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LP38511 unity gain cross over frequency

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LP38511, LP38511-ADJ
Hello,
 
Could you please kindly let me know the unity gain cross over frequency of LP38511 ?
 
---
LP38511 need feed forward capacitor Cff.
I understood that Cff make Fz  to between 20kHz ~ 40kHz..
 
In the page 4 of below Application note , there is explanation of feedforward zero as  
 "The optimum frequency location for the feedforward zero is typically about 1/3 of the unity gain crossover frequency."
 
I think that unity gain cross over frequency of LP38511 is about 60~120kHz.
 
Best Regards,
Ryuji Asaka
  • 0
    TI__Mastermind 22610 points

    The LP38511-ADJ datasheet fully describes the process for determining an appropriate value for Cff for the LP38511-ADJ. The datasheet also offers a table of suggested values for R1, R2, and Cff for a range of output voltages.

    " ... Cff make Fz  to between 20kHz ~ 40kHz ... "

    You need to know your R1 value used to set the output voltage, then use the formula from the datasheet

          Cff = 1 / (2 x π x R1 x fz) , where fz is 30kHz

    or,

    Calculate for the minimum and maximum Cff values at fz=40kHz and fz=20kHz, then select a capacitor value in-between the calculated min/max values.

     

  • 0 in reply to Donald Jones
    Guru 20090 points
    Hello Donald san,
     
    Could I have the unity gain cross over frequency and circuitly for the unity gain cross over frequency?
     
    I can't understood why we must set Fzero between 20kHz ~ 40kHz due to there is not  bode plot data.
    So, I would like to know unity gain cross over grequency.
     
    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Mastermind 22610 points

    The 20kHz to 40kHz takes into account the variations in UG crossover due the operating Vout range, the output current range, and a 10uF ceramic for Cout.

    In general, the UG crossover will be in the 75kHz to 150kHz range.

    There is no model, or circuitry, available for the LP38511-ADJ control loop.

     

  • 0 in reply to Donald Jones
    Guru 20090 points
    Hello Donald san,
     
    Thank you very much for the reply.
    I understood.
     
    Best Regards,
    Ryuji Asaka
  • 0 in reply to Donald Jones
    Guru 20090 points

    Hello Donald san,
     
    Sorry.  I got the question from customer again.
     
    Could you please kindly let me know the best Cff for each below six conditions ?
    Our customer must build circuit as stable as possible.
    Thus, they expect best Cff value for below conditions.
    In the datasheet , Cff make Fz to between 20 kHz ~ 40 kHz.
    There is tolerance from 20kHz to 40kHz, so we can't understand that  best Fz frequency.
    Fz should make to 20kHz? or 30kHz? or 40kHz?

    Customer conditions are following.
    ------------------------------
    *Cout is 10uF of multi layer ceramic capacitor
    (1)Vin:2.5V , Vout:1V , Iout:0.41A
     
    (2)Vin:2.5V , Vout:1.2V , Iout:0.16A
     
    (3)Vin:3.3V , Vout:1.2V , Iout:0.02A
     
    (4)Vin:3.3V , Vout:1.8V , Iout:0.19A
     
    (5)Vin:5V , Vout:3.3V , Iout:0.20A
     
    (6)Vin:2.5V , Vout:1.8V , Iout:0.56A
    ------------------------------
    If you need more information , please let me know.

    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Mastermind 22610 points

    Component selection and standard component values will make it unlikely to get a set of components that will calculate out to an fz of exactly 30.00 kHz.

    As a result, there is a range of acceptable values available. The intention of having a 'window' was to make Cff selection easier, not more difficult.

    There is only marginal difference in dynamic performance between fz= 20 kHz and fz= 40 kHz.. This results in a stable, but slightly under-damped, control loop.

    Setting fz lower than the recommend range will provide a stable, but possibly over-damped, control loop.

    To calculate the proper Cff, the value for R1 (the resistor between the OUT pin and the ADJ pin) must be known first.

    Please forward to me the R1 (and R2) values used for the six voltage options listed, and I will calculate the proper Cff per the formula given in the datasheet (#5, page 10)

     

  • 0 in reply to Donald Jones
    Guru 20090 points
    Hello Donald san,
     
    Thank you for the explanation.
    I understood.
     
    I calculate Cff by the datasheet information to aiming 30kHz Fzero.
    Thank you very much for your great support .
     
    Best Regards,
    Ryuji Asaka