How to tell logic low and logic high from a datasheet?
I don't blame you -- it's a tricky datasheet to read. I believe this is the answer you're looking for:



Normally, you'd look for a "SHDN_B VIH/VIL" specification or similar. In this case, since that table specifies all conditions are at "VIN=3.6V", I feel comfortable taking those numbers as the threshold levels.

I see that you can PWM the SHDN# pin (common on these types of parts) to get variable brightness -- this is where IMO, it would have been nice to get a labeled 'Logic Low/Logic High' threshold in the table, with the VIH and VIL numbers.

My read is that a voltage below 0.5V, at the tested conditions is guaranteed to be read as a logic '0', and a voltage at and above 1.5V, at the tested conditions is guaranteed to be logic '1'. So, your 3.3V drive will work. If this was a device where VIH was 0.7*VDD, then you might have needed to consider a level-shifter. (This is often the difference between CMOS/TTL inputs, whether VIH is a fixed number independent of VDD, or if it's a dependent value).

Additionally, I don't see a direct path between VIN and SHDN# from the block diagram, so I don't think you have too much to worry about with 5V making it to your 3.3V device -- it can't hurt to put a 0R resistor in line so you can change to say a 100R resistor in the future if you have issues.

This application note from TI is a really good resource in general for understanding datasheets. It's focused at the conventional 74xxx logic families, but I think there's a lot to be learned from it.