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Original question:

TPS23753A: PMP7355 design issue, 20hz saw tooth output on no load

TPS23753AEVM-004: Q2 part substitution

Kyran McGlasson73
Prodigy 240 points
Part Number: TPS23753AEVM-004

Hi,

Reading the users guide for the TPS23753AEVM-004 board, I am looking at validating the operation at 5V under 10W load, with a 12V AUX input.

First, I will change the components listed in the BOM for 5V output operation (for example T2 use a HA3802-BL)

Can I substitute Q2, Si4848DY for a Si4848ADY? I assume the A suffix denotes an improved part (higher current operation).

Can I also lower R18 from 390 mOhms to say 220mOhms to achieve extra current at 12V input (knowing that the flyback transformer can handle it)?

Thanks in advance,

Kyran McGlasson
SAGE Automation - Australia

  • 0
    TI__Genius 9230 points
    HI Kyran,

    1. There should be no problem with making that substitution.

    2. Yes, you can lower the current sense resistor for your higher current needs.

    Anything in the primary side power path (T2, Q2, R18) will need to be the correct current rating for lower Vin due to higher input current requirements.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer
  • 0 in reply to Thomas Amlee
    Prodigy 240 points

    Thanks Tom,

    Also I forgot to ask, this design uses freq of 125kHz, is there a reason for this? Can we increase it to 250kHz? Does anything need to be changed to do this?

    Regards,

    Kyran McGlasson
    SAGE Automation

  • 0 in reply to Kyran McGlasson73
    TI__Genius 9230 points

    Hi Kyran,

    I was not around during the definition of this 125kHz, design, so I would not. A reduction in switching frequency may yeld some benefits in lowering switching losses (by switching less often). You can increase the frequency to 250kHz with no changes, although you can choose a lower inductance transformer at this higher frequency which may lower cost.

    If you do planning on making any major changes to the power stage, I recommend looking at our flyback design guide.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer

  • 0 in reply to Thomas Amlee
    Prodigy 240 points
    Hi Tom,
    I will take the TPS23753AEVM-004 board, and evaluate this design using 250kHz and see how it goes.
    Thanks again,
    Kyran McGlasson
    SAGE Automation