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TPS54040 Inductor Value from SwitcherPro

Jim Gavin
Intellectual 920 points
Other Parts Discussed in Thread: TPS54040

In SwitcherPro Desktop Version 3.11.5.2, selecting the TPS54040 and providing the following inputs:

Vin_min: 23V

Vin_max: 40V

Vout: 5V

Iout_max: 0.01A

The designer calculates an inductor value of 330uH.  This value violates the specification such that the minimum inductor ripple current of 30mA is not met.  With the given input values and the calculated inductance, the inductor ripple current can be calculated.  From the data sheet,

This gives an inductor ripple current of approximately 19mA, which does not meet the 30mA spec.  Does anyone know how the designer actually calculates the inductance?

  • 0
    TI__Intellectual 2710 points

    Hello Jim,

    We are looknig into this now and get back to you soon.

    Thanks,

    Hetal Joshi

    Senior Developer and Project Manager

    Analog eLab Design Center

    HPA Design Tools

    Texas Instruments, Inc.

  • 0 in reply to Hetal
    TI__Guru**** 191445 points

    The 30 mA ripple current is a suggested value not a requirement.  The inductor calculation is just the equation you post solving for Lo with I ripple = Iomax *Kind where Kind is teh percentage of allowable ripple current (usually Kind = 0.2 -0.3, but you can use more or less depending on how much ripple current you can tolerate in your design).  So for very low output currents, large inductor values can result.  In this case the calculated value is around 2000 uH.  Switcherpro has a coded in limit of 330 uH as the largest allowed value so that is what it chose.  I does not explicitly check for the 30 mA limit.  I actual practice, I would be inclined to size my inductor value based on the DCM/CCM threshold at those very low output current levels.  I would probably design to always operate in DCM, which would set the inductor value to right around 330 uH anyway.  It amy all be moot as there is a high probability that this circuit will operate in eco-mode (which switcherpro does not comprehend at all) anyway.