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TPS57160-Q1: Selecting the Switching Frequency Fsw

noureddine tariba
Prodigy 180 points
Part Number: TPS57160-Q1


I need to calculate the switching frequency of  TPS57160 based on the datasheet (page 18  / 8.3.12  Selecting the Switching Frequency)

 I Have a question about the controlable ON time  Ton . How to define correctly the Ton time for the TPS57160 . in the datasheet indicate that the minimal Ton is 130ns  but i got i high switching frequency more than 2000khz 

Also how to match  between Fsw(div) and fsw(skip) ?

 In my case I need to design a step down converter  from 12V to 3.3V @ 1A

 BR 

 Noureddine 

  • 0
    TI__Guru**** 191445 points

    The minimum on time is fixed.  It is not frequency dependent.  You need to perform both calculations and use the lesser value as your switching frequency.

  • 0
    TI__Expert 5745 points

    When work in PWM mode , the Ton time is decided by frequency and D=Vout/Vin, Ton=D*T

    When work in PSM(pulse skip) mode, the Ton time is equal to the min Ton time.

    In your case, if the load is 1A , it should be in  PWM mode ,

  • 0 in reply to Vental Mao
    Prodigy 180 points
    Hey Vental
    ok thank you for your repply

    So the Ton , will be used to define Fsw_max(skip) as detailed in the datasheet , also Fsw(shift). and we choose the less value .

    other the question about Fsw(shift) how to define F(div), and Vout(SC)?

    thank you in advance
    Noureddine
  • 0 in reply to noureddine tariba
    TI__Expert 5745 points

    it depends on the Ton.  If the duty divide the frequency (the Ton )  is less than the Ton-min, it will be into pulse skip mode

  • 0 in reply to Vental Mao
    TI__Guru**** 191445 points
    It is a little more complicated than that. There are two cases that can cause pulse skipping. For this topic, pulse skipping occurs whent he required on time is less than the minimum controllable on time. Designing per equation 8 will prevent that from happening. You can also skip pules when the device is in eco-mode. this occurs when the COMP pin voltage is at the minimum clamp voltage. This is an automatic feature and cannot be overridden.
  • 0 in reply to JohnTucker
    TI__Expert 5745 points

    Hi John  thank you very much !

  • 0 in reply to JohnTucker
    Prodigy 180 points
    Hey john
    to avoid the pulse skipping, we need to define properly the fsw
    So in my case, I have the maximum input voltage vinmax=12.6V  which corresponds to the maximum frequency 2500khz  referred to the graph 33. 
    for Rdson=0.2ohm | DCR=0.078ohm|  Vd=0.5V| vout=3,3v @ 1A
    I think for now, I need just to define the fsw.shift referred to eq.9 to choose the less value between the two frequency.
    other things,  why we don't use just the curve 31 and eq (7) ,to define the the switching frequency?
    BR
    Noureddine
  • 0 in reply to noureddine tariba
    TI__Guru**** 191445 points

    Figure 31 and eq 7 define the switching frequency with respect to the RT set resistor.  It will not limit whether or not you are skipping pulses.  If both your EQ 8 and EQ 9 calculated frequencies are above 2500 kHz, you can use any frequency in the range of 100 to 2500 kHz.  if EQ 8 or EQ 9 are less than 2500 kHz, that EQ defines you maximum frequency.  You can also skip pulses due to eco-mode.  that will occur at lower currents, but is defined by the minimum COMP clamp voltage.  See this app note:

    www.ti.com/.../slva388

  • 0 in reply to JohnTucker
    Prodigy 180 points
    I have calculated the Fsw.skip, about 28390kHz, it's more than 2500kHz ,that means the skippings pulses will not occurs during PWM mode if I am not wrong. One thing need to be confirmed that switching freqeuncy correspond to Vinmax is 2500kHz, and Ton=Vout/(Vinmax*fswmax) =12.6V/(3.3V*2500khz)=105ns but this value is less than the miminum ON time (130ns) and that will occurs the skippings pulses mode. Any explications ?

    Thanks

    Noureddine
  • 0 in reply to noureddine tariba
    TI__Expert 5745 points

    Hi the D= Vout/Vin is an ideal condition. In fact, there are some parasitic parameters also need to be considerate. 

    And if you set the frequency to 2.5MHz, that means there is less margin for the PWM shift to the skip mode. So you can choose the frequency less than 2.5MHz, 2MHz may be better.

  • 0 in reply to Vental Mao
    Prodigy 180 points

    Hey Vental 

    Thank  you

     BR 

    Noureddine