TL431
TL431 | |
---|---|
Voltage regulator IC | |
Type | Adjustable shunt voltage regulator |
Year of introduction | 1977 |
Original manufacturer | Texas Instruments |
The TL431
Texas Instruments introduced the TL431 in 1977. In the 21st century, the original TL431 remains in production along with a multitude of clones and derivatives (TLV431, TL432, ATL431, KA431, LM431, TS431, 142ЕН19 and others). These functionally similar circuits may differ considerably in die size and layout, precision and speed characteristics, minimal operating currents, safe operating areas, and specific voltage reference.
Construction and operation
The TL431 is functionally equivalent to an ideal
On a functional level the TL431 contains an open-loop
Current–voltage relationship
When VREF is safely below the 2.495 V threshold (point A on current-voltage curve), the output transistor is off. Residual cathode-anode current ICA, feeding the front-end circuit, stays within 100 and 200 μA.
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.[10] 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.[11]
Precision
The nominal reference voltage, VREF=2.495 V, stated in a datasheet, is tested in zener mode at an ambient temperature of +25 °C (77 °F), and ICA=10 mA.[13] The threshold voltage and the boundary between low-transconductance and high-transconductance modes are not specified and not tested.[9] 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%;[14]
- Temperature. Thermal plot of a bandgap reference voltage has a hump-like shape. By design, the hump is centered on +25 °C (77 °F), where VREF=2.495 V; above and below +25 °C (77 °F), VREF gently decreases by a few millivolts. However, if a specific IC deviates substantially from the norm, the hump shifts to lower or higher temperatures; in the worst outliers it degenerates into a monotonously rising or falling curve.[15] [12]
- Owing to finite output impedance, changes in VCA voltage affect ICA and, indirectly, VREF, just like they do in transistors or triodes. For a given fixed ICA, a 1 V rise in VCA must be offset with ≈1.4 mV (2.7 mV worst-case maximum)[13] decrease in VREF. The ratio μ = 1 V / 1.4 mV ≈ 300–1000, or ≈ 50–60 dB is the theoretical maximum open-loop voltage gain at DC and low frequencies;[16]
- Owing to finite transconductance, a rise in ICA causes a rise in VREF at a rate of 0.5–1 mV/mA.[17]
Speed and stability
The open-loop
The slew rates of ICA, VCA and the settling time of VREF are not specified. According to Texas Instruments, the 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 0.5–1 μs more.[19]
Capacitive cathode loads (CL) may cause instability and oscillation.[20] According to stability boundary charts published in the original datasheet, TL431 is absolutely stable when CL is either less than 1 nF, or greater than 10 μF.[21][22] Inside the 1 nF–10 μF range the likelihood of oscillation depends on the combination of capacitance, ICA and VCA.[21][22] The 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.[22] However, even a regulator designed for high ICA and high VCA may oscillate at power-on, when VCA has not yet risen to a steady-state level.[21]
In a 2014
Applications
Linear regulators
Fixed Zener Mode
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 Ω.[24] Impedance begins to grow at around 100 kHz and reaches 10 Ω at around 10 MHz.[24]
Variable Zener Mode
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 times.[25] The maximum sustained, regulated voltage may not exceed 36 V; the maximum cathode-anode voltage is limited to 37 V.[26] 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".[27]
Additional Pass Transistor
Adding an
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).[8][7][2] For better control loop stability, optimal ICA should be set at around 5 mA, although this may compromise overall efficiency.[30][7]
Switched-mode power supplies
In the 21st century, the TL431, loaded with an
Design of a robust, efficient and stable SMPS with TL431 is a common but complex task.
Voltage comparators
The simplest TL431-based comparator circuit requires a single external resistor to limit ICA at around 5 mA.[39] Operation at lesser currents is undesirable because of longer turn-off transients.[39] Turn-on delay depends mostly on the difference between input and threshold voltage (overdrive voltage); higher overdrive speeds up the turn-on process.[39] Optimal transient speed is attained at 10% (≈250 mV) overdrive and source impedance of 10 kΩ or less.[39]
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.[40] Low-voltage CMOS (e.g. 3.3 V or 1.8 V logic) requires level conversion with a resistive voltage divider,[40] or replacing the TL431 with a low-voltage alternative like the TLV431.[41]
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
- ,[42]
provided that is larger than so that the spread between two trip voltages is wide enough.[42]
Undocumented modes
By 2010, DIY magazines published many audio amplifier designs that employed the TL431 as a voltage gain device.[43] Most were outright failures because of excessive negative feedback and low gain.[43] Feedback is necessary to reduce open-loop nonlinearity, but, given limited open-loop gain of the TL431,[44] any practical feedback level results in impractically low closed-loop gain.[43] The stability of these amplifiers leaves a lot to be desired, too.[43]
The inherently unstable TL431 may operate as a voltage-controlled oscillator for frequencies ranging from a few kHz to 1.5 MHz.[45] The frequency range and control law of such an oscillator strongly depends on the particular make of TL431 used.[45] Chips made by different manufacturers are usually not interchangeable.[45]
A pair of TL431s may replace transistors in a symmetrical astable multivibrator for frequencies ranging from under 1 Hz to around 50 kHz.[46] This, again, is an undocumented and potentially unsafe mode, with periodical capacitor charge currents flowing through input stage protection diodes (T2 on the schematic).[46]
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
The obsolete TL430 was an ugly sister [citation needed] 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.[49][50] The TL432 is electrically the same as TL431, manufactured in surface-mount packages only, and having a different pinout.[14]
In 2015, Texas Instruments announced the ATL431, an improved derivative of the TL431 for very high efficiency switch-mode regulators[51] that has a VREF of 2.5 V instead of 2.495 V. The recommended minimum operating current is only 35 μA (standard TL431: 1 mA); the maximum ICA and VCA are the same as standard (100 mA and 36 V).[52] 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 a very different instability area.[52] At low voltages and currents it is absolutely stable with any practical capacitive load, provided the capacitors are of a high-quality, low-impedance type.[53][54] The minimal recommended value of the series decoupling resistor is 250 Ω (standard TL431: 1 Ω).[55]
Apart from the TL431 and its descendants, as of 2015, only two shunt regulator ICs found wide use in the industry.[56] Both types have similar functionality and applications, but different internal circuits, different reference levels, maximum currents and voltages:[56]
- The bipolar LMV431 by Texas Instruments has a VREF of 1.24 V and is capable of regulating voltages up to 30 V at currents from 80 μA to 30 mA;[57][58]
- The
References
- ^ a b c Basso 2012, p. 383.
- ^ a b c d Brown 2001, p. 78.
- ^ a b c Zhanyou Sha 2015, p. 154.
- ^ a b c Basso 2012, p. 384.
- ^ a b c d e Texas Instruments 2015, pp. 20–21.
- ^ a b Basso 2012, pp. 383, 385–386.
- ^ a b c d e f Basso 2012, p. 388.
- ^ a b Texas Instruments 2015, p. 19.
- ^ a b c d e f Basso 2012, p. 387.
- ^ a b c Texas Instruments 2015, p. 20.
- ^ Zamora 2018, p. 4.
- ^ a b Texas Instruments 2015, p. 14.
- ^ a b Texas Instruments 2015, pp. 5–13.
- ^ a b Texas Instruments 2015, p. 1.
- ^ Camenzind 2005, pp. 7–5, 7–6, 7–7.
- ^ a b c d e Tepsa & Suntio 2013, p. 94.
- ^ Basso 2012, pp. 383, 387.
- ^ a b Schönberger 2012, p. 4.
- ^ Texas Instruments 2015, p. 25.
- ^ Michallick 2014, p. 1.
- ^ a b c Taiwan Semiconductor (2007). "TS431 Adjustable Precision Shunt Regulator" (PDF). Taiwan Semiconductor Datasheet: 3.
- ^ a b c d e Michallick 2014, p. 2.
- ^ Michallick 2014, pp. 3–4.
- ^ a b Texas Instruments 2015, pp. 5–13, 16.
- ^ Texas Instruments 2015, p. 24.
- ^ Texas Instruments 2015, p. 4.
- ^ Pippinger & Tobaben 1985, p. 6.22.
- ^ Dubhashi 1993, p. 211.
- ^ a b c d Dubhashi 1993, p. 212.
- ^ Tepsa & Suntio 2013, p. 93.
- ^ Basso 2012, p. 393.
- ^ Ridley 2005, pp. 1, 2.
- ^ a b Basso 2012, pp. 388, 392.
- ^ a b Ridley 2005, p. 2.
- ^ Ridley 2005, p. 3.
- ^ a b Basso 2012, pp. 396–397.
- ^ a b Ridley 2005, p. 4.
- ^ Basso 2012, pp. 397–398.
- ^ a b c d e Texas Instruments 2015, p. 22.
- ^ a b Texas Instruments 2015, p. 23.
- ^ Rivera-Matos & Than 2018, p. 1.
- ^ a b Rivera-Matos & Than 2018, p. 3.
- ^ a b c d Field, Ian (2010). "Electret Mic Booster". Elektor (7): 65–66. Archived from the original on 2020-06-15. Retrieved 2020-07-04.
- thermal voltage, is usually in the range of 3000-6000, or 20 dB higher than that of TL431.
- ^ a b c Ocaya, R. O. (2013). "VCO using the TL431 reference". EDN Network (10). Archived from the original on 2018-11-04. Retrieved 2020-07-04.
- ^ a b Clément, Giles (2009). "TL431 Multivibrator". Elektor (July/August): 40–41. Archived from the original on 2020-06-15. Retrieved 2020-07-04.
- ^ "Reverse-engineering the TL431: the most common chip you've never heard of". Ken Shiriff. 2014-05-26. Archived from the original on 2020-06-22. Retrieved 2020-07-04.
- ^ System General (2004). "SG6105 Power Supply Supervisor + Regulator + PWM" (PDF). System General Product Specification (7): 1, 5, 6. Archived (PDF) from the original on 2020-09-14. Retrieved 2020-07-04.
- ^ Texas Instruments (2005). "TL430 Adjustable Shunt Regulator" (PDF). Texas Instruments Datasheet (SLVS050D). Archived (PDF) from the original on 2020-06-20. Retrieved 2020-07-04.
- ^ Pippinger & Tobaben 1985, p. 6.21.
- ^ Leverette 2015, p. 2.
- ^ a b Leverette 2015, p. 3.
- ^ Leverette 2015, p. 4.
- ^ Texas Instruments 2016, pp. 7, 8.
- ^ Texas Instruments 2016, p. 17.
- ^ a b Zhanyou Sha 2015, p. 153.
- ^ Zhanyou Sha 2015, p. 157.
- ^ "LMV431x Low-Voltage (1.24-V) Adjustable Precision Shunt Regulators" (PDF). Texas Instruments. 2014. Archived (PDF) from the original on 2020-06-20. Retrieved 2020-07-04.
- ^ Zhanyou Sha 2015, p. 155.
- ON Semiconductor. 2009. Archived(PDF) from the original on 2020-06-21. Retrieved 2020-07-04.
Bibliography
Books and journals
- Basso, C. (2012). "Chapter 7. TL431-based Compensators". Designing Control Loops for Linear and Switching Power Supplies. ISBN 9781608075577.
- Brown, M. (2001). Power Supply Cookbook. EDN Series for Design Engineers. Vol. 22. pp. 229–237. )
- ISBN 9781589397187.
- Ridley, R. (2005). "Designing with the TL431 - the first complete analysis". Switching Power Magazine (August 1): 1–5.
- Ridley, R. (2007). "Using the TL431 in a Power Supply". Power Systems Design Europe (June): 16–18.
- Tepsa, T.; Suntio, T. (2013). "Adjustable Shunt Regulator Based Control Systems". IEEE Power Electronics Letters. 1 (4): 93–96. from the original on 2018-11-04. Retrieved 2020-07-04.
- Zhanyou Sha (2015). Optimal Design of Switching Power Supply. ISBN 9781118790946. Archivedfrom the original on 2020-11-11. Retrieved 2020-07-04.
Corporate publications
- TL43xx Precision Programmable Reference (PDF) (Datasheet). Rev. R. Texas Instruments. October 2023 [August 2004]. SLVS543R. Archived (PDF) from the original on 2020-06-13. Retrieved 2020-07-04.
- ATL431, ATL432 2.5-V Low Iq Adjustable Precision Shunt Regulator (PDF) (Datasheet). Rev. D. Texas Instruments. October 2016 [March 2015]. SLVSCV5D. Archived (PDF) from the original on 2018-11-04. Retrieved 2020-07-04.
- Leverette, A. (June 2015). Designing with the "Advanced" TL431, ATL431 (PDF) (Application Report). Texas Instruments. SLVA685. Archived (PDF) from the original on 2018-12-23. Retrieved 2020-07-04.
- Dubhashi, A. (1993). "AN-970: HEXFET Power MOSFETs in Low Dropout Linear Post-Regulators". HEXFET Designer's Manual Volume I. International Rectifier. pp. 211–214.
- Michallick, R. (January 2014) [September 2011]. Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet (PDF) (Application Report). Rev. A. Texas Instruments. SLVA482A. Archived (PDF) from the original on 2020-02-01. Retrieved 2020-07-04.
- Pippinger, D. E.; Tobaben, E. J. (1985). Linear and Interface Circuit Application. Volume I: Amplifiers, Comparators, Timers, Voltage Regulators. Texas Instruments. pp. 3–19, 6–10, 6-21–27.
- Rivera-Matos, Ricardo; Than, Ethan; Zamora, Marco (December 2019) [July 2018]. Using the TL431 for Undervoltage and Overvoltage Detection (PDF) (Application Report). Rev. A. Texas Instruments. SLVA987A. Archived from the original (PDF) on 2018-11-02.
- Schönberger, John (2012). Design of a TL431-Based Controller for a Flyback Converter (PDF) (Technical report). Plexim GMBH. Archived (PDF) from the original on 2015-11-23. Retrieved 2020-07-04.
- Zamora, Marco (January 2018). TL431 Pin FMEA (PDF) (Application Report). Texas Instruments. SNVA809. Archived (PDF) from the original on 2020-06-22. Retrieved 2020-07-04.
- The TL431 in the Control of Switching Power Supplies (PDF) (Technical report). ON Semiconductor. 2019-12-03.
Text books only describe op amps in compensators… The market reality is different: the TL431 rules!