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TPS54040: About Issavg

Tsukui Yusuke
Guru 21930 points
Part Number: TPS54040

Hi,

Could you tell me about Issavg.
Is Issavg the same value as Iss=2uA?
If that is not the case, please tell me how Issavg calculates.



Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Expert 6165 points
    Hello Tsukui,

    Issavg is inrush current, the current in excess of load current that the power supply designer allows the supply draw during startup. This current is the charge needed to apply voltage across the output caps divided by soft start time. It is a power supply design parameter, not an IC parameter.
  • 0 in reply to Robert Blattner
    Guru 21930 points

    Robert-san,

    Thank you for your support.
    Could you tell me how to calculate Issavg.
    Customer want to estimate Issavg.

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Expert 6165 points
    Hello Yusuke,

    Issavg = (load current not due to capacitor charging during startup, which is set by the power supply designer) – 0.5 A. This value can be used to calculate the size of the capacitor connected to the SS/TRK pin. If the power supply designer does not know what maximum load current is during soft start, we find that for Cout < 60 uF, 2 ms is a good soft start time. If Cout is > 60 uF, 4 ms works better. This rule of thumb is based on inrush being < 30% of rated current.