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TPS65131: Inductor value calculations

lemmiwinks
Expert 1050 points
Part Number: TPS65131
Other Parts Discussed in Thread: LM3310, LM2700,

Tool/software:

Vin = 3.3

Vpos = 15

Vneg = -15

Ipos = 0.1

Ineg = 0.1

Using the equations in the datasheet I get:

I_Lp = 710mA


I_Ln = 870mA


L1 = 14uH


L2 = 12uH

However, using the Webench Power Designer I get:

I_Lp = 196mA

L1 = 4.7uH

It seems the equations in the datasheet are wrong because even for a relatively low current of 100mA I end up with very large inductors.

  • 0
    TI__Mastermind 29590 points

    Hi,

    Is the I_Lp the inductor peak current? If so, the webench result looks not correct. With Vin=3.3V, Vpos=15V and Ipos=100mA, the average current will be already over ~500mA.

    BR

    Patrick

  • 0 in reply to Patrick Yao
    Expert 1050 points

    Yes, I_Lp is the peak inductor current.

    Perhaps somebody should fix Webench.

    However, the equations for the inductor values in the datasheet are suspect.  For a PWM converter the minimum inductance is usually defined by the duty cycle and Rds_on of the switch.

    For example, if I use the equations from, say, the LM2700 or LM3310 (typical PWM boost converters) I get a duty cycle of 78% and a minimum inductance of 7.5uH which seems more reasonable.

    The equations in the TPS65131 datasheet give an inductor value that is inversely proportional to the output current.  For 10mA of output current the formula yields an inductor value of 140uH!!!  Certainly something is wrong.

    Thanks.

  • 0 in reply to lemmiwinks
    TI__Mastermind 29590 points

    Hi,

    This is because the inductor current ripple is chosen as 20% of the I_Lp. Actually the recommended inductance is between 3.3uH to 6.8uH.

    BR

    Patrick