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I was studing a linear's datasheet, a step down DC to DC switching regulator.

More specific: the LTC3615.

I am a little bit confused with ripple curent which involved to inductor value calculation. In the datasheet, PAGE 25, "Design Example" a ripple current considered 1A.

And here is my problem. In my research on google i found plenty information about this and all of them refer the ripple current, for properly application, a value of 10 - 30% of maximum DC Load current on the Output. And in the datasheet, Linear choosing that value 1A, for 3A maximum output. 1/3 of maximum current.

Is there some information that i haven't notice?

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  • \$\begingroup\$ Ripple current can be much bigger than load current so please provide links to what you read. \$\endgroup\$ – Andy aka Sep 19 '15 at 23:13
  • \$\begingroup\$ electronics.stackexchange.com/questions/49280/… \$\endgroup\$ – MrBit Sep 19 '15 at 23:36
  • \$\begingroup\$ cooperindustries.com/content/dam/public/bussmann/Electronics/… \$\endgroup\$ – MrBit Sep 19 '15 at 23:39
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    \$\begingroup\$ I think the value of the ripple current is one of the initial design choices ! So saying that the ripple current should typically be 10 - 30% of the DC load current can only be true for a limited set of cases. For example when the load current does not very a lot (for example: 0 A (off) or 1 A). But when the load current does vary a lot (for example, 0 A, 1mA, 100 mA or 1 A) and for all these currents you want a decent efficiency then a different choice for the ripply current (under full load) might be more advantageous. \$\endgroup\$ – Bimpelrekkie Sep 20 '15 at 11:32
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    \$\begingroup\$ I think that for the 1A output you can choose a 1A ripple current as the maximum current will always be below the maximum current the switch can handle. In te same way 3A DC + 1A ripple might just be OK but 3A DC + 3 A ripple will be too high for the switch so the ripple must be reduced to 1A. \$\endgroup\$ – Bimpelrekkie Sep 20 '15 at 11:40
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The term "ideal ripple current" is valid. I have used the term "porching factor" to define this. Marty Brown defines a ripple factor in his book which is correct.

The ripple factor is important because if it's too small the orthodox current mode chip won't have a proper ramp to sense and you will get instability and/or subharmonics, and you may have to fight with slope compensation to get the thing to behave.

There is good credence to the fact that if the slope compensation was large and ripple factor or porching factor was very small then you are almost back to voltage mode anyway. So, too little ripple is BAD in current mode.

Now, if ripple is big then it is true that peak currents are higher than "necessary" for a given output current.

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