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ADM1272's ISTART current limit and Start-Up CB(circuit breaker)

Dear ADI Expert,

What is the difference between ADM1272's ISTART current limit and Start-Up circuit breaker?

Thanks.

Best regards.

Andy Yang.

  • Hi Andy,

        The ISTART current limit sets the SENSE voltage (SENSE+ - SENSE-) at which the ADM1272's analog current control loop will actively throttle current flowing through the MOSFET. The ADM1272 will pull down on the GATE node to prevent current from rising beyond the ISTART limit during the initial inrush event.

        The CB threshold, on the other hand, is the SENSE voltage above which the ADM1272 determines that the current has exceeded its limit. This CB threshold is set just below the current limit so that CB is guaranteed to be active while the ADM1272 is operating in current limit. If the ADM1272 operates with current above the CB threshold until the ESTART pin voltage exceeds 1V then the CB trips and the FET shuts down.

    Thanks!
      Nathan

  • Dear Nathan,

    Thanks for your answer. I have 4 other questions.

    1. Is there a hidden Register for ADM1272 to adjust the START-UP CURRENT LIMIT, just like 0xF6. It's just that 0xF6 is adjusted to be smaller, what my customer wants is to increase.

    2. Does ADM1272_Workbook_0.94 calculate the tolerances of +/-20% when calculating Inrush Current?

    3. The ADM1272_Workbook_0.94 calculation tool shows "ISTART should be> 2x above the Inrush Current". Should I use Typ or Max for Inrush Current? 

    4. The ADM1272_Workbook_0.94 calculation tool does not have the Start-Up CB (circuit breaker) field. Is it not necessary to consider this item when calculating?

    Thanks.

    Best regards.

    Andy Yang.

  • Hi Andy,

    1) There is no hidden register for adjusting ISTART upward. You can only adjust downward with PMBus. In order to increase the current limit you must adjust the voltage at the ISTART pin.

    2) Yes, the workbook uses the 20% tolerance on capacitance when calculating inrush currents.

    3) I suggest using MAX inrush current when setting ISTART, but this is a judgement call based on your knowledge of system tolerances. If you have sufficient SOA margin at startup you should keep ISTART as high as possible to avoid false startup problems.

    4) This is a good point, as at startup the CB can be significantly lower than the current limit. This is one of the reasons that we recommend a >2x margin on ISTART. I will consider adding the CB calculations to future design tools.

    Thanks!

      Nathan

  • Dear Nathan,

    Thanks for your answer. I have 1 more question.

    To use MAX inrush current when setting ISTART, do I use "INRUSH CURRENT BY DVDT (MAX)" or "INRUSH INCLUDING START-UP R and I LOADS (MAX)"?

    Thanks.

    Best regards.

    Andy Yang.

  • Hi Andy,

        The full inrush current includes R and I loads, so you should use INRUSH INCLUDING START-UP and I LOADS (MAX).

    Thanks!
      Nathan