Despite its age ( introduced in 1978, still in full production in 2020 ), the TL431 is very useful for a lot of applications ( and it is cheap ).
Their datasheet is not very clear ( even some manufacturer's application engineers gives wrong advise based on the datasheet ).
I have measured a Tl431 to clear up the 2 points below.
The absolute maximum input current into the Ref pin is specified as -50 µA to 10 mA. No voltages are specified.
The ref pin voltage can go from around 1 diode voltage below the Anode voltage to around 1 diode voltage above the Cathode voltage.
The adjust pin voltage is around 0.6 V below the Anode voltage with an input current of -40 µA. This varies very little with Cathode-Anode voltages from 0 V to 20 V.
I have no measurements for the Ref-Cathode diode.
To confuse things, some TL431 datasheets specify the Cathode current vs. Cathode voltage to a Ref-Anode voltage of -1.3 V - clearly far beyond the absolute maximum rating.
The reference voltage noise of the TL431 ( measured like in Fig.1(a) below ) is specified between 50 nV/√Hz and 230 nV/√Hz, depending on which datasheet you use.
Some measurements are shown below:
Fig.1: TL431 noise measurement schematics.
Fig.1(a) is the setup used for the datasheet specification.
Fig.1(b) is a circuit with higher anode voltage. The increase in output noise matches the increase in gain reasonable well ( the contribution of thermal noise from R2, R3 is around 23 nV/√Hz.
Fig.1(c) maintains the DC gain of fig.1(b) while reducing the AC gain ti 1. The noise should be the same as the circuit in fig.1(a), but I forgot to measure the circuit with a 10 µF bypass capacitor.
An issue with this circuit is that if it is possible to short Vout to GND, C2 will discharge through the regulators Ref-Anode diode, possibly destroying it.
Fig.1(d) use diode D1 to prevent C2 from discharging through the regulator. The trade-off for this is the temperature stability of Vout.
400 Hz..22 kHz
22 Hz..22 kHz
400 Hz..22 kHz
22 Hz..22 kHz
|Noise voltage density|
|1(a)||-91.9 dBu||-91.9 dBu||19.7 µV||19.8 µV||140 nV/√Hz|
|1(a)||10 µF||-92.1 dBu||-92.0 dBu||19.3 µV||19.5 µV||138 nV/√Hz|
|1(b)||-73.4 dBu||-73.3 dBu||166 µV||168 µV||1.19 µV/√Hz|
|1(b)||10 µF||-74.3 dBu||-74.5 dBu||149 µV||147 µV||1.04 µV/√Hz|
|1(b)||100 µF||-82.5 dBu||-81.7 dBu||58.2 µV||63.5 µV||449 nV/√Hz|
|1(c)||100 µF||-93.3 dBu||-93.2 dBu||16.8 µV||16.9 µV||120 nV/√Hz|
|1(d)||100 µF||-93.4 dBu||-93.3 dBu||16.5 µV||16.7 µV||118 nV/√Hz|
|||The 400 Hz..22 kHz filter is used to identify mains hum in the measurement. It should read within 0.1 dB of the 22 Hz..22 kHz filter.|
|||The 22 Hz..22 kHz filter has a noise bandwidth of 20 kHz.|
|||Noise voltage density is calculated from the 22 Hz..22 kHz measurement.|
|||Noise measurements on fig.1(b) are very sensitive to external noise fields.|
Spread-sheet with TL431 measurements.
|||Texas Instruments: TL431, TL432 datasheet, SLVS543P – August 2004 – Revised November 2018|
|||On Semicunductor: TL431A, B Series, NCV431A, B Series, SCV431A datasheet", January 2019, Rev. 40
Includes a simple model for stability calculations.
|||Diodes Incorporated: TL431/TL432 datasheet, Document number DS35044 Rev.6 - 2, April 2012|
|||ST Microelectronics: TL431/TL432 datasheet, Document number D4467 Rev. 12, December 2017|
|||Texas Instruments: Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet, SLVA482A–September 2011–Revised January 2014|
|||Texas Instruments: Setting the Shunt Voltage on an Adjustable Shunt Regulator, SLVA445–September 2011|
|||Audio Perfection: Realistic SPICE model for TL431: stability, noise, impedance and performance simulation of TL431 shunt regulator|
|||edaboard.com: Spice model for TL431|
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