Hi,
I am using TL431QDBZR shunt regulator with a NMOS transistor (drain pin of NMOS to enable the circuit).
Case 1 - With Vin= -5V and resistor divider ckt b/w ref pin and anode (R1 = 24.9K and R2 = 20K), Vref = -2.227V which is less than int Vref = 2.495V. The NMOS is turned low and Drain is pulled high through pull up resistor i.e 21V
Case 2 - Again if I use the same resistor divider circuit with -4V supply, Vref at node = -1.782V which is again less than int Vref but NMOS is pulled high and drain is pulled low to ground.
Case 3- But if I change 24.9K to 34K value, Vref changes to -1.5V and NMOS is off and drain pulled high through pull up resistor.
My question is why NMOS is turned high in case 2 and not in case 3 although Vref at node in case3 is higher than in case 2?
Thanks
Tanveer
