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LM3409: DCM operation

David Naviaux
Intellectual 590 points
Part Number: LM3409

Hi,

Ideally, I would be able to operate the LM3409 in DCM since my minimum output current is less than 1/2 of the inductor ripple current.  Does the LM3409 output current still follow a linear relationship with the voltage applied to iAdj pin in DCM?

Thank you,

David

  • 0
    TI__Mastermind 28655 points

    Hello David,

    No, it does not.  The LM3409 regulates peak current.  If the current ripple is fixed while in CCM the change in iadj will be linear with an offset of 1/2 of the current ripple.  As you drop into DCM there is a portion of the time when there is no current.  Example; if you are regulating the peak at half of the CCM ripple half of the time there would be no current (you only get the top half of the CCM ripple, the bottom half there is no current.  This makes the average current 1/4 of the peak instead of 1/2 if the ripple if it was at the CCM/DCM transition.  That is just this one operating point.  It may be easier to see graphically if you draw it out.  It can be used in this mode, you just have to account for where it is operating.

    Figure 32 on page 32 of the datasheet shows an example.

    An equation to calculate this shouldn't be too hard to come up with, the average current is the (peak current/2)*ontime%, ontime% will be (peak current)/(pkpkcurrent).  Pkpkcurrent is the CCM peak to peak current ripple.

    Best Regards,