LM3812 is the wrong chip to employ for this application. It converts current to duty cycle in a positive relationship which is probably the opposite of what you are looking for. Besides it's the wrong operating voltage range (2-5.25) and the wrong frequency (12.5-25 Hz), so I doubt it would be much use to you.
What you probably need is a constant current LED driver. National, ON-Semi, Luxdrive, Luxeonics, Catsemi, Linear, Maxim-IC, Melexis, Sipex, TaskLED, Texas Instruments, Xetex and The Sandwich Shoppe are some of the places where you can find LED drivers.