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LM20143DESIGN-CALC: Inductance calculation

Part Number: LM20143DESIGN-CALC
Other Parts Discussed in Thread: TPS62813, LM20143

Tool/software:

Hello

I would like to have precision regarding the calculation used for the Inductance in the datasheet and in the LM20143DESIGN-CALC.

The formula given is : 

Our data are the following : 

Vin = 3.3V

Vout = 2.5V

F_sw = 755kHz

Iout = 0.3A

D = Vout / Vin = 0.75

Therefore L should be included : 5.35uH and 10.7uH

However on the LM20143DESIGN-CALC : the value recommanded for L is : 0.9uH.

I also tried to simulate with Webench and the Self recommanded is also around 1uH.

How is it calculated ? Which formula is used to get these values ?

Thanks in advance,

Regards,

Florent

  • Hi Florent,

    Have you checked out TPS62813? This is a much newer part compared to LM20143.

    For your calculations, you need to use 3A for IOUT(max) instead of 0.3A. Then the calculated inductance will be between 0.535uH and 1.07uH. The IOUT(max) refers to the max current this part can source.

    Best regards,

    Varun

  • Hi Varun, 

    We already used the LM20143 but for next design I'll keep in mind the TPS62813.

    No I shouldn't use 3A for Iout because in my application the max current is 0.3A and not 3A.

    The self size will result in a current ripple in the Self and my self must be sized for 0.3A and not 3A.

    The formula used to calculate the inductance is the following : 

    In general we consider Delta_IL = 0.3 x Ioutmax so Delta_IL = 0.09A.

    It is explained in the datasheet the Delta_IL must be in the range 25% to 50% of the I_out current.

    With f_sw = 755kHz // Vin = 3.3V and Vout = 2.5V,

    => L >= 5.35uH 

    Regards,

    Florent

  • Hi Florent,

    Sorry if this confused you, but what the datasheet means is the rated output current. This part is rated for 3A output current. The reason to have the rated current in the formula is to ensure that the inductor chosen will give enough current ripple for the control loop. The control loop requires a ripple of at least 10% of rated current (3A).

    Best regards,

    Varun