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UCC27523: Driving Mosfet amount question

Chao Fu Chen
Intellectual 1150 points
Part Number: UCC27523
Other Parts Discussed in Thread: UCC27524

Hi Team,

My customer just curious how many mosfet could be driving by UCC27523 ?

For example , if they parallel 4 mosfet together to used UCC27523 to drive ? Is it met sense ?

Or any better amount suggestion in here. Thanks!

Eddie

  • 0
    TI__Expert 4070 points

    Hi Eddie,

    Thanks for reaching out!

    ------

    Yes, this is possible, BUT, it depends on how much gate charge the driver has to drive.

    ------

    They must consider the following:

    1)  Here is an application note going into Schematic and Layout considerations (The gate trace which splits to each gate should be length matched to each gate input to prevent possible mismatch)

    2)  Here is an E2E thread on how gate charge is an important variable for Power Dissipation and Temperature Considerations

    3)  Having a large amount of gate charge will increase turn-on and turn-off times.  Here is another E2E post that has an example of taking rise/fall times into account.

    4)  Reference the datasheet for best practices as well (consider using a small 0.1uF capacitor along with a 1uF capacitor as well; and place them physically close to the driver)

    5)  Gate resistors are highly recommended, and they should be placed as close as possible to each gate input (of the power MOSFET) to minimize ringing/coupling.

    -------

    I hope this answers your question.

    Thanks!

    Aaron Grgurich

  • 0
    TI__Intellectual 1150 points

    Hi Aaron,

    Just created two quick question come form same customer.

    If they decide to use the UCC27524 and try to added the RC in INA /INB pin(R: 350~400R , C: 20~33pF). Is it have any effect ?

    Then they see the UCC27524 his INA/INB pull high and floating has the same output status. Is it can do the both way and get the same result ?

    the Cload is necessary to have in this part ? or it can be ignore if you want it ?

    Eddie 

  • +1 in reply to Chao Fu Chen
    TI__Expert 4070 points

    Hi Eddie,

    1)  Regarding the input RC filtering, this is normal.  At a minimum, it is always good to have placeholders for these components if possible.

    • RC filter will delay the signal slightly  (as the capacitor to GND charges up and the resistor limits that charge rate, and therefore delays how quickly the voltage can hit the input threshold voltage)
      • Therefore, this can slightly increase the time it takes for the controller to change the output
      • This difference will likely be negligible  (application dependent)
    • I recommend that the RC filter not have a RC time constant larger than 20ns
      • i.e.  <100 Ohm & <200 pF

    -------

    2)  The INA / INB question is slightly confusing; but I think this may answer your question (This is in Section 8.2 of the datasheet):

    If the customer is using the UCC27523, the inputs are internally pulled up to VDD.  So, if left floating, the pin will go to the VDD value.

    - Unused pins should be tied to VDD or GND, as appropriate.

    --------

    3)  If you are talking about this C_load, we simply just put a capacitive load on the output of the device, to emulate having a MOSFET gate on the output to characterize the driver.  (So yes, you should ignore the "C_load")

    The only load that should be on the output is the MOSFET gate and a gate resistor to limit current / noise as appropriate.  A good starting point for a gate resistor is somewhere between 1 Ohm to 10 Ohms.

    -------

    I hope this answered your question; if so, please press the green button.  Otherwise, please feel free to follow up.

    Thanks,

    Aaron Grgurich