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BQ25910: BQ25910 PMID capacitor

michael YANG
Prodigy 195 points
Part Number: BQ25910

Hi Team,

About the capacitor on PMID of BQ25910,Ti suggest us to select 10uf .  is it possible to select a small one ? how about 2.2uf ?

i know the capcacior on PMID pin is the input capacitor of charger(buck) circuit ,also it is the output capacitor of the boost circuit (OTG mode).

my question is .

a) about the boost mode , small output capcitor mean higher RH zero , also it mean it will be higher than 10*fcross.

    so the 2.2uf is ok for boost mode . am i right ?

b)  about the charger circuit (buck mode), seems the capacitor on PMID will lead a RC  circuit (the R is the Ron of the mosfet on input side ) in input current loop , it is related with  the stability of input current regulation loop .

     is it  OK to reduce it ?

c)    if the input capacitor is too small, will it lead a unstable for CCM during charging ?

       it is mean the circuit can't  mantain the Volt-second conservation in single PWM period ,so there will be a low frequncy envelope line on output .

      if the scan frequncy of oscilloscope is high enough , we can see more detail with a short time ,the noise will only be switch ripple .

      if the scan  frequncy of oscilloscope is slow , we will see the wave with a longer time ,than we will find the  envelope line ,and the output nosie will be higher than before (the scan frequncy of oscilloscope is high    enough)

d )  the capacitor will increasing  the input ripple .which mean the input current DPM will be more easy to be actived .

e)   is there any other influnce on reducing the capacitor of PMID ?

Best regards

Michael .Yang

  • 0
    TI__Prodigy 250 points

    Hi Michael,

    In general, TI recommends 10uF capacitor by taking into consideration heavy DC de-rating, which will yield a final capacitance value around 2uF. Reducing the capacitance value will affect various aspects of the converter as you pointed out. Here are my specific comments for bq25910:

    a) bq25910 has no OTG functionality, so this is no concern.

    b) IINDPM relies on having average input current information. Smaller cap means there is more ripple in the current through the blocking FET, which will affect the efficiency of the converter (higher RMS current in blocking FET) as well as the IINDPM loop response and accuracy. All of our design and testing assumes a 10uF capacitor, so TI can guarantee the stability of the converter at that point. Using a smaller capacitor may still work, but is not guaranteed by design or internal test.

    c) Any buck converter needs a certain amount of input decoupling, this charger is no different. Reducing the input decoupling means that the VBUS seen by the converter will vary across a cycle and potentially slower (dictated by cable resistance and inductance). Again, with all this in consideration, TI recommends 10uF capacitor to fit all operation modes of the bq25910.

    d) There is more risk in activating the IDPM at lower currents if the ripple in BLKFET is high enough.

    e) The one you did not consider is efficiency. In the extreme where PMID capacitor is not there, the BLKFET current will be equal to HS FET switching current, which has significantly more RMS content than average input current. This will degrade the efficiency of the part.

    In general, there are many things to consider when designing the input filter of a converter. TI recommends 10uF based on design and bench evaluation. Reducing the PMID capacitor to 2.2uF is not tested in the lab, and may result in undesired effects such as higher power loss and less accuracy in IDPM.  

    Regards,

    Alvaro   

  • 0 in reply to Alvaro Aguilar
    Prodigy 195 points
    Hi Alvaro ,
    Great thanks for your reply .
    Yes ,the Eff. should be considered .
    we have tested the input ripple after reducing the input capacitor .
    it changed from 225mv to 335mv.

    i still need confirm it will take infulence on the stability of input current regulation loop or not .
    could you please share more detail about this loop ?
    great thanks for your support!


    Best Regards
    Michael.Yang
  • 0 in reply to michael YANG
    TI__Prodigy 250 points

    Hi Michael,

    The input current regulation loop stability will not be affected (but the accuracy may be affected).

    However, the input voltage regulation loop may be affected because of the increased voltage ripple on the input side given the 4x lower input decoupling cap. Additionally the input-to-output transfer function is affected, which may introduce the low frequency envelope line on output you mentioned before.

    In general, buck converter stability depends on the input filter design. By changing the input filter capacitor by 4x and leaving everything else the same, there is a risk of exhibiting instability in the control loop. This is common to all buck converters, not just chargers (see Fundamentals of Power Electronics by Robert Erickson pp377 - 405). 

    Thanks,

    Alvaro

  • 0 in reply to Alvaro Aguilar
    Prodigy 195 points
    Hi Alvaro,
    Great thanks for your reply .
    yes , if there is a input filter ,such as LC filter , the stability of two systems in series should be considered .
    thanks for your reply again.


    Best Regards
    Michael.Yang