Hello,
I am interest in driving a IR constant current device (VCSEL) using the LT3477. I am going to use the device in buck or buck/boost mode as my Vf < Vin. My Vin range is 2.7V-6.2V (1 or 2 non-recharable Li Cells). My question is in regards to the application example design shown on the datasheet. 
In this example however the LED string cathode is connected to Vin. This will not work for me because as per my VCSEL, I have to have the cathode grounded to chasis ground for heatsinking
As per the design note by Daniel Chen found here: https://agelectronica.lat/pdfs/textos/L/LT3477IFEPBF.PDF the reason for this is "The cathode end of the LED string is tied back to the input voltage, which allows it to operate from a wide input voltage range."
Does my input voltage range fall into this category? Is it considered "wide" as per the design note? The LT3477is rated for Vin = 2.5V-25V and I'm only doing roughly 1/4 of that.
If I do hook up my design just like this, and I ground the cathode of the VCSEL, what may happen at different Vin levels?
