Calculating the voltage difference between a switching pin out and a constant Vout
From Spec


  VSWCE(DROP) Darlington Switch Collector to
  Emitter Voltage Drop
  (ISW = 1.0 A, Pin 2 to GND,
  TJ = 25°C)
  1.0Vtyp 1.3 Vmax
  Note 7. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient temperature as possible


This implies Rce= 300 mohms + 0.8V of Vbe or using linear regression,
 Vce= 0.8 + Ic * 0.3Ω