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TPS43060: What will happen if VIN is higher than the desired VOUT?

Jiang Boqian
Intellectual 655 points
Part Number: TPS43060
Other Parts Discussed in Thread: TPS55340-Q1, TPS61253A, TPS63070, TPS55340

I see that TPS43060 doesn't support 100% duty cycle for high side mosfet, what will happen if VIN is higher than the desired VOUT?

I think unlike the device which can support 100% duty cycle that can have VIN bypassed to VOUT via high side switch (VOUT=VIN), TPS43060 will always have VOUT a little higher than VIN, right?

  • 0
    TI__Guru* 78400 points
    Hi Boqian,

    When VIN is higher than VOUT, the sync FET will be off and the body diode conducts. The VIN can be slightly higher than Vout ( a body diode drop).

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Intellectual 655 points
    Hi Youhao,
    Thank you, I just didn't read the datasheet carefully, especially "8.4.2 Pass Through (VIN > VOUT)" chapter.
    By the way, if VIN will be larger than VOUT, can I connect a powerful schottky diode externally parallel to the sync FET?
  • 0 in reply to Youhao Xi
    Intellectual 655 points
    Hi again Youhao,
    Does these two boost converter/controller support pass through when VIN>VOUT?
    TPS611781(sync rectify) and TPS55340-Q1(non-sync)
    I see some devices' datasheet explicitly says it support pass through, like TPS61253A.
    However, these two above devices' datasheet didn't. I know maybe the datasheet implicitly says, but I cannot figure it out...

    My input voltage will some times higher or lower than my output, like 3.4~14V, output 3.8V. I couldn't find the appropriate buck/boost converter(TPS63070 is good, but cannot support my system's maximum current(GSM PA 2.3A pulse) during low input voltage). So I am now considering pre-boost + buck like what is recommended in the car start-stop condition. However, I want device that can support pass through function to maximize system efficiency. So this is why I ask this question.
    By the way, if there's no appropriate pre-boost converter for my application, my last hope will be SEPIC topology.
    Thank you.
  • 0 in reply to Jiang Boqian
    TI__Guru* 78400 points
    Hi Boqian,

    When we way pass through, we mean the sync boost with its sync FET turned on 100% duty to pass the input voltage. In such sense, TPS55340 does not apply. The LM611781 would be similar to TPS43060 in such case.

    We do have a boost controller LM25122 which support bypass (100% duty of the sync FET). You may take a look at it.

    Thanks,
    Youhao