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Original question:

TPS40170: Check the TPS40170 schematic

TPS40170: What is the maximum current into the Vin pin?

Bill Argabright
Prodigy 250 points
Part Number: TPS40170
Other Parts Discussed in Thread: LM5145

The datasheet mentions an Iq of 4.5mA max, but that's with the drivers not switching.  I'm looking for the maximum current this pin could sink (with switching drivers) because I'm looking to confirm that the 1-ohm series resistor I have in line with the Vin pin is appropriately sized.

If you can't give me a definitive number, can you help me understand all the things that go into finding it?

Thanks!
-Bill

  • 0
    TI__Mastermind 49317 points

    Hi Bill,

    The DC current associated with driving each FET is Qg*Fsw, where Qg is the gate charge at the applicable gate drive amplitude (8V in this case). Note, however, that VIN to SW is used for high-side current sensing, so a 1 Ohm resistor in series with VIN may affect this and is generally not recommended.

    Note: the LM5145 is the next generation of controller (with the same control architecture) and is preferred given its advantages of better deadtime performance, lower quiescent current, easier implementation, etc. Please use the quickstart calculator available by download from the product folder to assist with component selection.

    Regards,

    Tim

  • 0
    Prodigy 250 points

    Tim,

    We only added the 1-ohm resistor because that's how the EVM is built :(  And like the EVM, we also have 1uF of decoupling on the Vin pin.

    The 2 FETs together have a worst-case Qg of 30.1nC (with 21nC being worst-case for 1 of the FETs), and we switch at ~350kHz.  So even if the FETs switch together (which doesn't happen), it only adds ~10mA to the Vin pin, and therefore a ~15mA total current and ~15mV total drop across the 1-ohm resistor.

    Do you have any guidance for how "off" things can be before it causes problems?  I assume that a voltage difference between the drain of the high-side FET and the Vin pin just manifests as an error in the sensed current?

    Thanks!

    -Bill

  • 0 in reply to Bill Argabright
    TI__Mastermind 49317 points

    Bill,

    Yes, both FETs are switching and their currents are averaged from a DC standpoint in terms of consumption from VIN. 15mA should be fine. However, there can be a pulse of current at startup. Latest guidance is to remove that 1 Ohm resistor to avoid impacting Vds sensing of the high-side FET. You can keep it as a placeholder if you prefer as it is a convenient point to measure the IC current consumption.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 250 points

    Oh, duh, because we're talking average current with Qg*Fsw!

    My whole reason for asking about this part is to ensure it is appropriately sized.  Let's assume I'm not having problems with Vds sensing as you've described.  Do you know of a situation where the Vin pin could draw something like 250mA sustained long enough to exceed the 1/16th Watt rating of the resistor?

    -Bill

  • +1 in reply to Bill Argabright
    TI__Mastermind 49317 points

    That power rating should be fine. The current shouldn't exceed 15mA based on your FET choice and switching frequency.

  • 0 in reply to Bill Argabright
    TI__Mastermind 49317 points

    Thanks, Bill. I'll close out this thread.

    Regards,

    Tim