• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 585 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.

TPS92515: TPS92515 Frequency Problem

user5956241
Intellectual 565 points
Part Number: TPS92515

Hi Sir,

We plan use TPS92515-Q1 designd for Sequential Turn with BC846B.
There are frequency problem need your comfirm. In datasheet figure result description fixed input voltage, the device operates at the maximum frequency at 50% duty cycle and the frequency reduces as the duty cycle becomes shorter or longer.

Why we use equation(25), (26) calculate the frequency and draw graphical in excel result different datesheet figure 12?

  • 0
    TI__Expert 3590 points

    Hello,

    You should not use the equation you quote for calculating the Fsw. That equation uses a target Fsw and then calculates the required off time.

    Here is how you should proceed to get the Fsw variation across Vout, using the basic Buck volt-second balance equations to a first order:

    1. Ton = Iripple*L/(Vin-Vout)

    2. Toff = Iripple*L/Vled

    3. Tsw = Ton + Toff = Iripple*L*[1/(Vin-Vled) + 1/Vled] i.e. Iripple*L*Vin/[(Vin-Vled)*Vled]

    4. Fsw = 1/ Tsw = Vled*(Vin-Vled)/(Vin*Iripple*L)

    The converter operates such that the off time and Vled product is constant, and therefore the ripple is constant. Hence Iripple*L is constant.

    Therefore Fsw = Vled*(Vin-Vled)/(Vin*constant). Now, if you keep Vin constant and vary Vled you will get Fsw = Vled*(Vin-Vled)/constant which has the characteristic shows in the datasheet - where the Fsw = 0 at Vled=0 and at Vled=Vin, and is maximum at Vled=Vin/2.

    Hope this helps.