This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3488: Regulating method for current limit on TIDA-010057

Guru 19475 points
Part Number: LM3488
Other Parts Discussed in Thread: TIDA-010057

One customer is create LM3488 board the same as TIDA-010057 design.

At first, start up was not completed. 

Waveform is attached below;

I guess that the reason is insufficient input current from 5V input source.

      

When RSEN (TIDA-010057's R41) changed 0.05Ω to 0.1Ω, this problem was cleared and normal operation.

Please let me know about three points below about current limit setting;

・Is these start up problem on default TIDA-010057 with insufficient input current, valid operation or not?

・Is there any risks for change the large RSEN value?

・About the idea for normal start up, which is recommend for change the RSEN value or RSL (TIDA-010057's R38) value?

 If there any other idea, please let me know.

Best regards,

Satoshi

  • Hello Satoshi,

    I want to apologize for the delay. Friday and Monday were bank holidays.

    Is this the same project than this one?    LM3488: Problem for PSpice for TI simulation based on TIDA-010057

    What exactly do you mean, when you say:  insufficient input current ?  
        Do you mean the current limit of the bench supply?  
        Which current limit is used for the supply?

    Which load is connected to the outputs?

    Are tey using the exact same components as on our TIDA-010057 design ? 

    Can we please get the whole schematics and information about the selected parts (FET, inductor/transformer, etc.)

    Can we also get the layout?

    Thanks a lot,

    Harry


  • Here are some more comments.

    The LM3488 is using current mode control, so it needs to rely on a proper feedback from the current sense resistor at every cycle.

    Indeed the LM3488 has a maximum duty cycle of 100%.
    Therefore, if there is a problem with the feedback at the ISEN pin it is expected that this device can stay in a mode where the DR pin is permanently on.
    This is an expected behavior of the LM3488.
    But it is not a valid operation mode of the whole SEPIC converter.

    To avoid such problems you need to take care of two things:

    1)
    Current limit of the bench supply:
    ・Is these start up problem on default TIDA-010057 with insufficient input current, valid operation or not?

    Indeed, if the current limit of the bench supply is set to a low limit where the current feedback loop will no longer be triggered, there will be no event that can
    shut off the PWM and so the DR pin will stay permanently on.
    Again: This is an expected behavior of the LM3488. But it is not a valid operation mode of the whole SEPIC converter.

    2)
    The current sense resistor has to be correctly calculated.
    Please follow section 8.2.2.2.4 in the datasheet for a SEPIC configuration.

    Please also note that the choice of the inductor, FET, traces on the PCB etc. will have an influence on the voltage across the sense resistor.
    So, the resistor may need to have a different value than what you see on our board.

    ・Is there any risks for change the large RSEN value?

    RSEN has to be chosen in a way that the current feedback loop will work properly under all conditions.

    But please be careful and do not chose a too big value.
    When the voltage across the sense resistor (measured on ISEN Pin) exceeds 350mV, short-circuit current limit gets activated.
    This should not happen during regular operation.


    ・About the idea for normal start up, which is recommend for change the RSEN value or RSL (TIDA-010057's R38) value?

    R38 is used for the slope compensation setting.
    As per the formula in the datasheet it is dependent on the chosen value of RSEN.

    As a rough estimate: To keep the same slope compensation: when you double RSEN, also double R38.

    Best regards,
    Harry

  • Hi Harry

    I apologize my reply delay,

    Thank you for many advice, I understood.

    As a result, the reason for this problem is based on 1A current limiting on input side (current limit switch), TIDA-010057 is 1.5A.

    Current situation, LM3488 board is stable on RSEN: 0.1Ω.

    Please refer modified information for LM3488 information; 

    ・The thread of LM3488 PSPICE for TI is not same condition.

     Project and customer are the same, but simulation and board design are separately working. 

    ・Insufficient input current is 1A limitation by current limit switch.

    ・Output side is load resistance. 

    ・Customer's board is not completely the same as TIDA-010057.

      Parts value are the same (expect for RSEN), but parts number are not same by BOM list. 

    Best regards,

    Satoshi

  • Hello Satoshi,

    Thanks for the information.

    >> LM3488 board is stable on RSEN: 0.1Ω

    Please make sure that the prototype board will be tested with all combinations of worst case conditions to confirm that it is really working stable.

    Do I understand correctly that there is currently no more open action item on TI side for this project?

    Thanks and regards,

    Harry