• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 663 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS73501, Feedback Capacitor

Rhino
Prodigy 190 points
Other Parts Discussed in Thread: TPS735

Dear All

 

I have a question on TPS73501.

 

According to the datasheet, the feedback capacitor is required for stability and its recommended value is between 3pF and 1nF based on R1//R2 = 250 kOhm.

And our customer wants to change the R value.

 

Q1. When R1//R2 value is changed from 250 kOhm, what is the recommended capacitor value?

Staying between 3pF and 1nF?

Or should be changed accordingly to keep the time constant (C * R1//R2) the same as before?

 

Q2. Regarding soft start, S.S. time constant depends only on feedback capacitor value?

Or depends on ( C * R1//R2 )?

 

Q3. Could you please give me “turn on response curve” when soft start is activated.

Curves of several conditions are fine.

 

Best Regards,

Rhino

  • 0
    TI__Expert 8890 points

    Hello Rhino,

    Q1 Response: The original test/design engineers decided that under certain load and line conditions that the phase margin was too low, and so they recommend that a zero & pole combination be added to the control loop (accomplished by adding the feedback capacitor, Cfb).  Yes, you are correct, if the parallel feedback resistor combination is changed then the feedback capacitor must be appropriately changed so as to keep the zero-pole frequency approximately constant.  We recommend that setting the pole frequency to 175kHz for the parallel resistor combination (plus feedback capacitor, Cfb) will take care of any potential problems: F_pole = 1/(2 x pi x (R1||R2) x Cfb)   =>  Cfb = 1/(2 x pi x (R1||R2) x 175k).

    Q2 Response: The softstart ramp-time characteristic - due to the affect of the feedback capacitance value -  will be primarily a function of the instantaneous output voltage, Vo(t), and the voltage occurring at the feedback pin, Vfb, due to current through the feedback capacitor, Cfb x dVo/dt x R2:  Vfb = Vo(t) + ( R2 x Cfb x dVo/dt).  In cases where the feedback capacitance is not used then the softstart is determined by the slew rate (or large signal bandwidth) of the control loop.

    Q3 Response:  Basic turn-on characteristics for the TPS735 are given in Figures 15 and 16 for the case where a noise reduction capacitor is employed and has the added effect of slowing the startup.  Generally the startup characteristic will vary dramatically depending on the feedback capacitor selected.  If you would like to select a feedback capacitor we can generate an oscope plot for you.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 190 points

    Hello Bill,

     

    Thank you for the answer above.

     

    Today I received successive question from the same customer.

    The questions or requirements are as below;

     

    1.      smallest size of Cfb

     

    After several considerations by the customer, finally they decided to turn-on as first as possible.

    At the datasheet of TPS73501, “the feedback capacitor is required for stability” is described.

    So the question is what the minimum value of Cfb is.

     

    2. Turn-on response curves are requested.

     

    The customer has chosen the candidates of Cfb value as 1pF and 3pF.

     

    So the request is to send me the 3 kind of turn-on response curve which condition is 1pF, 3pF, and your recommended minimum value.

     

    Best Regards,

    Rhino