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TL431 notes.

Contents


Introduction.

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.


TL431.

Reference pin input voltage and current

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.

Noise

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:

TL431 noise measurement schematics.
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.

Table 1: TL431 noise measurement.
FigC1Noise
400 Hz..22 kHz
Noise
22 Hz..22 kHz
Noise
400 Hz..22 kHz
Noise
22 Hz..22 kHz
Noise voltage density
1(a) -91.9 dBu-91.9 dBu19.7 µV19.8 µV140 nV/√Hz
1(a)10 µF-92.1 dBu-92.0 dBu19.3 µV19.5 µV138 nV/√Hz
1(b) -73.4 dBu-73.3 dBu166 µV168 µV1.19 µV/√Hz
1(b)10 µF-74.3 dBu-74.5 dBu149 µV147 µV1.04 µV/√Hz
1(b)100 µF-82.5 dBu-81.7 dBu58.2 µV63.5 µV449 nV/√Hz
1(c)100 µF-93.3 dBu-93.2 dBu16.8 µV16.9 µV120 nV/√Hz
1(d)100 µF-93.4 dBu-93.3 dBu16.5 µV16.7 µV118 nV/√Hz

[1]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.
[2]The 22 Hz..22 kHz filter has a noise bandwidth of 20 kHz.
[3]Noise voltage density is calculated from the 22 Hz..22 kHz measurement.
[4]Noise measurements on fig.1(b) are very sensitive to external noise fields.

Downloads.

Spread-sheet with TL431 measurements.


References.

[1] Texas Instruments: TL431, TL432 datasheet, SLVS543P – August 2004 – Revised November 2018
[2] On Semicunductor: TL431A, B Series, NCV431A, B Series, SCV431A datasheet", January 2019, Rev. 40
Includes a simple model for stability calculations.
[3] Diodes Incorporated: TL431/TL432 datasheet, Document number DS35044 Rev.6 - 2, April 2012
[4] ST Microelectronics: TL431/TL432 datasheet, Document number D4467 Rev. 12, December 2017
[5] Texas Instruments: Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet, SLVA482A–September 2011–Revised January 2014
[6] Texas Instruments: Setting the Shunt Voltage on an Adjustable Shunt Regulator, SLVA445–September 2011
[7] Audio Perfection: Realistic SPICE model for TL431: stability, noise, impedance and performance simulation of TL431 shunt regulator
[8] edaboard.com: Spice model for TL431

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