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LTC3644 Applications example in error

Category: Datasheet/Specs
Product Number: LTC3644 LTC3644-2

Lo and behold the Applications example had a similar requirement as do I. on page 15 of the datasheet the current requirement (OUT) is stated Iload1=Iload2=400 mA, Iload3=1a Iload4=1.25A. However in the calculations the ripple current is 40% of Iout or 0.5A for determining the inductor value. which results in higher values of inductors than calculated. When putting in the correct values I get an inductor value of 15uH for both 5V and 3.3V. Is this an error? Isn't a 15uH too large for a 400 mA current. 

When putting the actual values Iload at 5V of 0.1A and 3.6V of .3A I get inductor values of 33 and 15 uH. Is this part not good for low currents? I like it because I can tie the phases of the regulators to be anti-phase.The 2.5 and the 1.8V circuits are fine. I'm just concerned about the low current inductors.Could I get away with a 10uH inductor?

Is there a better way to get the four outputs in phase or anti-phase?

Thanks

  • Hello,

    if you check the formulas on page 15 you will see that smaller output currents will lead to higher inductor values to follow the 40% ripple current guideline.

    If you want to use smaller inductors you will increase the ripple current and end up operating in discontinuous mode. Higher ripple current might demand for more output cap as well (when maintaining same voltage ripple with larger inductor.

    You should play around with the LTSpice example of LTC3644 to see the difference when changing inductor values.

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