power supplies (SMPS). A resistive voltage divider driving the control input of the TL431, and the LED's cathode are normally connected to the regulator's output; the optocoupler's phototransistor is connected to the control input of the Pulse-width modulation (PWM) controller. Resistor R3 (around 1 kOhm), shunting the LED, helps keep ICA above the 1 mA threshold. In a typical power supply/charger supplied with a laptop computer, average ICA is set at around 1.5 mA, including a 0.5 mA LED current and a 1 mA shunt current (2012 data). Design of a robust, efficient and stable SMPS with TL431 is a common but complex task. In the simplest possible configuration, frequency compensation is maintained by an integrating network C1R4. In addition to this explicit compensation network, the frequency response of the control loop is affected by the output smoothing capacitor, the TL431 itself, and the parasitic capacitance of the phototransistor. The TL431 is governed by not one, but two control loops: the main, "slow lane" loop connected to an output capacitor with a voltage divider, and a secondary "fast lane" connected to the output rail with an LED. The IC, loaded with the very low impedance of the LED, operates as a current source; undesirable voltage ripple passes from the output rail to the cathode almost unimpeded. This "fast lane" dominates at mid-band frequencies (ca. 10 kHz–1 MHz), and is usually broken by decoupling the LED from the output capacitor with a zener diode or a low-pass filter. Voltage comparators The simplest TL431-based comparator circuit requires a single external resistor to limit ICA at around 5 mA. Operation at lesser currents is undesirable because of longer turn-off transients. Turn-on delay depends mostly on the difference between input and threshold voltage (overdrive voltage); higher overdrive speeds up the turn-on process. Optimal transient speed is attained at 10% (≈250 mV) overdrive and source impedance of 10 kOhm or less. On-state VCA drops to around 2 V, which is compatible with Transistor–transistor logic (TTL) and CMOS logic gates with 5 V power supply. Low-voltage CMOS (e.g. 3.3 V or 1.8 V logic) requires level conversion with a resistive voltage divider, or replacing the TL431 with a low-voltage alternative like the TLV431. TL431-based comparators and invertors can be easily cascaded following the rules of relay logic. For example, a two-stage window voltage monitor will turn on (switching from high-state to low-state output) when , provided