• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 389 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TPS54325: A question about the transfer function of On-time Timer.

user5809830
Prodigy 50 points
Part Number: TPS54325

Hi,

Recently, I have read an application report, named "D-CAP2TM Frequency Response Model based on frequency domain analysis of Fixed On-Time with Bottom Detection having Ripple Injection"(SLVA546). In this report, it writes "The Duty ratio cannot change while fixed on-time, so it should be considered as  delay expressed as  Hd(s)=e^(-sTon/2)". Can you explain to me why is it like this? Second, I am wondering how to get poles and zeros through this transfer function?slva546.pdf

  • 0
    TI__Mastermind 19080 points
    Hi,

    1. In my understanding, the delay transfer function stands for the control loop will re-start to monitor the Vout and do something to track the reference only after Ton. This is just personally understanding. Let's see if anyone else can give a better explanation.
    2. Different parts use different ripple injection circuit, so the calculation of ripple injection poles and zeros will be different. Usually it's fixed in internal IC design. The zero locates at about tens of kHz, which is well below the typical bandwidth of D-CAPx.

    Regards,
    Sam
  • 0
    TI__Genius 10070 points

    Hi,

    The on time can not change during on time even though the output voltage change, the device should wait until the on-time is over, Therefore, there is a delay.

    Shawn

  • 0 in reply to Sam(MPAA) Zhang
    Prodigy 50 points
    Hi Sam,

    Thanks for your reply and it helps a lot. However, I'm not clear that

    1. The delay transfer function only considers on time but doesn't consider off time , why?

    2. Why Hd(s) is e^(-sTon/2) but not e^(-sTon)?

    3. Can we get the exact poles and zeros solution of Hd(s)? (e.g. to convert exponential form to (1-s/z)/(1-s/p)).

    Best regards，
    Tintin
  • 0 in reply to user5809830
    TI__Genius 10070 points
    Hi Tintin,

    This is a average value, some papers use e^(-sDTon). e^(-sTon/2) is the delay transfer function, there are no poles and zeros.

    Shawn
  • 0 in reply to Shawn Nie
    Prodigy 50 points
    Hi, Shawn

    Thank you for the reply.
    During the on-time period, the high side MOSFET is turned on and the system is working. I don't understand which device is “the device”. Will you specifically tell me about the device? In addition, I wonder why Hd(s) equals to e^(-sTon/2) rather than e^(-sTon).

    Best regards,
    Tintin
  • 0 in reply to user5809830
    TI__Genius 10070 points

    Hi Tintin,

    The device means the IC.

    In my opinion, because the vout may change at any point  in the on-time. Ton/2 is the average value.

    Shawn

  • 0 in reply to Shawn Nie
    Prodigy 50 points

    Good morning, Shawn

    Thank you very much for your prompt reply. I may understand what you said, then I made a little reasoning according to your prompt, please help me check it out.

    I will wait for your reply. Thank you again for your patience.

    Best regards,

    Tintin

  • 0 in reply to user5809830
    TI__Genius 10070 points
    Hi,

    I think it is a roughly estimated model, somebody use e^(-sDTon), and for this paper, the writer think it is e^(-sTon/2). Or you can research it deeply and drive a more accurate model.

    Shawn