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LM5022: Notes when using a large capacity for Soft Start Cap

Taito Takemura
Intellectual 2310 points
Part Number: LM5022

Hi support team,

My customer would like to use 1uF for Css, which is recommended for 10nF on the data sheet. Can they use LM5022 with Css of 1uF?

Could you please let me know if you have any concerns and notes about using CSS of 1uF?

Thank you for your help.

Best Regards,

Taito Takemura  

  • 0
    TI__Genius 14415 points

    Hi Takemura，

    There is no hard limit as below.

    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/700962/lm5022-about-soft-start-cap

    I would also suggest to check the startup behavior and restart behavior when using excessive capacitance.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    TI__Intellectual 2310 points

    Hi Feng,

    Thank you for your answer. I have an additional question about Css.

    I found there are no strict restrictions on Css, but how much capacity is actually allowed?
    My customer would like to know the upper limit of Css. Also, I would be grateful if you could tell me about the factors that form the upper limit. (e.g.  the startup behavior and restart behavior, as you said before )

    Thank you for your help.

    Best Regards,

    Taito Takemura

  • +1 in reply to Taito Takemura
    TI__Genius 16905 points

    Hello Takemura,

    As you can see from the datasheet, worst case the SoftStart pin of the LM5022 provides only 7µA of current to charge the externally connected capacitor.


    Some part of this current will be used to charge the capacior, so that its Voltage will increase and ultimately exceed the Soft start COMP offset voltage.


    The other part of the current will be lost and "burned" inside the (parasitic) inner resistance of the capacitor (ESR).

    The customer needs to select a high quality capacitor with a very low ESR to make sure that there is enough current left to properly charge the capacitor.   

    Best regards,
    Harry

  • +1 in reply to Harry Hofner
    TI__Genius 16905 points

    Hello Takemura,

    I am sorry, I need to correct a mistake of my previous post.

    I meant to say:
    The other part of the current will be lost and "burned" inside the (parasitic) inner resistance of the capacitor (Isolation Resistance).
    The customer needs to select a high quality capacitor with a very low Leakage Current (High Isolation Resistance) to make sure that there is enough current left to properly charge the capacitor.  

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Intellectual 2310 points

    Hi Harry,

    Thank you for your answer.

    Does TI have a test result of LM5022 using Css of 1uF or more? My customer would like to know how much room the LM5022 with 1uF has. Could you please tell me it?

    (For example, up to 2uF is guaranteed to work by actual measurement, or up to 10uF has been confirmed to work and 1uF has a sufficient margin.)

    Best Regards,

    Taito Takemura

  • +1 in reply to Taito Takemura
    TI__Mastermind 47871 points

    Hi Takemura-san,

    we do not have more information on that available beside the data mentioned in the datasheet.

    From the device itself there is not limitation but please consider that the device does provide a charge current of ~10uA. The higher the capacitor used the higher the leakage current will be therefore this could interfere and result in not working as expected.

    Also keep in mind that the slower the ramp of the startup cap charging the more sensitive it will be against noise and therefore have a larger time window where the device starts operating.

    Best regards,

     Stefan