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TPS54062: TPS54062 low power buck converter

Dhananjay3021
Prodigy 110 points
Part Number: TPS54062
Other Parts Discussed in Thread: LM5166

Hi,

I am going to use TPS54062 for a low power step down conversion. Input voltage range is from 36V to 52V.  There is a limitation on input current of 5mA. i.e. input current shall not exceed 5mA. Required Output is 12V/ 10 mA. 

Now for this, application, is TPS54062 suitable? TPS54062 needs external bootstrap capacitor to turn on high side N channel MOSFET. During each PWM cycle, this capacitor will charge from input supply and input current will exceed 5mA for certain amount of time right? Can you please suggest solution to this problem? 

Is adding a series resistor to bootstrap capacitor a good solution? This will limit the switching frequency and duty cycle.

  • 0
    TI__Mastermind 25590 points

    Dhananjay,

    This is an interesting problem. Here are my thoughts:

    1. You could use a PFET converter/controller. This may hurt efficiency but it will eliminate the boot cap.

    2. The boot cap may not be the biggest issue. The boot cap will only be discharged a little bit every switch cycle whereas the FET parasitic caps are smaller but will be charged/discharged to VIN. This may be more of an issue but will be difficult to remedy with any DC/DC IC.

    3. You could use a diode in parallel with an RBOOT. That way it CBOOT charges through the resistor but discharged through the diode. 0.5V diode drop taken from ~8V may be acceptable for your design. I'm just concerned that the current through the RBOOT may hit boot UVLO and cause the part to malfunction before you can get RBOOT high enough to hit your 5mA consideration.

    4. You can also add significant input filtering but that will cost money.


    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 110 points
    Hello Samuel,

    Thanks for reply. Can you please suggest a low cost buck converter which will have PFET as a high side MOSFET?
    It is preferred if it has a current control loop.

    Regards,
    Dhananjay
  • 0 in reply to Dhananjay3021
    Prodigy 110 points
    Hello Samuel,

    Even if we are using TPS54062 for this application, initially input bulk caps will get charged. There will be surge current because of this but once these caps are charged, input bulk bypass capacitors will supply charging current for MOSFET parasitic and boot cap, right? so it wont be problem once input caps are charged right? surge current will be provided by input caps.

    Regards,
    Dhananjay
  • 0 in reply to Dhananjay3021
    TI__Mastermind 25590 points
    Dhananjay,

    LM5166 is a good PFET part but it's COT. If you need current-mode then I'd recommend sticking with TPS54062.

    You can use WEBENCH (TPS54062 product folder, on the right-hand side if you scroll down a bit) and export a TINA/Spice sim. From there you can run simulations to set up your system to your specs.

    -Sam
  • 0 in reply to Samuel Jaffe
    Prodigy 110 points
    Hello Samuel,
    What is the max. duty cycle TPS64062 can support?

    -Dhananjay
  • 0 in reply to Dhananjay3021
    TI__Mastermind 25590 points
    Dhananjay,

    The parametrics on the TPS54062 product folder say 98%. You will need to do some math in the datasheet to figure out the exact value for your specs (min on time is 120ns, dead time is 30ns...)

    -Sam