According to the data sheet, the number of LEDs per channel of the TPS92638-Q1 device is 3. Why is the channel limited to 3 LEDs? With LEDs in series being and a Vf or 3 volts, you could get up to 10-12 LEDs in series assuming a 40V Supply input. I have seen applications using this device where the number of LEDs in series was more than 3.