Hello all
Please could you advise on this.
We use a TL431 (TI part TL431AIDBZT, not sure what the “T” represents) as a voltage sensor. There is a 33k resistor (also considering 27k, actual value T.B.A.) between pin 2 (Reference) and pin 3 (anode, also ground in our circuit). There is a 1M resistor from pin 2 to a voltage rail whose level we need to detect which is over 70V (it can rise to 350V or so, hence the high resistor value to minimise power dissipation).
According to the data sheet, the “AI” part has a reference voltage of 2.47V to 2.52V, and there is a thermal drift of 34mV for the SOT23 package which we use. Thus the lowest I expect the device to switch on is 2.47-0.034=2.436V. Yet at room temperature (23°) the device switches on with 2.05V on the reference pin (measured with an oscilloscope). This is around 16% lower than expected.
Please could someone advise urgently what I have missed.
Thank you.