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Bootstrap capacitor selection

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: TPS54425, TPS54331

Many DCDC converters require 0.1uF ceramic capacitor for bootstrap capacitor.

How can I select capacitor? I would like to know judgement method of capacitor spec as voltage rating, DC bias, size etc...

What kind of capacitors can we use actually?

Best Regards,
Kohei Sasaki

  • 0
    TI__Genius 11040 points
    Hello,

    This information should be recommended in each datasheet. A ceramic capacitor should be used which meets the capacitance and voltage rating that is detailed in the datasheet. Generally, too large of a boot capacitor that it may take a few switching cycles to charge CBOOT to the steady-state value. On the other hand, generally too small means the cap might not have enough energy to maintain the steady state voltage during switching cycles. This would reduce the effective gate voltage on the FET which would not turn on the FET as well which would lead to more power dissipation in the FET / worse efficiency overall.

    Are you looking at a specific part and its boot capacitor?

    Thank you,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 10200 points
    Katelyn-san,

    I'm looking at TPS54425 and TPS54331.
    On the datasheet, it is written only 0.1uF ceramic capacitor is required. I think it is not described about size or effective capacitance value?
    Generally, how level effective capacitance value is allowed? Until 50% or 70% of 0.1uF?

    Best Regards,
    Kohei Sasaki
  • 0 in reply to K. Sasaki14
    TI__Genius 11040 points
    Hi Sasaki-san,

    Thank you for sharing the part numbers. I looked at the datasheets, and I agree that a 0.1uF is required. I would recommend using an X7R or X5R capacitor.

    I will also copy the product expert to comment.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 10200 points

    Katelyn-san,

    Thank you for your support.

    Capacitance characteristics is difference by size or voltage rating etc...

    So we would like to know what kind of capacitor we should select. Even allowed effective capacitance value is helpful.

    Best Regards,
    Kohei Sasaki

  • 0 in reply to K. Sasaki14
    TI__Guru**** 191445 points

    Sasaki-san,

    The BOOT capacitor selection is not too critical.  The trade off is lower value charges quicker while large value will hold charge longer.  Typically, the IC BOOT charge circuit is designed with a particular nominal BOOT cap value such as 0.1 uF  The BOOT charge may be as simple as a diode drop from VIN for low voltage parts, a diode drop from an internal regulator (such as VREG5 for your TPS54425 example), or more complex BOOT charge circuit for wide VIN parts such as TPS54331.

    Lets look at TPS54425 for an example.  That BOOT cap is 0.1 uF, 50 V, X7R, 10%, 0603.  The actual voltage across the BOOT cap is a little less than 5 V.  The DC bias characteristic for that capacitor at 5 V is actually above 0.1 uF.  I have seen 16 V capacitor used in TPS54425  applications ( I used to directly support TPS54425 several years ago).  Even for 16 V rating, the capacitance is still over 95% of 0.1 uF.  As a practical matter I would not be concerned with 70 to 75 % of the typical capacitance for BOOT capacitor, but as you can see from above, it is relatively easy to pick a capacitor with not much capacitance loss.

    Hope this helps.  Let me know if you need more guidance.

  • 0 in reply to JohnTucker
    Guru 10200 points

    John-san,

    Thank you for your answer.

    When capacitor size becomes smaller, DC bias characteristic becomes worse. It is about 50% of 0.1uF etc...

    In this case, should we change capacitance value to like 0.22uF from 0.1uF because we have larger effective capacitance value?

    Or 50% effective capacitance of 0.1uF is no problem?

    Best Regards,
    Kohei Sasaki 

  • 0 in reply to K. Sasaki14
    TI__Guru**** 191445 points

    Sasaki-san,

    It is always an engineering trade off.  I could not find a specific example capacitor where the DC bias effect was that severe.   Even 0.1 uF,  0402, 10 V still have over 85 % capacitance at typical BOOT voltages.  It really only comes into play for low frequency/high duty cycle applications or when operating in low headroom drop out mode where the input voltage is lower than the nominal BOOT voltage and duty cycles are exceeding 100%.  In that case the BOOT cap can discharge below the BOOT UVLO (if the device has a UVLO on teh BOOT voltage) during the high side on time and the high side on time will then be terminated early.  If you are worried that there is not sufficient capacitance, by all means select a different part.  I have not had difficulty finding appropriate parts at teh recommended value.