inductor current ripple in uModules

Hi 

historically buck regulators was designed with inductor and switching frequency to reach a inductor current ripple of 30-40%.

is it relevant for new uModules with integrated inductors? asking in a matter of EMI noise etc..

for example, in my design i use the LTM4622 uModule with 2 seperete rails (3.3 and 1.8V).

i have a system requirement to work in synchronization mode and to configure the frequency to 800kHz.

so the device theoritaclly can work with 1120-2080kHz external clock.

the problem: when in low frequency (1120kHz), the ripple for the 3.3V rail is around 76%

when in high frequency (2080kHz) the ripple for 1.8V rail is 29%.

my question is what the design goal? if the inductor is internal, can i rely on the package of the device to reduce the EMI noise with ripples above 40%?

Thanks.

Parents
  • +1
    •  Analog Employees 
    on Sep 17, 2020 5:03 PM

    Hi Shimon, 

    Keeping the inductor ripple within a reasonable range helps guide the design to have sufficiently large current sense signal for proper operation, but not too large so that you may potentially trigger reverse current threshold at light load. The datasheet mentions to keep inductor ripple to 1.2A pk-pk or lower for this reason.

    For the LTM4622 at 12VIn to 3.3Vout the datasheet recommends to use 2MHz, 800kHz would be too low for this output voltage.

    Instead you may consider the LTM4622A which has a larger (2.2uH) inductor so pk-pk ripple is lower for same output voltage. You can operate this one at 800kHz  for 12Vin, 3.3Vout & 1.8Vout rails and it will have good design regarding inductor current ripple concerns.

    Thanks 

Reply
  • +1
    •  Analog Employees 
    on Sep 17, 2020 5:03 PM

    Hi Shimon, 

    Keeping the inductor ripple within a reasonable range helps guide the design to have sufficiently large current sense signal for proper operation, but not too large so that you may potentially trigger reverse current threshold at light load. The datasheet mentions to keep inductor ripple to 1.2A pk-pk or lower for this reason.

    For the LTM4622 at 12VIn to 3.3Vout the datasheet recommends to use 2MHz, 800kHz would be too low for this output voltage.

    Instead you may consider the LTM4622A which has a larger (2.2uH) inductor so pk-pk ripple is lower for same output voltage. You can operate this one at 800kHz  for 12Vin, 3.3Vout & 1.8Vout rails and it will have good design regarding inductor current ripple concerns.

    Thanks 

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