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Question about TPS27801A equation(3)

BX27
Genius 3900 points
Other Parts Discussed in Thread: CSD25213W10, TPS27081A

Hi there, 

we are using TPS27801A, could you please explain how equation(3) is derived ? 

We also considering using power MOSFET,such as CSD25213W10,  to build a load switch with similar structure with TPS27081A, and soft-start feature is expected, therefore we hope to know why t_rise is calculated as so.

Thanks.

  • 0
    Genius 3900 points
    Another question is , why toff > 2 × R1 × C1 (on page 6)? I think the charging time of C1 should also be affected by C(load).
  • 0 in reply to BX27
    TI__Genius 15770 points

    Hi Yi Xiao,

    I have contacted the appropriate engineer and they should get back to you soon.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Genius 3900 points
    Hi Alek, is there any update on this issue? Thanks.
  • 0 in reply to BX27
    TI__Guru**** 164586 points

    Yi,

    The turn on formula has some correction factors (3.9 and 2/3). ON/OFF level (same as VIN for formula) sets voltage across R2 resistor (OF/OFF)-VGS(Q2). R2 resistance sets current that charges C1. C1 voltage is in proportion to rising output voltage.   

    Toff formula shows the time for the switch turning off. The TPS27081A will not discharge the output capacitor.

  • 0 in reply to Ron Michallick
    Genius 3900 points

    Hi Ron,

    Thank you for reply.

    At first I thought the formula(3) is a theoretical one, but from your answer, could I think it's actually a experimental formula with some correction factors? 

    When ON/OFF =High, C1 is charged through R2, is there any process to analyze from circuit principle and lead to this formula? I thought the "time" should be with R*C, but when divided by VIN(2/3), the unit seems to be incorrect.

    We are refering to TPS27081A's similar architecture in order to build loadswitch with power mosfet, besides of simulation, we hope to make theory calculation to define the rising time of Vout, so we can compare the results of theroy, simulation, and hands-on test.

    Hope you could kindly explain more. Thanks very much!