TL431


The TL431 is a three-terminal adjustable precision shunt voltage regulator integrated circuit. With the use of an external voltage divider, TL431 can regulate voltages ranging from 2.5 to 36 V, at currents up 100 mA. Typical initial deviation of reference voltage from nominal 2.495 V level is measured in millivolts, maximum worst-case deviation is measured in tens of millivolts. The circuit can control power transistors directly; combinations of TL431 with power MOS transistors are employed in high efficiency, very low dropout linear regulators. The TL431 is the de facto industry standard error amplifier circuit for switched-mode power supplies with optoelectronic coupling of input and output networks.
The TL431 was introduced by Texas Instruments in 1977. In the 21st century the original TL431 remains in production along with a multitude of clones and derivatives. These functionally similar circuits may differ considerably in die size and layout, precision and speed characteristics, minimal operating currents and safe operating areas.

Construction and operation

The TL431 is a three-terminal bipolar transistor switch, functionally equivalent to an ideal n-type transistor with stable 2.5 V switching threshold and no apparent hysteresis. "Base", "collector" and "emitter" of this "transistor" are traditionally called reference, cathode and anode. Positive control voltage VREF is applied between reference input and anode; output current ICA flows from cathode to anode.
On a functional level the TL431 contains a 2.5 V voltage reference, and an open-loop operational amplifier that compares input control voltage with the reference. This, however, is merely an abstraction: both functions are inextricably linked inside the TL431 front end. There is no physical 2.5 V source: actual internal reference is provided by a 1.2 V Widlar bandgap, driven by input emitter followers T1, T6. This enables correct operation even when cathode-anode voltage drops below 2.5 V, down to around 2.0 V minimum. Differential amplifier is made of two current sources ; positive difference of their currents sinks into the base of T10. Output open collector transistor T11 can sink currents up to 100 mA, and is protected from polarity reversal with a reverse diode. The circuit does not provide protection against excessive current or overheating.
When VREF is safely below the 2.5 V threshold, output transistor is closed; residual cathode-anode current ICA, feeding the front-end circuit, stays within 100 and 200 μA. As the rising VREF approaches the threshold, ICA rises to 300500 μA, but output transistor remains closed. Upon reaching threshold output transistor gently opens up, and ICA begins to rise at a rate of around 30 mA/V. When VREF exceeed the threshold by around 3 mV, and ICA reaches 500600 μA, 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. Current rises until negative feedback loop connecting cathode with control input stabilizes VREF at some point above the threshold. This point is, strictly speaking, the reference voltage of the complete regulator. Alternatively, the TL431 may work without feedback as a voltage comparator, or with positive feedback as a Schmitt trigger; in these application ICA is limited only by anode load and 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 ; operation with hanging REF input is prohibited but will not damage the TL431 directly. It will survive open circuit at any pin, short circuit to ground of any pin, or a short circuit between any pair of pins, provided that the voltages across pins remain within safety limits.

Precision

Nominal reference voltage VREF=2.495 V, stated in datasheet, is tested in [|zener mode] at ambient temperature of +25 C, and ICA=10 mA. Threshold voltage and the boundary between low-transconductance and high-transconductance modes are not specified and not tested. 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:
Open-loop frequency response of a TL431 can be reliably approximated as a first-order low-pass filter; dominant pole is provided by a relatively large compensation capacitor in the output stage. Equivalent model contains an ideal 1 A/V voltage-to-current converter, shunted with a 70 nF capacitor. For a typical cathode load of 230 Ohm, this translates to open-loop cutoff frequency of 10 kHz and unity gain frequency of 2 Mhz. Owing to various second-order effects, actual unity gain frequency is only 1 MHz; in practice, the difference between 1 and 2 MHz is unimportant.
Slew rates of ICA, VCA and settling time of VREF are not specified. According to Texas Instruments, power-on transient lasts for around 2 μs. Initially, VCA quickly rises to ≈2 V, and then locks at this level for around 1 μs. Charging internal capacitances to steady-state voltages takes up 0.51 μs more.
Capacitive cathode loads may cause instability and oscillation. According to stability boundary charts published in the original datasheet, TL431 is absolutely stable when CL is either less than 1 nF, or larger than 10 μF. Inside the 1 nF 10 μF range the likelihood of oscillation depends on the combination of capacitance, ICA and VCA. Worst-case scenario occurs at low ICA and VCA. On the contrary, combinations of high ICA and high VCA, when the TL431 operates close to its maximum dissipation rating, are absolutely stable. However, even a regulator designed for high ICA and high VCA may oscillate at power-on, when VCA has not yet risen to steady-state level.
In a 2014 application note Texas Instruments admitted that their stability boundary charts are unreasonably optimistic. They describe a "typical" IC sample at zero 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 resistor lie between 1 Ohm and 1 kOhm.

Applications

Linear regulators

The simplest TL431 regulator circuit is made by shorting control input to 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 R2R1; cathode voltage and output impedance increase 1+R2/R1 times. Maximum sustained regulated voltage may not exceed 36 V, 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 at best, because single npn-type transistors or Darlington pairs require fairly 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 1 V voltage drop. N-channel power MOSFET device enable the best possible combination of low drive current, very low dropout voltage and stability. However, low-dropout MOSFET operation requires an additional high-side voltage source for driving the gate.
Closed-loop regulator circuits employing the TL431 are always designed to operate in high transconductance mode, with ICA no less than 1 mA. 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, is the de facto industry standard solution for regulated switched-mode power supplies. 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 PWM controller. Resistor R3, shunting the LED, helps keeping 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 0.5 mA LED current and 1 mA shunt current.
Design of a robust, efficient and stable SMPS with TL431 is a common but complex task. In a simplest possible configuration, frequency compensation is maintained by an integrating network C1R4. In addition to this explicit compensation network, 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 output capacitor with a voltage divider, and a secondary "fast lane" connected to the output rail with a LED. The IC, loaded with 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 midband frequencies, 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 due to longer turn-off transients. Turn-on delay depends mostly on the difference between input and threshold voltage ; higher overdrive speeds up turn-on process. Optimal transient speed is attained at 10% overdrive and low signal source impedance of 10 kOhm or less.
On-state VCA drops to around 2 V, which is compatible with TTL and CMOS logic gates with 5 V power supply. Low-voltage CMOS requires level conversion with a resistive voltage divider, or replacing 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 when
provided 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 due to 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. 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. Frequency range and control law of such oscillator strongly depend 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.

Variants, clones and derivatives

Integrated circuits marketed by various manufacturers as TL431, or having similar designations like KA431 or TS431, may substantially differ from the Texas Instruments original. Sometimes the difference may be only revealed by testing in undocumented modes; sometimes it is publicly declared in datasheets. For example, the Vishay TL431 has abnormally high 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 regularors 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 through-hole package only, and having VREF of 2.75 V. Its bandgap reference was not thermally compensated, and was less precise than that of TL431; the output stage had no protection diode. The TL432 is electrically same as TL431, manufactured in surface-mount packages only, and having a different pinout.
In 2015 Texas Instruments announced ATL431, an improved derivative of the TL431 for very high efficiency switch-mode regulators. Recommended minimum operating current is only 35 μA ; maximum ICA and VCA are the same as standard. Unity gain frequency is reduced to 250 kHz to attenuate high frequency ripples so they are not fed back to the controller. The ATL431 has very different instability area. At low voltages and currents it is absolutely stable with any practical capacitive load, provided that the capacitors are of high-quality, low-impedance type. Minimal recommended value of series decoupling resistor is 250 Ohm.
Apart from the TL431 and its descendants, as of 2015 only two shunt regulator ICs found wide use in the industry. Both types have similar functionality and applications, but different internal circuits, different reference levels, maximum currents and voltages: