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LT8490 / LT8491 multiphase questions

Hi,

In looking through some other answers for LT8490/LT8491 it seems that the recommended way to create a multiphase converter is to use one LT8491 and (N-1)x LT8705A parts. I have a few questions regarding this:

1. Is it possible to only use one error amplifier or combine error amplifiers in this application? We require a low-tolerance output voltage of 0.1% or better and were planning on achieving this in a single phase application through a digipot making up part of the Rfbout2 resistor as specified on the LT8491 datasheet, but I'm not sure how this will work with multiple error amplifiers in different ICs involved.

2. What connections need to be made from the master LT8491 to the LT8705A parts? Is it just CLKOUT on LT8491 to SYNC on LT8705A? Is SWENO on LT8491 recommended to be connected to LT8705A SWEN? Any others? Is DC2285A the only example board that ADI produces to demonstrate multiphase LT8705A? Can you elaborate on the "Optional LT8490 Interface" as seen on the DC2285A schematic?

3. Unfortunately, our input capacitance is limited to 100 µF max, so our nominal input capacitance is about 83 µF. Because of this, our DC input voltage (Vin on LT8491) will fluctuate from about 35 to 45 VDC at around 18 kHz. Do any considerations need to be made in regards to setting the LT8491 switching frequency due to this input voltage ripple? Will the ICs be able to handle this and still produce a stable output voltage? Our output will be 72 VDC, 10.5 A.

Thank you for your time,

Carter

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  • The recommended paralleling method when adding LT8705A phases to the LT8491 puts the LT8491 in control and the LT8705As as slaves. Please see attached for recommended paralleling method. With regard to the input ripple, the LT8491 and parallel LT8705As should be able to handle this and most of the ripple should be filtered out.

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  • Hi Charlie,

    Thanks for the info. The one last question I have now is in regards to the switching frequency to minimize ripple at the output. I was originally planning on using 120 kHz switching frequency to help with efficiency, but would you recommend increasing that due to the 18 kHz input ripple? By how much?

    Thanks again,

    Carter

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  • Hi Charlie,

    Thanks for the info. The one last question I have now is in regards to the switching frequency to minimize ripple at the output. I was originally planning on using 120 kHz switching frequency to help with efficiency, but would you recommend increasing that due to the 18 kHz input ripple? By how much?

    Thanks again,

    Carter

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