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LP38798: CP capacitor

Ella KIM
Expert 4082 points
Part Number: LP38798

Hi Team,

I wonder if CP pin capacitor is mandatory. I checked it's pass FET is PMOS. Does this device still need a charge pump for its basic operation?

The CAP pad is damaged in my board so I wonder if I can open CP pin without the storage capacitor.

Is this how the internal charge pump looks like? Then could you kindly let me know which capacitor (C2 I guess?) is corresponding to CP capacitor of LP38798?

Thank you.

  • 0
    TI__Mastermind 35465 points

    Hi Ella,

    I am reviewing your question and will have feedback within 2 business days.

    Thanks,

    - Stephen

  • 0
    TI__Mastermind 35465 points

    Hi Ella,

    I was unable to locate any additional information regarding the charge pump topology.  However, charge pumps are usually employed in linear regulators to give a higher positive voltage.  An E2E post written years ago by an Apps engineer closer to the team who design this LDO, stated that the charge pump is used for gate drive of an NMOS pre-regulator inside the "Active Ripple Rejection" block.  So without the CP capacitor, the charge pump will be unable to generate the voltage needed to drive the circuitry inside the "Active Ripple Rejection" block.  Without the CP capacitor, at a minimum the PSRR will deteriorate.  At most the LDO will not regulate properly.  I do not have test data to review but this could be done using the EVM.  I would not recommend operating the device without the CP capacitor installed.

    The previous E2E thread is located here:

    e2e.ti.com/.../lp38798---charge-pomp-and-pin-descriptions

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 4082 points

    Stephen,

    Appreciate for your quick feedback!

    Is it impossible to know the internal structure of the device?

    Then if you have EVM now, could you please check if it doesn't regulate or if it regulates just with deteriorated PSRR when CP pin is open? I'd like to check it by myself but I afraid that the response to customer would be delayed due to the EVM delivery.

  • 0 in reply to Ella KIM
    TI__Expert 4082 points

    Stephen, 

    I found a EVM in the lab I can check if it regulates or not. Still, it would be very helpful to know what the internal CP structure is like and what the external CP capacitor's function is. Thank you!

  • 0 in reply to Ella KIM
    TI__Mastermind 35465 points

    Hi Ella,

    I will ask the IC design team for further insight into the internal CP structure of this device.  Please allow us some time to investigate and reply.  Once I have an estimate on time from the designers, I will reply back and let you know how long you should expect.

    Thanks,

    - Stephen

  • 0 in reply to Ella KIM
    TI__Genius 13965 points

    Hey Ella,

    I wanted to follow up while Stephen is out. This is an old design and so it will take some time to recover the database and get the information requested, but we are still working on othis. 

  • 0 in reply to Kyle Van Renterghem
    TI__Expert 4082 points

    Thanks Kyle for your follow-up action here!

  • 0 in reply to Ella KIM
    TI__Genius 13965 points

    Hey Ella,

    Design has been very overloaded and this is taking much longer than expected but I'll see if they have an update. 

  • 0 in reply to Kyle Van Renterghem
    TI__Genius 13965 points

    Hey Ella,

    Thank you for your patience we've now gotten the information you requested. 

    1) Can you or someone in your team please review this device and give us some understanding of the charge pump structure. 

     The charge pump boosts the sub-regulator voltage 3X times. It is a 2 stage cross coupled charge pump architecture running at 3.5MHz clock.

    2) The impact of leaving the CP capacitor out of the circuit

    The charge pump does not have any load capacitor internally and relies on the external capacitor to regulate the charge pump output. Without the capacitor there will be a very high ripple on the charge pump output which could significantly impact the LDO performance so we do not recommend that this pin be left floating.