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LM3409-Q1: Minimum Ripple Current

Part Number: LM3409-Q1
Other Parts Discussed in Thread: LM3409


The LM3409-Q1 datasheet (SNVS602L) section 8.3.4 gives a minimum ripple current requirement in order to achieve reliable current regulation:

The LM3409 Demonstration Board User's Guide (SNVA391D) section 8 shows a table of values for alternate designs:

Design 2 and Design 4 appear to violate the minimum ripple current requirement.

Design 2 should have a min ripple = 24mV / 80mOhm = 300mA (but the table has delta_I_LED of 70mA).

Design 4 should have a min ripple = 200mA (but the table has delta_I_LED of 60mA).

Is there an error with Design 2 and Design 4, or, is there an error with the minimum ripple requirement, or, is there an error with my understanding?

Thank You!

  • Hi, Seth, 

       is inductor ripple current,    is LED ripple current. If no C5, they are same value, while if there is C5 after the inductor, then LED ripple current will be reduced and the values are not the same. 

    For example, in design 2, inductor ripple current is calculated as 350mA, higher than 300mA.