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UCC256301: how to use External Bootstrap IC UCC27714 along with UCC256301

venkata subash73
Prodigy 60 points
Part Number: UCC256301
Other Parts Discussed in Thread: UCC27714, UCC27517A

Hi,

we want to use UCC256301 along with external Bootstrap IC UCC27714 to increase the Peak current capability.

In this case how to connect the bootstrap connections of UCC256301? please give me any reference if you have.

  • 0
    TI__Mastermind 40550 points
    Hello Venkata,

    Thank you for your interest in the UCC256301 LLC controller. It will be a big problem to use the UCC27714 driver for your particular application. The UCC256301 already has a floating high-side drive, that needs to have HS connected to the switched node for adaptive timing, and its output must then be connected to the GND-referenced input on the UCC27714. There will be very complex interfacing required for these parts.

    A simpler alternative is to use two low-side drivers UCC27517A which have higher current capability, but the external high side can be bootstrapped just like the internal driver. Connect the high-side UCC27517A GND and IN- pins to the switched node.
    Connect the VDD cap and VDD input to a bootstrap diode and series resistor from RVCC. Connect HO output to the IN+ input of the high-side external driver.

    And the smaller separate packages can improve your pcb layout flexibility.
    I hope this solves your design requirements.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 60 points

    Dear Ulrich,

    Thanks for the reply,

    we have power board and control board separate.

    If we use  UCC27517A with UCC256301,in power board UCC27517A will be there and LLC ic will be there in control board.

    where should we place the boot strap capacitor and diode, to have small gate loop and proper operation.

    just want to understand, in case we want use external bootstrap IC with out adaptive timing is it possible ? in that case how should we connect.

  • 0 in reply to venkata subash73
    TI__Mastermind 40550 points
    Hello Venkata,

    I recommend to place the bootstrap diode, a "bootstrap" series resistor, and the bootstrap cap all on the power board next to the driver.
    There will be some peak current, limited a bit by the series resistor, drawn from RVCC on the control board. Try to arrange that RVCC path to keep its loop area to GND as small as is reasonably possible, although that path will go through the bottom MOSFET.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 60 points
    Thanks Ulrich,
    Can you share any reference layout for this .
  • 0 in reply to venkata subash73
    TI__Mastermind 40550 points
    Hello Venkata,

    I'm sorry I don't have a reference design that specifically addresses this recommendation.
    The UCC27517A datasheet shows a layout guideline in section 12 (pages 21 and 22) for the typical application.
    This can be directly applied for the low-side MOSFET. For the high side, the bootstrap diode and series resistor can be placed to the left of the driver IC, where the VDD and RTN vias are next to C1.

    Then the RVCC input and GND return connections would be to the left of the bootstrap diode.
    Also, the bootstrapped VCC and GND of C1 will need to be routed back to the HB and HS pins of the UCC256301.
    The main idea of the routing is to follow the guideline of the datasheet to keep the loop areas of the current paths as small as possible, to minimize or eliminate noise coupling into neighboring circuits.

    Since the UCC27517A is doing the gate drive function for the high-side, the peak currents for the HS, HO, and HB nets are rather low, so the track widths can be relatively narrow, but still keep the loop areas small on general principles.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 60 points

    Thank you Ulrich for the response.