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Original question:

DRV8308: Unused pin of DRV8308 for unused FG INTERFACE

DRV8308: unused FG function

Yusuke Mukuno
Expert 7395 points
Part Number: DRV8308

Hi expert,

From following E2E session,

Datasheet is descripted as below,

5. Pin Configurations and Functions, FGINN_TACH pin, ~~ If unused, connect FGFB to FG–.~~

However, I found the comment as below in above session.

"If not using the FG interface, you can leave the FGFB, FGINN_TACH, and FGINP as floating. "

Which is correct? Both OK?

Thanks

Muk

  • 0
    TI__Mastermind 18900 points

    Hey Muk,

    In most cases, both are okay. I say this because most people will use the HALLU setting with the CLOCK FREQUENCY mode, instead of the "FG amplifier input". As such, most people don't care or use FG amplifier input pins if they don't use an FG input and they are considered "unused" (which is what I assume you are doing).

    Internally, the FG amplifier input circuit is just an op amp with some of the signal conditioning components integrated within the DRV. So, shorting the negative to the feedback node makes the effective Gain = 1V/V or voltage follower, while leaving them floating creates "infinite gain" or "open or no feedback" scenario.

    In a worse case scenario, if the inputs are noisy due to bad layout, the noise might be able to flip and rail the output of the FG amplifier input. While this should affect anything from a performance standpoint, it will burn some current inside the device because the state of the amplifier will change (so the entire system is less efficient). Again, this is very... hypothetical and not a problem for most customers. As such, we have seen customer systems (and our reference designs, as shown in the post you linked), that can leave the inputs floating with no problems. 

    Best,

    -Cole