I don't understand the formula for the thermal dissipation of the STCS1A. The datasheet in chapter 8.2 states:
Pd = (Vd - Vfb) * Id + (Vcc * Icc)
Vcc - VLED.
Here the connection scheme:
When the mosfet turns on, the current flows through the LEDs and the feedback resistor. Internally, the feedback voltage is fed to a comparator that turns off the mosfet when the preset current is reached.
Now, while the mosfet is on, the voltage across the diodes depends on their forward voltage with the actual current. The voltage across the mosfet is due to the Rds_on (ideally it would be a short-circuit) so it also fixed by the current. The only component that would receive the remaining voltage is the feedback resistor.
Of course, at steady-state this would be
Vcc - VLED - Vds but this is never reached because the mosfet is turned off when the Vfb is ~ 100 mV in order to achieve the current regulation.
If this is correct, I don't understand why the voltage across drain-source receive the gap between VLED and Vcc. The mosfet is not used in the linear region, it's a PWM control so it's off or completely on.
If I'm wrong and the datasheet is correct (and I bet that's right!) it means that if you use a high voltage supply with few LEDs, the mosfet will dissipate a lot more. But this makes no sense to me. I selected this device because I understood it regulates the current turning off the mosfet whenever the current rises over the preselected value, so the power dissipation (during the on-state) would be just
Vds * Id where
Vds = Rds_on * Id.