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TPS40304 Calculation for Inductor

Christine Chuah
Prodigy 160 points
Other Parts Discussed in Thread: TPS40304

Hi

Attached is an image of the design schematic generated from SwitcherPro from Vin of (12V - 16V) to an output of 1.2V @10A using TPS40304.

 

From here, the inductor L1 is 1.2uH. However, if we calculate it based on the formula in Page15 of TPS40304 datasheet, the inductor value that i get is 0.6uH.

L = (Vin_max - Vout)/(0.3 x Iout) x Vout/Vin_max x 1/fsw = (16V - 1.2V) / (0.3 x 10A) * 1.2V / 16V x (1 / 600kHz) = 0.617uH

 

I would like to confirm whether the tool is generating the correct design with the correct value or did I make a mistake in my calculations as the value shown in the schematic from the SwitcherPro tool (1.2uH) seems to be twice the calculated value (0.617uH).

Thanks.

 

Regards,
Christine

 

  • 0
    TI__Genius 17240 points

    Hi Christine,

    I think the discrepancy may come from the allowance of current ripple -- the factor 0.3 in equation (3) of the datasheet. As a rule of thumb, the synchrounous buck power inductors are sized for approximately 20-40% of the full load current as the peak-to-peak ripple current. Higher the ripple, more loss on the inductor but smaller inductance is needed, which leads to faster transient response and possibly smaller size. The SwitcherPro may use the factor of 0.2 to allow a 20% current ripple. But the factor of 0.3 is also good. So you can pick up an inductor with smaller inductance.

    Regards,

    Na

     

  • 0 in reply to Na Kong
    Prodigy 160 points

    Thanks for the clarification!

  • 0 in reply to Christine Chuah
    Expert 1985 points

    Hi,

    Based on the TPS40304 datasheet, I see that the minimum duty cycle can go upto 6% approximately. In the above example of Vo = 1.2V and Vinmax = 14V, D = 8.5%, can we go further down on the Duty cycle to 6.6% (Vo=0.8V, Vinmax=12V) or so? please confirm the same, it will help a lot in my design. 

     

    Regards,

    Khader Shareef

  • 0 in reply to Shareef
    TI__Genius 17240 points

    Hi Khader,

    I think you can use TPS40304 for your application of 12V to 0.8V. My concerns are

    (1) During start-up, there might be pulse-skipping and cause some noise on the output.

    (2) For load step-down transient, since you will not have much margin for the duty cycle to further go down, the overshoot on output voltage  may be high.

    It should work fine for normal operation.

    Regards,
    Na