that is larger than so that the spread between two trip voltages is wide enough. Undocumented modes By 2010, DIY magazines published many audio amplifier designs that employed the TL431 as a voltage gain device. Most were outright failures because of excessive negative feedback and low gain. Feedback is necessary to reduce open-loop nonlinearity, but, given limited open-loop gain of the TL431, any practical feedback level results in impractically low closed-loop gain. The stability of these amplifiers leaves a lot to be desired, too. The inherently unstable TL431 may operate as a voltage-controlled oscillator for frequencies ranging from a few kHz to 1.5 MHz. The frequency range and control law of such an oscillator strongly depends on the particular make of TL431 used. Chips made by different manufacturers are usually not interchangeable. A pair of TL431s may replace transistors in a symmetrical astable multivibrator for frequencies ranging from under 1 Hz to around 50 kHz. This, again, is an undocumented and potentially unsafe mode, with periodical capacitor charge currents flowing through input stage protection diodes (T2 on the schematic). Variants, clones and derivatives Integrated circuits marketed by various manufacturers as TL431, or having similar designations like KA431 or TS431, may differ substantially from the Texas Instruments original. Sometimes the difference may only be revealed by testing in undocumented modes; sometimes it is publicly declared in datasheets. For example, the Vishay TL431 has abnormally high (ca. 75 dB) DC voltage gain, which starts to roll off at 100 Hz; at frequencies above 10 kHz gain falls back to standard and reaches unity at the standard 1 MHz frequency. The SG6105 SMPS controller contains two independent regulators marked as TL431, but their maximum ICA and VCA are only 16 V and 30 mA, respectively; the manufacturer does not test these regulators for precision. The obsolete TL430 was an ugly sister of the TL431, manufactured by Texas Instruments in a through-hole package only, and having a VREF of 2.75 V. Its bandgap reference was not thermally compensated, and was less precise than that of the TL431; the output stage had no protection diode. The TL432 is electrically the same as TL431, manufactured in surface-mount packages only, and having a different pinout. In 2015, Texas Instruments announced the ATL431, an improved derivative of the TL431 for very high efficiency switch-mode regulators. The recommended minimum operating current is only 35 μA (standard TL431: 1 mA);