on current-voltage curve), the output transistor is closed. Residual cathode-anode current ICA, feeding the front-end circuit, stays within 100 and 200 μA. When VREF approaches the threshold, the ICA rises to 300–500 μA, but the output transistor remains closed. Upon reaching its threshold (point B), the output transistor gently opens up, and the ICA begins rising at a rate of around 30 mA/V. When VREF exceeds the threshold by around 3 mV, and ICA reaches 500600 μA (point C), transconductance sharply jumps to 1.01.4 A/V. Above this point the TL431 operates in its normal, high transconductance mode and may be conveniently approximated with a differential voltage to single-ended current converter model. The current rises until the negative feedback loop connecting the cathode with the control input stabilizes VREF at some point above the threshold. This point (Vref) is, strictly speaking, the reference voltage of the complete regulator. Alternatively, the TL431 may operate without feedback as a voltage comparator, or with positive feedback as a Schmitt trigger; in such applications ICA is limited only by the anode load and the power supply capacity. Reference input current IREF is independent of ICA and fairly constant, at around 2 μA. The network feeding reference input should be able to source at least twice this amount (4 μA or more); operation with hanging REF input is prohibited but will not damage the TL431 directly. It will survive an open circuit at any pin, a short circuit to ground of any pin, or a short circuit between any pair of pins, provided that the voltages across the pins remain within safety limits. Precision The nominal reference voltage, VREF=2.495 V, stated in a datasheet, is tested in zener mode at an ambient temperature of +, and ICA=10 mA. The threshold voltage and the boundary between low-transconductance and high-transconductance modes are not specified and not tested. The actual VREF maintained by a specific TL431 in a real-world application may be higher or lower than 2.495 V, depending on four factors: Individual initial deviation of a specific chip. For different grades of TL431, deviation at normal conditions is within ±0.5%, ±1%, or ±2%; Temperature. Thermal plot of a bandgap reference voltage has a hump-like shape. By design, the hump is centered on +, where VREF=2.495 V; above and below +, VREF gently decreases by a few millivolts. However, if a specific IC deviates substantially from the norm, the