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LM5180EVM-S05: Light load operation and transient response

Michiaki Tanii
Expert 4610 points
Part Number: LM5180EVM-S05
Other Parts Discussed in Thread: LM5181, LM5180

Hi team,

 

Customer is trying to evaluate LM5181 using LM5180EVM.

And we got inquires as following.

 

  1. Compared quickstart calculator between LM5181 and LM5180, they found the difference for min Iout which needs exceed 10mA for LM5180.
    Why does LM5180 needs Iout exceed 10mA ?
  2. Customer saw the operation changing the load to 10mA from no load with evaluation board and found the output at 5.5V between no load and 5mA and 5V stable output more than 5mA.
    What is caused ?
  3. Customer saw the transient response when the load was to 100mA from to 10mA and to 10mA from 100mA.
    As the below figure, the behavior is quite difference. Could you kindly the reason why ?

 

The test condition is 16V input and 5V output for both question 2 and 3.

 

Thank you and best regards,

Michiaki

  • 0
    TI__Mastermind 34875 points

    Hi Michiaki,

    As instructed on data sheet, at even lighter loads, the primary-side peak current set by the internal error amplifier decreases to a minimum level of 0.3 A, which sets a minimum load requirement of approximately 0.5% full load, the transformer storage energy will charge up output voltage if output is no load. 

    At light load condition, the operation freq is foldback to very low freq of 12KHz, so it takes more cycles to increase transformer current to higher level, this will cause little bigger output ripple. 

    B R

    Andy

  • 0 in reply to Andy Chen
    TI__Expert 4610 points

    Hi Andy,

     

    Why can LM5181 be configured from output current of 1mA in Quickstart tool ?

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Mastermind 34875 points

    Hi Michiaki,

    For LM5181 at lighter loads, the primary-side peak current set by the internal error amplifier decreases to a minimum level of 0.15 A, the storage energy reduced to 25% compared with 0.3A for same inductance. so LM5181 has lower requirement on minimum light load current.

    B R

    Andy

  • 0 in reply to Andy Chen
    TI__Expert 4610 points

    Hi Andy,

     

    Please let me double check on the output current configuration.

     

    I understand that the internal error amp decreases to a minimum level of 0.15A for LM5181 compared to 0.3A for LM5180.

     

    But in the Quickstart tools, LM5181 can configure from 1mA against 10mA for LM5180 even though the minimum level is getting half.

     

    Could you kindly advise the reason why ?

     

    And if customer change to LM5181 from LM5180, customer will not see charging up output voltage at 4-5mA output current ?

     

    Thank you and best regards,

    Michiaki

  • 0 in reply to Michiaki Tanii
    TI__Guru 74052 points

    Michiaki,

    The minimum current delivered to the load is Iout-min = 0.5*Lmag*Ipk-FFM^2*Fsw-min/Vout, where Ipk in FFM is 0.3A and 0.15A for the LM5180 and LM5181 respectively, and Fsw-min is 12kHz.

    Note that a higher mag inductance is required for the LM5181 - see datasheets and quickstart calculators for more detail. This is to meet the converter toff requirement. Please check that the LM5181 transformer was suitably selected (see table in the datasheet of parts provided by Wurth).

    Regards,

    Tim