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I need the datasheet of an SMD component, on which I can read

74YH 
S21B

The chip is about 3mm x 3mm and, given its position on the board, has likely something to do with voltage regulation (but I can't be 100% sure). It has 8 leads. Please see the attached picture if that helps.

the component to identify is marked 74YH S21B

I have already explored the component-identification procedures proposed in the following post:
How do I identify SMD components? (or how do I identify any component)
but nothing clear came out (at best, hundreds of potential results...).

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Probably an LM3488, a DC/DC converter controller IC; those are marked S21B. The top line of the markings varies.

Not entirely sure, though.

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    \$\begingroup\$ I think you are correct e.g. OP's photo shows pins 4 & 5 connected, which fits with that IC you identified (pin 4 = AGND, pin 5 = PGND) and pin 1 for that IC is I_SEN which we expect to be connected to a low value resistor - and we do see it connected to a 0.1 ohm resistor in the photo. The capacitor & resistor in the lower left of the photo, would match the RC network expected to be connected to pin 2 (COMP). Some traces (and more of the surrounding components) aren't visible in the photo, but on the supplied info from the OP, this get my vote :-) \$\endgroup\$
    – SamGibson
    Nov 19 '20 at 12:25
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    \$\begingroup\$ Thanks, FYI I find all the components about it making a SEPIC, so this makes a lot of sense! \$\endgroup\$
    – Bru
    Nov 19 '20 at 15:19

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