I'm a little bit surpriced that you have used the same resistor between Vcc and the cathode of the TL431 for current source and current sink. In the current source you have much less voltage across this resistor compared to the current sink. I also don't understand that in the case of larger emitter currents of the TIP120 your TL431 gets hot. In principle more of the current from Vcc to the cathode of the TL431 is used for higher emitter currents of the TIP120. So the TL431 ist less loaded for higher emitter currents of the TIP120.
Sir could you please explain how to count value of resistor between Vcc and the cathode of the TL431 (in your case 4.7K) for constant current sink circuit? In TL431 data sheet I found formula for this resistor in constant current source circuit but there is no similar for constant current sink. Second question is how can I to count Ika?
This resistor isn't critical it simply supplies KA current for the TL431 and the base current for the transistor. If using a lower gain transistor, the 4.7K can be dropped to 2.2K.
