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LTM4644 & LTM4646 minimum output current

Hello,

I'm investigating the devices LTM4644 & LTM4646 as a solution to power a Zynq MPSoC device. Thanks to the multiple output capability, it'd be a nice solution.

However, power analysis from tools and datasheets from Xilinx show various current needs (high and low).

For high currents, I plan to combine outputs so this is not a problem.

However, for lower currents, I was wondering about the minimum output current for each ouput. Analysis shows 0.2A need for some voltage (0.9V).

Is there a drawback/limitation to use such power device for such low current consumption? If so, what would be the consequences.

Thanks.

Regards,

Sébastien.

  • Hi Sebastien,

    For lower output current/power consumption, the suggestion would be to run the LTM4644 in DCM mode for better efficiency at light load. However, the slight drawback is your output voltage ripple will be slightly larger compared to the output voltage ripple in FCM mode. For your higher load requirements, the LTM4644 can be left in DCM for the light loads, as the LTM4644 will transition from DCM mode to FCM mode as the inductor current is allowed to reverse during the off time.

    For your application, do you have any fixed frequency operation or output voltage ripple percentage requirements? If there are tight constraints, FCM mode is recommended by wagering the tradeoff of efficiency for smaller voltage ripple and fixed frequency operation.

    Hope this helps, thanks!

  • For very light load condition like 0.2A, the efficiency of the uModule will be rather low. Same as LTM4644, LTM4646 can also operate in DCM mode to improve the efficiency at light load. Drawback at DCM mode is large output voltage ripple.

  • Hello,

    Thanks for your answer. I'm no specialist of power supply to be honest. I believe i don't need specific frequency requirements. 1MHz default frequency should be ok.

    Regarding the ripple, I see the Vout(dc) is ~0.023V. What would be the ripple in DCM mode?

    Thanks.

  • Hi Sebastien,

    The LTM4644 cannot regulate below 0.6V. Is the 0.023V correct? 

    If you're interested in looking at the output voltage ripple in DCM, LTSpice can help give you a ball-park estimation of what the output voltage ripple might look like. Make sure the MODE pin is tied to GND (ensures DCM for light loads) and size the bottom feedback resistor and the load resistance to the proper output voltage and output current for your application.

    Thanks!

  • The lowest voltage I need to provide is 0.85V.

    when I look at the datasheet (electrical characteristics), I see the line output voltage variation min/max. I see 0.023V between min and typical. about the same between max and typical.

  • Hi Sebastien,

    Ah... that is a typical range for the output voltage. The module is trimmed well to be programmed to match the typical value (the min/max is drawn from large batches of modules they test using histograms to determine the variation).

    For your 0.85V case, I would still recommend using LTSpice to simulate to give you a estimated ball-park of what the output voltage ripple might be. Depending on the output capacitance, the ripple can vary. 0.2A is close to the threshold where the module is in CCM so it might be better to run in CCM for that load condition.

    Thanks!