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LP38511 / Slow output rise time

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: LP38511, LP38511-ADJ

Hi,

Our customer hope to slow the output rise time because LP38511 has no soft start function and they need to meet FPGA's power up requirement. To slow the rise time, they plan to add output capacitor. Then can you show the formula to estimate rise time with selected capacitance?

Now they evaluated LP38511EVM with 10uF of output capacitor and confirmed 0.05msec of rise time. They need to achieve slower than 10msec.

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Mastermind 22610 points

    "... To slow the rise time, they plan to add output capacitor ... show the formula ..."

    C = (I / (dV / dt)) 

    where:

       C= Cout

       I = Isc

       dV = change in Vout

       dt = Vout charge time

    Example : Cout required to limit Vout rise time from 0.0V to 3.3V to 10ms ...

       Cout = (1.5A / (3.3V / 0.01s)) = 4.545455E-03F = 4545µF = 4.5mF

     

  • 0 in reply to Donald Jones
    TI__Mastermind 22610 points

    Is the 'LP38511-1.8' version, or the 'LP38511-ADJ' version ?

    If this is the 'LP38511-ADJ' version, then the Cff value can be increased to create the same soft-start effect, and Cout can stay at 10uF.

  • 0 in reply to Donald Jones
    Guru 21675 points

    Hi Donald-san,

    Thank you for your comments. Our customer will use LP38511-ADJ, so they can increase Cff value. Then how about the maximum value of Cff you can recommend? And can we keep the stability if we increase Cff value?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Mastermind 22610 points

    This is a function of the pole frequency (Fp) as set by the Cff value.

    Output voltage rise will look like RC time constant as Cff charges.

    As a starting point ...

    You will need 5 RC time constants to be within 1%. This is a totally subjective value, adjust if you desire.

    If 5 RC time constants is 10ms, then 1 RC time constant is 2ms, and Fp = 1/0.002s = 500.Hz.

    The Cff value depends on the values used for R1 and R2 to set Vout.

        Cff =1 / (2 x 3.14159 x 500Hz x ((R1 x R2) / (R1 + R2)))

  • 0 in reply to Donald Jones
    Guru 21675 points

    Hi Donald-san,

    Thank you for your additional comment. I'll advise our customer to increase Cff value and then re-calculate voltage divider to meet recommended Fz range.

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Mastermind 22610 points

    "... increase Cff value and then re-calculate voltage divider to meet recommended Fz range ..."

    No.

    Forget about Fz and the datasheet recommendations for Cff.

    Since you want to extend the Vout ramp time, you will need to violate the datasheet recommendations. This application will force Fz and Fp to be lower than the recommended datasheet Cff values give. Fz will be lower than the 'optimum' which includes fastest possible start-up time, but stability will still be good.

       1) Calculate required R1 and R2 voltage divider for desired Vout.

       2) Calculate required Cff value for Fp= 500Hz (NOT Fz).

       3) Test, observe the behavior, and adjust Cff value as needed.

     If you know the R1 and R2 values I can calculate a Cff value and provide results from a simple op-amp spice simulation.

     

  • 0 in reply to Donald Jones
    Guru 21675 points

    Hi Donald-san,

    Our customer plan to use LP38511-ADJ for 1.0V and 1.2V as attached. Now I would like to calculate Cff value for 0.2msec, 10msec and 50msec for soft-start effect.

    It's appreciated if you will calculate for each circuit.

    Best Regards,

    Sonoki

    LP38511-ADJ schematic.pdf
  • 0 in reply to S.Satoshi
    TI__Mastermind 22610 points

    For R1= 1000 ohms, R2= 1000 ohms, Cff is

       0.2msec = 0.064 uF          63.662 nF
       10msec  = 3.183 uF     3183.099 nF
       50msec =15.915 uF   15915.494 nF

    For R1= 1680 ohms, R2= 1000 ohms, Cff is

       0.2msec=  0.051 uF       50.778 nF
       10msec = 2.539 uF    2538.900 nF
       50msec =12.695 uF 12694.501 nF

    My opinion is that these particular Cff values might be a bit odd. I would scale the R1 and R2 values up to give more useable values for Cff.

  • 0 in reply to Donald Jones
    Guru 21675 points

    Hi Donald-san,

    Thank you for calculating Cff values. I ask our customer to try these value and recommend to change R1 and R2 if they hope to use general capacitor value.

    Best Regards,

    Sonoki