phase margin; in practice, robust designs should target at least 30 degree phase margin. Usually, inserting a series resistance between the cathode and load capacitance, effectively increasing the latter's ESR, is sufficient for suppressing unwanted oscillations. Series resistance introduces a low-frequency zero at a relatively low frequency, cancelling most of the unwanted phase lag that was caused by load capacitance alone. Minimal values of series resistors lie between 1 Ohm (high CL) and 1 kOhm (low CL, high VCA). Applications Linear regulators The simplest TL431 regulator circuit is made by shorting the control input to the cathode. The resulting two-terminal network has a zener-like current–voltage characteristic, with a stable threshold voltage VREF≈2.5 V, and low-frequency impedance of around 0.2 Ohm. Impedance begins to grow at around 100 kHz and reaches 10 Ohm at around 10 MHz. Regulation of voltages higher than 2.5 V requires an external voltage divider. With divider resistors R2 and R1, the cathode voltage and the output impedance increase 1+R2/R1 times. The maximum sustained, regulated voltage may not exceed 36 V; the maximum cathode-anode voltage is limited to 37 V. Historically, TL431 was designed and manufactured with this application in mind, and was advertised as an "extremely attractive replacement for high cost, temperature-compensated zeners". Adding an emitter follower converts a shunt regulator into a series regulator. Efficiency is mediocre because single npn-type transistors or Darlington pairs require fairly a high collector-emitter voltage drop. A single common-emitter pnp-type transistor can operate correctly in saturation mode, with only ≈0.25 voltage drop but also with impractically high base currents. A compound pnp-type transistor does not need as much drive current, but requires at least a 1 V voltage drop. An N-channel power MOSFET device enables the best combination of low drive current, very low dropout voltage, and stability. However, low-dropout MOSFET operation requires an additional high-side voltage source (ΔU in schematic) for driving the gate. ΔU can be obviated if a depletion mode MOSFET is used. Closed-loop regulator circuits using the TL431 are always designed to operate in high transconductance mode, with ICA no less than 1 mA (point D on the current-voltage curve). For better control loop stability, optimal ICA should be set at around 5 mA, although this may compromise overall efficiency. Switched-mode power supplies In the 21st century, the TL431, loaded with an optocoupler's light-emitting diode (LED), is the de facto industry standard solution for regulated switched-mode