wrong if the PWPD is not connected, but we do mention several times in the datasheet that it is strongly recommend. We cannot guarantee correct functionality if the recommended configuration is not followed. There is an app note that details the attachment of these types of devices, and mentions the thermal pad. I would highly recommend reading through this document for more information. The document can be found here. 

  • 0 in reply to Shawn Hinkle
    Prodigy 75 points

    Hi Shawn,

    I went through the document you suggested and couldn't find any relevant information about the electrical connection of the thermal pad. I will be happy if you could let me know the basis of the recommendation for the electrical connection of thermal pad and VSS pin on the IC. I think the same will apply to any other IC as well. 

    Thanks 

  • 0 in reply to Shubham Kumar1
    TI__Expert 7035 points

    Hi Shubham,

    In the document, it is mentioned: "The standard QFN and SON package has an exposed pad that enhances the thermal and electrical characteristics, enabling high-power and high-frequency applications." As I mentioned before, without this connection, we cannot guarantee correct functionality of the device. I could theorize what might occur, but we have not done any testing of this scenario. I would highly recommend this connection. 

  • 0 in reply to Shawn Hinkle
    Prodigy 75 points

    Hi Shawn,

    I would really be interested in the implications of not doing that. Please put forward your opinions related to that. 

    Albeit I will connect PWPD to VSS for proper working as you told. Your opinion is for my subject knowledge enhancement.

    Thanks for helping me through.

  • 0 in reply to Shubham Kumar1
    TI__Expert 7035 points

    Hi Shubham,

    I am unable to describe exactly what would occur if the PWPD is not connected to VSS, as we have not tested it. I would imagine the device would not work properly, but I am unsure of the details.