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Original question:

LM2731: Can SHDN pin do soft-start function?

LM2731: adjust the start up time

kevin p
Intellectual 470 points
Part Number: LM2731

Hello team,

I use  LM2731 in my design.

I want  to slow start to reduce inrush current on the power supply when power on,

I add RC on the SHDN pin.

the R=47k, C=1UF，the input power is 5V, output  is 12V,I want to delay ~20ms  to start up when power on,

Could you help to review and give your comments,

Could  you give your solution ?

Thank you,

kevin

  • 0
    TI__Genius 17600 points

    Hi Kevin,

    Please see the following thread regarding this topic:

    https://e2e.ti.com/support/power-management/f/196/p/817310/3025148#3025148?jktype=e2e

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Intellectual 470 points

    Hi Richard,

    Thanks for you support.

    Can I  precisely control the  SHDN  delay timing?

    The Vin is powered first and put SHDN low, after delaying 20ms,put SHDN to high. Is it ok?

    Thank you.

  • 0 in reply to kevin p
    TI__Genius 17600 points

    Hi Kevin,

    The engineer responsible for this part has been assigned to this question. Please wait for his/her response.

    If it may help, I think this may be doable, but the timing will have to be done very carefully.

    Thanks,

    Richard

  • 0 in reply to kevin p
    TI__Mastermind 34201 points

    Thanks Richard.

    Hi Kevin,

    Below is soft start circuit for non synchronous boost converter that has not built-in soft start function.

    During the start up, power supply Vin starts charges soft start capacitor C1 through resistor R1, Diode D1 and bottom feedback resistor. Since the initial capacitor voltage is 0V, the FB pin voltage at start up is high so the device duty cycle is very small. When C1 voltage rises up, the FB pin voltage decreases slowly. Finally the C1 is charged up to Vin and boost converter works.

    Once the device is shutdown, D2 is the used to help discharge C1.

  • 0 in reply to Zack Liu
    Intellectual 470 points

    Hi Zack,

    Thanks for you quickly response.

    Is the circuit reliability when power on and power off repeately?

    Could you recommend the MPN of  D1 and D2,is the MBR0530 ok for this circuit?

    How to calculate the startup time? If R1 is 10K,C1 is 1UF,VIN=5V,VOUT=7V,the rise time on the power supply VIN is ~2ms,

    I want to calculate the time from VIN to VOUT rise up to7V.

    Could you give the calculation formula to me.

    Thank you.

    kevin

  • 0 in reply to kevin p
    TI__Mastermind 34201 points

    Hi Kevin,

    A 30V/100-500mA current rating diode would be sufficient because these diode only work during start up and shutdown.

    The startup time is related to the output capacitance. A big output capacitance cause long start up time. Have you determined the Cout value? 

  • 0 in reply to Zack Liu
    Intellectual 470 points

    Hi Zack,

    Thanks for you quickly response.

    The Cout value is 20uF.

    Thank you.

    kevin

  • 0 in reply to kevin p
    TI__Mastermind 34201 points

    Hi Kevin,

    I can do a bench test next week. I'll let you know once the I complete the test.

  • 0 in reply to Zack Liu
    Intellectual 470 points

    Hi Zack,

    Thanks for you support.

    Maybe I needn't  add soft start function on the LM2731,I only want LM2731 to delay startup when the power on.

    I want to reduce the inrush current when power on, due to there are several LDOs, DC-DCs in my design.

    I want to start each power according to the timing sequence.

    Can I precisely control the SHDN delay timing?The Vin of LM2731 is powered on firstly and the SHDN  pin is low, after delaying 20ms,I put the SHDN pin to high. Is it ok?

    Please help to review and give your commends.

    BTW,if I add soft start function, Is it  reliable to delay output? Will there be other issues？

    Thank you.

    Kevin

  • 0 in reply to kevin p
    Intellectual 470 points

    Hi Zack,

    There may be some misunderstandings, I clarify it.

    I needn't  want to add soft start function on the LM2731,I only want LM2731 to delay startup when the power on.

    I want to reduce the inrush current when power on, due to there are several LDOs, DC-DCs in my design.

    I want to start each power according to the timing sequence. There are two ways  as below. 

    1. Can I precisely control the SHDN delay timing? The Vin of LM2731 is powered on firstly and the SHDN  pin is low, after delaying ~20ms,I put the SHDN pin to high. Is it ok?

    2. I want to add RC on the SHDN pin, please refer LM2731_SCH as the attachment, Is it ok?

    Please help to review and give your commends.

    Thank you.

    Kevin

    LM2731_SCH.pdf

  • 0 in reply to kevin p
    TI__Mastermind 34201 points

    Hi Kevin,

    Adding RC circuit on SHDN pin is OK. You could adjust the resistor and capacitor value to get the 20ms delay time. 

    What's the maximum inrush current could you accept in your design?

  • 0 in reply to kevin p
    Intellectual 470 points

    Hi Zack,

    I test the IC on the demo, the two ways was failed.

    The SHDN pin can't be controlled.

    I want to the inrush current is less than 100mA, is there any solution.

    Thank you.

    Kevin

     

  • 0 in reply to kevin p
    TI__Mastermind 34201 points

    Hi Kevin,

    LM2731 is a non-synchronous boost converter. Even SHDN pin is low when power pn, the power supply will charge output capacitor through inductor and diode. So output equals Vin minus voltage drop of diode, not 0V. That's a normal behavior for a non-synchronous boost converter. The charging current is related to output capacitance.

    When you add RC on SHDN pin, after SHDN pin reaches logic high, LM2731 starts swtiching to charge output capacitor to programmed Vout. By adding the soft-start circuit that I shown you, the switching inrush current could be reduced.