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LM61495: light load operation

Tomer Ashkenazi
Prodigy 10 points
Part Number: LM61495

Hi,

i intend to use LM61495 for  (15-32v 24 nominal) 24v-12v conversion at 0.6A\0.3A (max load\light load).

would you recommend this use? , could you estimate the efficiency of this solution? (using 400Khz  switching frequency) 

also, assuming that the solution will conduct in light mode - does this change the inductor selection proses? (or still the eq in the datasheet regarding ripple and min value are still valid)

note that, when simulating in PSPICE I've got a waveform matching CCM conduction mode, while I was expecting DCM / frequency reduction ( ive used the PFM version).

best regards, 

Tomer Ashkenazi 

  • 0
    TI__Guru 50120 points

    Hello Tomer,

    Is there  particular reason why you are using a 10A DC current rated device for a 600mA max load application?

    Choosing this 10A would warrant picking an inductor with high enough saturation current to account for high-side current limit.

    If you are still early in your design, I think it would be more cost effective to use a lower current device like the LMR43606.

    You can use the WEBENCH Designer tool (in the product folder) to estimate the efficiency of the solution. 

    The selection of the inductor should always follow the common equation below where K is the ripple factor and Ioutmax is the DC current rating of the device. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    hello Jimmy, 

    my project already uses lm61495, and we would like to keep the amount of P.N. to a minimum.

    the need for an inductor with a saturation current as big as the high-side current limit baffles me  (20 A in this converter). 

    Doesn't the saturation current cause problems only if the inductor reaches it in a steady -state?

    In this design assuming I will conduct it at PFM, isn't the Ipk_min a sufficient lower bound for the saturation current? 

    (note that Webence also recommends inductors with Isat~2.5A ) 

    my biggest concern regarding this design is that the efficiency will be significantly lower than a converter that will work in CCM, 

    I've found it hard to validate using Webence and PSPICE. 

    could you estimate the efficiency difference  between using this and other more suitable DC2DC ?(I've thought about LMR51420XDDCR)

    thanks a lot for the response, 

    Tomer. 

  • 0 in reply to Tomer Ashkenazi
    TI__Guru 50120 points

    Hello Tomer,

    So long as the DC average current through the inductor doesn't significantly derate the effective inductance by 50%+ I think in steady-state operation it should be okay.

    The concern and need to have a higher Isat inductor is in the fault condition where VOUT is shorted to GND and the inductor current rails to high-side current limit. Picking a higher DC current rated (10A) device would naturally mean high-side short circuit current limit is higher.

    WEBENCH Designer should be adequate to get the efficiencies at different current levels and would be how I compare between devices.

    Regards,

    Jimmy