The part is a max of 8Amps. I am using it to convert a 40V supply to 37V. Would the max current still be 8Amps in this configuration? Will a 3V drop be workable with this part?
in principle the high dutycycle is ok with switching frequencies below 600kHz (toff,max=110ns). But: 37V*8A are nearly 300W!!! Even with the high efficiency of approx 95% at slow switching frequencies we talk about 15W of power losses, at your Pmax. The thermal resistance (junction- ambient and junction-case) values from the package (see page 2 in the ds) are quite good, but this won't work, not even at -45C.
For such power ranges you probably need several units (LT8646S is intended for parallel use) in parallel to handle the thermals.
I recommend to use LTPowerCAD to get a better understanding of the design process with the LT8645/LT8646 and what results to expect with the configuration of choice.
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