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Question about EN and SW pin of LMR62421

Guru 19475 points
Other Parts Discussed in Thread: LMR62421

Please let me know about 2-points below of LMR62421XMFX.

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・Is there a spec in Min ON/OFF time of EN pin?

・When a load current isn't flowing above certain  of SW pin, is SW output keep while being off(shutdown)?

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【Background】

・Application:Backup power to 8×POSCAP

・When EN changed H⇒L⇒H at 10ms span, a few devices were disable condition

  (158 unit per 3unit)

・Customer circuit is attached.

・When discharge by resister, EN was enabled(High state).

・When R56 connect 10kΩ pulldown, EN was enabled(High state).

 

Best regards,

Satoshi

 

  • Hello Satoshi,
    I am sorry, but the question is not very clear to me. Are you saying that with H - L - H transition, not all the devices are being disabled and then enabled?
    When the device is disabled, because of the boost topology, the Vout never completely goes to zero. It will stay at Vin.

    Regards,
    Akshay
  • Dear Akshay Mehta-san

    Thank you for reply.

    I want to know H - L - H transition.

    Customer waveform is attached.

    When POSCAP(Vout) is dropped, switching was restart.

     

    I think that this issue depend on transient response.

    Is there information of  transient response of LMR62421?

    (I find SLVA372C and SLVA274A, but calcuration method is differ from LMR62421 datasheet)

    Best regards,

    Satoshi

  • Hello,
    I think we have understood your issue now. If I understand it correctly, when you do a H-L-H transition on EN, some devices do not turn on again and that causes the output to droop. Eventually when the output droops enough, the part starts switching again. I do not believe this has any thing to do with the load transient.

    I think we need to have more information.


    1) Can you please give us more details on the EN transitions you are doing? Please probe the EN pin and zoom in on the oscilloscope. You said you are doing the entire EN transition from high to low to high in 10ms.
    2) Please send scope shots of a working board for comparison without making any changes on the non working board. Please provide close up images of the EN pin.
    3) When you said you have 10k pulldown resistor, do you mean that it is connected as a load?
    4) In your first post you said this: "・When discharge by resister, EN was enabled(High state).". What do you mean by "discharge by resistor"?
    5) Take scope shots with the 10k ohm pull down resistor connected.
    6) What is the green trace on the scope shots you sent?
    7) How critical is the voltage droop on the 14V output? Are the POSCAPS keeping regulation at some other node and the regulation needs to be very tight?

    Regards,
    Akshay

  • Dear Akshay Mehta-san

    Sorry for delay, I'll answer your question later, please wait.

    Additional information is below.

    ・Load circuit is attached.

    ・Inductor:0520CDMCDS-100MC (Sumida)

    ・This move is depend on device, 3% of LMR62421 only occuring.

    ・Charge current:200mA, actual:50mA~150mA.

     

    Best regards,

    Satoshi

  • Dear Akshay Mehta-san

    Sorry for delay, I answer your question, please see below.

    -------------------------------------------------------------------------

    1) Can you please give us more details on the EN transitions you are doing? Please probe the EN pin and zoom in on the oscilloscope.

        You said you are doing the entire EN transition from high to low to high in 10ms.

    【Answer】

    It is dropped zero-level during about 8.2ms.

     

    2) Please send scope shots of a working board for comparison without making any changes on the non working board.

     Please provide close up images of the EN pin.

    【Answer】

    Please see attached below. (CH1(yerrow) is "DC/DC EN".)  

    ※CH2(pink):Vout,CH3 and CH4:Control signal

    3) When you said you have 10k pulldown resistor, do you mean that it is connected as a load?

    【Answer】

    Yes, this is bleeder resistance of 14.4Vout to GND.

     

    4) In your first post you said this: "・When discharge by resister, EN was enabled(High state).". What do you mean by "discharge by resistor"?

    【Answer】

    Discharge resister connected POSCAP.

     

    5) Take scope shots with the 10k ohm pull down resistor connected.

     【Answer】

    Please see attached below.

    4.7k ohm pull down is attached,too.

    ※Enable was recover charging after 50ms to Low→High.

    Customer thinking 2 way cause.

    ①Charge voltage drop fast because of bleeder resistance.

    ②DC/DC output made fixed load.

    (Which seems right?)

    6) What is the green trace on the scope shots you sent?

    【Answer】

    Green waveform is Vout of AC-waveform.

     

    7) How critical is the voltage droop on the 14V output? Are the POSCAPS keeping regulation at some other node and the regulation needs to be very tight?

    【Answer】

    There are no requests of droop and regulation in particular.

    -------------------------------------------------------------------------

    Additional information,

    ・The probability that this phenomenon occurs is 5pcs/158pcs.

    ・For occured device, recover time is proportional to pull down resister value.

     ⇒Customer take a guess that LMR62421 have internal 20k ohn resister, is it correct?

       (Sorry for no waveform because of customer confidential data)

    Best regards,

    Satoshi

  • Is there other points I should confirm?
  • Any update on this?
    I think that the reason which occurs by 5pcs/158pcs depends on dispersion of internal resistance, is it correct?

    Best regards,
    Satoshi
  • Additional information, the customer guessed time from EN:Low⇒High to be short(8ms), set Low to 200ms and measured.

    but, the voltage of 14.4V#1 will go down slowly.

     

    ※Is a reason because it isn't charged from a LMR62421?

    As a result, time of EN_Low isn't related to a this factor.

    【Additional Question】

    As well as EN, when Vload is being seen and output on/off is switch, how long is LMR62421 need a min detection current?

    I'm infering that it can make the fixed resister(min) connected to Vload, and avoid.

    Is it correct?

    Best regards,

    Satoshi

  • Any update on this?
    Information became a lot, so may I summarize?