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LM2735: Maximum Output Current

Julien Riou17
Prodigy 160 points
Part Number: LM2735

Hello, 

I want to use LM2735 in SEPIC configuration with :

Vout = 3.3V 

3V < Vin < 5V

according to the datasheet, D/D' = Vout/Vin in SEPIC. Therefore : Iout = Ilpk * Vin / (Vin + Vout) if I neglect the ripple current

 Ilpk  = 2.1 A over full temperature range so I should be able to have Iout = 1.25 A over full temperature range.

and at 25°C Ilpk = 3 A so at 25°C, I should be able to have  Iout  = 1.8 A

 

Could you confirm or infirm my calculations ?

 

Thank you in advance,

 

Kind regards

 

  • 0
    TI__Mastermind 31315 points
    Hi Julien,

    I've found that Iout/Ipk = D'/D. This gives a result of Iout = Ipk*(Vi/Vo). Can you show how you calculated the output current?

    Thanks,
    Richard
  • 0
    TI__Intellectual 2655 points
    Because of devices’ current limit has variation, we need take min value to do calculate. I suggest you use 2.1A to calculate no matter at 25degC or -40 to 125 degC.
    Richard is right, you should use this to calculate and take efficiency in to consideration. Iout = Ipk*(Vi/Vo)*eff
  • 0 in reply to Richard Fu22
    Prodigy 160 points

    Hello,

    Thank you for your answers.

     :

    On page 16 ILpk = Iout / D' (if we neglect ripple)

    In SEPIC design, on page 34, the equation is D / D' = Vout / Vin. With D = Vout / (Vout + Vin) and D' = 1 - D

    thus : D' = Vin / (Vout + Vin)

    And : Iout = ILpk * Vin / (Vout + Vin)

    How have you found Iout/Ipk = D'/D ?

    Kind regards,

    Julien

  • 0 in reply to Jing Ji
    Prodigy 160 points

    Hello,

    thank you for your answer,

    On page 16 ILpk = Iout / D' (if we neglect ripple)

    In SEPIC design, on page 34, the equation is D / D' = Vout / Vin. With D = Vout / (Vout + Vin) and D' = 1 - D

    thus : D' = Vin / (Vout + Vin)

    And : Iout = ILpk * Vin / (Vout + Vin)

    Taking into account the loss elements :

    I hadn't calculated the efficiency so I solved the equation (26) on page 36, with the values of the components in figure 33. 

    Vout = 3.3V :

    Using the equations in the datasheet, we find :

    Iout = (Ilpk*Vin*eff)/(Vin*eff + Vo)

    With Vin = 3V, the efficiency is 66.6% => Ioutmax = 0.79 A

    With Vin = 5V, the efficiency is 80% => Ioutmax = 1.1A

    Kind regards,

  • 0 in reply to Julien Riou17
    TI__Intellectual 2655 points

    Hi Julien,

     1 You can't use this equation: On page 16 ILpk = Iout / D' (if we neglect ripple)

     because this is for boost topology not SEPIC.

     2 If we neglect, MOS peak current is about Iin+Iout.

     I simulate it by SIMPLIS

     Test conditon:

      Vin=3V Vout=3.3V Iout=500mA,

      By the waveform, switching peak current is about 1.64A(blue IQ line) (this is no take loss into consideration).

      If I change Iout=1A, switching peak current is about 3.96A

    Iout=1A

  • 0 in reply to Jing Ji
    Prodigy 160 points
    Thank you Jing Ji