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LM3405: How to set the ratio of ripple current to LED current

Takao Yamamoto5
Expert 1690 points
Part Number: LM3405

Dear all,

Could you teach about the way to set the ratio of ripple current to LED current?

You say that 'The ratio of ripple current to LED current is optimized when it is set between 0.3 and 0.4 at 1A LED current.'.

And you also say that 'At a current of 0.2 A, r can be made as high as 0.7'.

In addition, you also say r = 24%~119% at Table 1.

I cannot understand how to choose this ratio.

Best regards,

  • 0
    TI__Mastermind 25500 points

    Hello Takao,

    Table 1 is showing what % the peak-to-peak inductor current ripple is compared to the average inductor (and output current). So with 1A output and a 4.7uH inductor the inductor peak-to-peak ripple current will be 510mA (or 51% of 1A). The ripple can become higher at lower output currents because you are farther away from the current limit, the max ripple ratio r specified in the datasheet is just to make sure you do not trip current limit.

    Note that the LED ripple current is further reduced by the output capacitance. But a lower inductor ripple current will result in lower LED current ripple given the same output capacitance which this device recommends.

    Regards,

    Clint