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TPS40210: 510kohms parallel resistor with Css

Kai Nomiyama
Expert 5285 points
Part Number: TPS40210
Other Parts Discussed in Thread: PMP10026

Hi Team,

I have a question about TPS40210 Css.
in order to discharge Css quickly, my customer wants to put 510kohms resister in parallel with Css.

could you let me know are there problem if we put 510kOhm resistor in parallel with Css Vss?

Regards,
Kai

  • 0
    TI__Mastermind 18320 points

    Hi Kai,

    By adding a 510KOhm resistor in parallel with Css, you will affect the soft-start time, and have constant power dissipation through that resistor, reducing your efficiency.

    Additionally, you will interfere with the internal discharge resistor that is connected to Css when over-current condition happens. Additionally, when charging the soft-start capacitor, you may have a longer soft-start since the 510KOhm resistor will slow down the charging time. 

    During shutdown, the internal pull-up resistor to V_BP will discharge Css.

    Overall, it seems like adding the 510KOhm resistor will make the overall design effort much more intensive for similar gain as if you did not place it there at all. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Expert 5285 points

    Hi Richard,

     

    From your comment, it seems we have following disadvantage when we put 510kohms(Rss) in parallel with Css.

    • Lower efficiencyby Rss loss

    • Longer soft start time due to Rss

    • Faster discharge due to Rss||Rss(dchg)

     

    Regarding following comment “During shutdown, the internal pull-up resistor to V_BP will discharge Css.”, could you explain more detail?

    From datasheet, it looks there is no relationship beween V_BP and Css.

    In addition, this is customer use case but when Vin goes low, TPS40210 cannot keep shutdown mode but are there any problem?

     

    Actually what I want to know here is

    • Are there any problem when we use 510kohms in parallel with Css?

    • I list up above disadvantage but are there any problem for TPS40210 operation except such disadvantage?

    • Relationship between V_BP and Css?

     

    Regards,

    Kai

  • 0 in reply to Kai Nomiyama
    TI__Mastermind 18320 points

    Hi Kai,

    1) There should be no problems when you use 510KOhms in parallel with Css, but it does not make much sense to do so, per the reasons I listed before. Additionally, when looking at reference designs using the TPS40210, I have not yet seen any designs utilize the 510KOhm in parallel with Css.

    2) From my analysis, there should be no problems besides the following disadvantages. 

    3) Please see equation 1 which shows that Css is related to Vbp. 

    Sorry for the misclarification, I meant the pull-down resistor discharging SS. 

    For the customer use case, the customer can supply VDD using the output, in which case, even if the input voltage goes low, the output can still be maintained. This can be implemented as seen in PMP10026 as seen here: https://www.ti.com/tool/PMP10026 with D100.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Expert 5285 points

    Hi Richard,

    thank you for your comment.

    Actually the reason why customer wants to put 510kohms in parallel with Css is when TPS40210 turns on/off, depending on Vss_bias,
    TPS40210 detect fake over current condition.
    In order to do fast discharge, they want to put 510kohms.

    I will let you know if I need more help.

    Regards,
    Kai