I am currently evaluating the TPS61183EVM-528 module and have found that the controller will not regulate to a voltage below 13.5V on the output when adjusting down the string voltages. If say, I light 6 strings of 3 series LEDs (Vf = ~9V), the controller will regulate to 13.5V on the output (across C3) and there will be ~4.5V on each of the IFB pins. If I light 6 strings of 5 series LEDs (Vf = 15V), the controller will function as normal, regulating to the largest string Vf + ~600mV accounting for the headroom on the drain of the current source.
I can not find in any of the documentation a reference to a minimum boost voltage, though I can see some logic in maintaining the 13.5V regulation point as this is also the threshold for determining a shorted string. Other similar TI controllers (TPS61180/1/2) explicitly state in the datasheet that the boost can not regulate below 15V, however I can not find the same declaration for the TPS61183.
Some clarity here would be greatly appreciated as it would be hard to design in this part knowing that with low output voltage applications we would be burning excessive power in the controller.