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LMR14020: side effect of Css floating

Nam, Dino
Expert 6145 points
Part Number: LMR14020

Dear team, 

My customer want to know side effect when soft start port is floating status.

In fact, they connected Css on SS port but Css is not connected GND. Is there any side effect? 

1. For this case, Does LMR14020 start-up normally? , Is there internal soft start time on LMR14020? 

2. Does this status cause damage of LMR14020 due to inrush current or others? 

Please let me know your opinion. 

Thank you. 

  • 0
    TI__Mastermind 27045 points

    Hi Dino,

    1.  there is no internal soft start time if the part has SS pin out, soft time is almost zero if Css is floated. the converter may not startup normally as the output cap voltage can't track the Css voltage because of limited output current capability. 

    2. LMR14020 may stuck in over current protection mode in this status, there maybe risk for LMR14020 if it starts working at high Vin condition, the inrush current may also has risk for other system devices, like the fast fuse.

    B R

    Andy

  • 0 in reply to Andy Chen
    TI__Expert 6145 points

    Hello Andy, 

    I am confused about the relationship soft start waveform and normal start up. I am sorry but please review below question and let me know your opinion.  

    Andy Chen said:
    1.  there is no internal soft start time if the part has SS pin out, soft time is almost zero if Css is floated. the converter may not startup normally as the output cap voltage can't track the Css voltage because of limited output current capability

    - During start up, Is Css voltage used as reference voltage? I understood If Css is floated, soft time is zero but I didn't understood the reason of no startup. Please explain again regarding your comment "the output cap voltage can't track the Css voltage because of limited output current capability" 

    - If LMR14020 is no load condition, Is it available to startup normally?

    Andy Chen said:
    2. LMR14020 may stuck in over current protection mode in this status, there maybe risk for LMR14020 if it starts working at high Vin condition, the inrush current may also has risk for other system devices, like the fast fuse.

    - If LMR14020 is not in over current protection mode during startup, Is it available to startup normally?

    - The over current limit of LMR14020 is typ 3.2A(High side MOSFET). Does it change during startup? 

    If possible, would you share document regarding on the relationship between Vfb, Vss, output of Error Amp and switching waveform?

    Thank you. 

  • +1 in reply to Nam, Dino
    TI__Mastermind 27045 points

    Hi Dino,

    1. Yes, during start up, Css voltage is used as reference voltage till it exceed 0.75V.

    2. If Css is floated, the SS pin voltage is immediately equal to internal pull up voltage(>>1.0V) even the source current is only 3uA, this cause the soft start time almost zero

    3. assume SS pin jump to 0.75V within 5us, Cout is 20uF, Vout=5V, then it require 20A to charge up 20uF within 5us, this will cause over current protection

    4. High side current limit is 3.2A at any operating time, includes soft start.

    5. Data sheet has typical soft start up waveforms.

    B R

    Andy

     

  • 0 in reply to Andy Chen
    TI__Expert 6145 points

    Hi Andy, 

    Thank you for your kind explanation. 

    Do you mean the reason of abnormal startup is only over current protection due to zero start up time when Css is floated?  

    Thank you.