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LM7480-Q1: Layout for minimum conductor clearance

af36
Prodigy 250 points

Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM74500-Q1

Hello,

I am using the LM74801QDRRRQ1 in a reverse battery protection / ideal diode application (snapshot below/attached) in an application up to 45V and 70A. Note R117 is not stuffed by default. IPC-2221B clearance requirements are 15.75 mils between component leads/terminations at this voltage (and 23.62 mils between external conductors). After progressing to layout, I have run into some issues with this schematic:

  1. The datasheet for the LM7480-Q1 says that if VSNS and SW are not used, short them and connect to VS. The OV pin is connected to GND because the secondary HGATE is not used. This places 45V between SW and OV at well less than 15.75 mil. Can any of these VSNS, SW, and OV connections be changed to maintain 15.75 mil clearance between 45V signals? Similarly, if R117 were installed then OV would be too close to the EN/UVLO connection.
  2. The Figure 9-2 application in the datasheet shows OUT pin 9 grounded. This places OUT next to the 45V connection at VS pin 10. Similar question: can the OUT pin 9 be connected differently?

Alternative questions to the above for my application may be: what would happen if VSNS or SW were grounded? What would happen if OV were connected to VS? What would happen if OUT were connected to VS?

  • 0
    Prodigy 250 points

    One clarification: this is for a reverse polarity application but not a battery application. An off the shelf AC-to-DC bulk front end converter is used to supply the input voltage up to 45Vdc.

  • 0 in reply to af36
    TI__Guru 57737 points

    Hi ,

    Thanks for reaching out to us. I will work on your query and get back to you by early next week.

  • 0 in reply to Praveen GD
    Prodigy 250 points

    Thanks Praveen.

    Looking at this a little closer, the only way to avoid clearance issues that I can see is to do the following:

    1. Leave VSNS and SW or OV disconnected (to allow clearance between low-voltage EN/UVLO and high-voltage A and intermediate pins)
    2. Connect OUT to VS or leave open

    Based on the functional block diagram, it seems like OV needs to be grounded for my application to put the OV comparator in a known state and keep HGATE off. It seems VSNS and SW might be able to remain disconnected / float (or short externally but no connection to VS or GND)?

    I'm not sure if it matters whether OUT is connected to VS or left disconnected, since with OV grounded the HGATE driver will remain off.

    An alternative schematic snapshot is shown below, with a question as to whether OV should be connected to VS or left floating. VSNS and SW are externally shorted below (but no other connections), but could alternatively be left floating independently.

  • 0 in reply to af36
    TI__Guru 57737 points

    Hi,

    In case you want only reverse polarity protection and no other protection feature, I would recommend you to have a look at LM74500-Q1. 

    LM74500-Q1 has greater pin to pin clearance, smaller package, lower cost when compared to LM7480-Q1 

  • 0 in reply to Praveen GD
    Prodigy 250 points

    Hi Praveen,

    I originally looked at the LM74500-Q1 but there is no stock available. I have already secured initial stock of the LM7480-Q, and unfortunately cannot wait for LM74500-Q1 stock.

    So sticking with the  LM7480-Q, seems like I could connect OUT to VS and float VSW/SW (externally shorted or not) for my application, correct?

    Thanks,

    Aaron

  • 0 in reply to af36
    Prodigy 250 points

    Hi Praveen,

    Below are two specific schematic option examples I am thinking of. I am assuming OUT should be connected to VS rather than float.

    Option A (externally short VSNS/SW but leave unconnected from anything else). Alternately, as stated previously, these two pins could be left disconnected from each other as well (completely floating).

    Option B (added R125/R126 voltage divider to keep VSNS/SW pin voltages < 30 V from adjacent pins). This option is less preferable because it requires the two additional resistors in an already tight layout:

    Please let me know if either of these (or something else) would work with the LM7480-Q1. Or if either of these designs would be problematic for the LM7480-Q1, please let me know that and I will adjust the best I can utilizing the original schematic in my first post. We follow IPC standards as best-practice but it is not specifically required for our application.

    Thanks much!

  • 0 in reply to af36
    TI__Guru 57737 points

    Hi,

    Can you let us know your end application ?

    Also, can you confirm if you do 'conformal coating' as this relaxes the pin spacing need for the exposed conductors..

    The Option A in your above comments is an acceptable solution.

  • 0 in reply to Praveen GD
    Prodigy 250 points

    Hi Praveen,

    End application is an IVD accessory for medical laboratory use. No conformal coating.

    Thank you for the follow up; unless you have anything further I will proceed with Option A and mark this thread as resolved.