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TPS7A92: Soft-Start time

Sam Ting
Expert 3590 points
Part Number: TPS7A92

Tool/software:

Hi team,

We've got a SS time problem, please see the schematic with Css= 470nF.
When SS_CTRL= GND, it is ok that the measured SS time is 54.7ms close to the calculated value of 61ms.
When SS_CTRL= VIN, the measured SS time is 1.3 sec, far from the calculated value of 4ms. Could you help to explain this gap? Thanks!

SS_CTRL= VIN

SS_CTRL= GND

Regards,
Sam Ting

  • 0
    TI__Mastermind 37065 points

    Hi Sam,

    Can you show me Vin and Vout on the same plot?  I would like to see if Vin is drooping during turn on and causing the LDO to enter dropout.  If so, adding more Cin or removing the ferrite bead should help.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 3590 points

    Hi Stephen,

    It does not seem to be a dropout problem because of VIN= 1.8V, VO= 1V and max load current= 0.61A.

    Regards,
    Sam Ting

  • 0 in reply to Sam Ting
    TI__Mastermind 37065 points

    Hi Sam,

    If the ferrite bead has significant inductance at low frequencies, it will fight the change in current while charge is transferred from Cin to Cout during turn on.  When SS_CTRL is tied to VIN this charge transfer will happen in a shorter time and Cin will droop.  If the source impedance is high (either from the ferrite bead or from the input power supply) then Vin will droop significantly. 

    Using the charge transfer equation: Q = CV, then the charge "Q" required to charge up Cout is Q = CV = 42.1uF * 1V = 42.1uC

    If the impedance on the line is high, then all of this charge comes from Cin.  The droop on Cin is thus: Vdroop = Q / Cin = 42.1uC / 33.1uF = 1.27V.

    We need to measure Vin with Vout to confirm that Vin is not drooping too much during turn on and placing the LDO into dropout.

    Thanks,

    Stephen