Do 100% duty cycle buck converters have no voltage drop?
Remember a typical buck converter configuration:







simulate this circuit &ndash; Schematic created using CircuitLab

The product page indicates that duty-cycle can be %100.

If duty-cycle is %100 then the MOSFET (M1) will not act as a switch (because it's fully on). Instead, it will turn into a resistor with a value of \$R_{ds-on}\$.

So a small voltage of \$V_{DS} = R_{ds-on} \cdot I_{LOAD}\$ will drop across this resistor. For example, according to the datasheet, on-resistance is 85mR. And for a load current of 3A, the voltage drop will be about 270mV. Note that there will also be a small voltage drop across the inductor due to its DC resistance.


  Would it be possible to detect a low voltage and bypass the converter if the voltage is too low?


This is called Brown-Out Detection and a lot of chips on the market have this feature. But instead of bypassing, they tend to turn themselves off to protect the converter from drawing excessive currents.