TL431 calculator: output voltage, divider and cathode resistor

The cathode voltage a TL431 shunt regulator sets from its two resistors, or the divider for the voltage you want in E24 or E96, with the reference current left in and the worst case each grade allows; then the cathode resistor that keeps the TL431 between its minimum regulating current and 100 mA from the lowest input and heaviest load to the highest input and lightest, and the power in both. Every limit is from TI's datasheet; the defaults are its 24 V, 10 kΩ + 10 kΩ example, 5.010 V.

Fig 1 — TL431AC set to 5.01 V by R1 = 10.0 kΩ and R2 = 10.0 kΩ, fed from 21.6 V to 26.4 V through R_S = 3.60 kΩ with a 0 to 1.00 mA load. The cathode current runs from 3.36 mA (lowest input, heaviest load) to 5.69 mA (highest input, lightest load), inside the 1 mA to 100 mA of the recommended operating conditions. R_S dissipates up to 127 mW, the TL431 up to 28.5 mW.

Calculate

Grade (initial accuracy)

Temperature range

Part and package

Top resistor R1 (cathode to REF) (kΩ)

Bottom resistor R2 (REF to anode) (kΩ)

Divider resistor tolerance t_R (%)

Minimum input voltage V_IN(min) (V)

Maximum input voltage V_IN(max) (V)

Minimum load current I_LOAD(min) (mA)

Maximum load current I_LOAD(max) (mA)

Minimum cathode current to keep I_KA (mA)

Cathode resistor R_S (kΩ)

Ambient temperature T_A (°C)

Cathode voltage V_KA = V_ref (1 + R1/R2) + I_ref × R1, typical: 5.010 V · 20.0 mV of it from I_ref
With the typical ΔV_ref/ΔV_KA of −1.4 mV/V (derived): 5.003 V · −7.02 mV
At 25 °C: V_ref 2470–2520 mV, I_ref 0–4 µA, R1 and R2 ±0.1 %: 4.935 V to 5.085 V
Worst case over 0 to 70 °C, with V_I(dev), ΔV_ref/ΔV_KA and |z'| (derived bound): 4.883 V to 5.129 V · −2.5 % / +2.4 %
Divider current V_ref/R2 + I_ref · circuit impedance |z'| = |Z_KA| (1 + R1/R2), typ / max: 251 µA · 400 mΩ / 1.00 Ω
R_S range: I_KA ≥ 1.00 mA at 21.6 V and full load, ≤ 100 mA at 26.4 V and least load: 213 Ω to 7.37 kΩ · E24 6.80 kΩ
Cathode current with R_S = 3.60 kΩ: at minimum input and maximum load, to maximum input and minimum load: 3.36 mA to 5.69 mA
R_S current · R_S dissipation at 26.4 V, (V_IN − V_KA)²/R_S: 4.61 mA to 5.94 mA · 127 mW
TL431 dissipation V_KA × I_KA · junction at 70 °C in SOT-23-3 (DBZ), 206 °C/W: 28.5 mW · 75.9 °C (P_D max 388 mW)
Stability chart for this part, if a capacitor goes across the cathode: Figure 6-18

How this is calculated

Standard: TI TL431, TL432 Precision Programmable Reference, SLVS543S (May 2024): §6.1, §6.3–§6.13, Figure 7-2, §7.2, §8.1, §9.2.2

$V_{KA} = V_{ref}\left(1 + \frac{R_1}{R_2}\right) + I_{ref}\, R_1$: Figure 7-2 (p. 19). V_ref is 2495 mV typical, with 25 °C limits of 2483–2507 mV (B), 2470–2520 mV (A) and 2440–2550 mV (standard); I_ref is 2 µA typical, 4 µA maximum (§6.5–§6.13, pp. 6–14). Page 26 prints the same equation with a minus before I_ref × R1; the node equation agrees with Figure 7-2's plus.
$R_1 = \frac{V_{KA} - V_{ref}}{V_{ref}/R_2 + I_{ref}}$: Figure 7-2 solved for R1, at the typical V_ref and I_ref. The stocked value offered is the E24 or E96 neighbour whose output lands closest to the target.
$\frac{V_{IN(max)} - V_{KA}}{100\ \text{mA} + I_{LOAD(min)} + I_{R1}} \le R_S \le \frac{V_{IN(min)} - V_{KA}}{I_{KA(min)} + I_{LOAD(max)} + I_{R1}}$: Kirchhoff at the cathode node, (V_IN − V_KA)/R_S = I_KA + I_LOAD + I_R1 with I_R1 = V_ref/R2 + I_ref, at its two corners: the lowest input with the heaviest load, and the highest input with the lightest. I_KA is held between the 1 mA and 100 mA of the recommended operating conditions (§6.4, p. 5), and the grade's "Minimum cathode current for regulation", 1 mA maximum for the standard grade and 0.6 or 0.7 mA for A and B. Figure 9-5, note A: "R should provide cathode current ≥1mA to the TL431 at minimum V(BATT)."
$P_{R_S} = \frac{(V_{IN(max)} - V_{KA})^2}{R_S}, \qquad P_{TL431} = V_{KA}\, I_{KA(max)}, \qquad P_D = \frac{T_{J(max)} - T_A}{\theta_{JA}}$: The dissipation at the highest input with the lightest load; the last is §6.4 note (1), with T_J(max) = 150 °C (§6.1) and θ_JA from §6.3: SOT-23 206 °C/W, TO-92 140 °C/W, SOIC-8 97 °C/W, SOT-89 52 °C/W.
$|z'| = \frac{\Delta V}{\Delta I} \approx |Z_{KA}|\left(1 + \frac{R_1}{R_2}\right)$: §7.2 (p. 20). |Z_KA| is 0.2 Ω typical, 0.5 Ω maximum, for f ≤ 1 kHz and I_KA = 1 mA to 100 mA. V_ref is specified at I_KA = 10 mA, and "Any deviation from I test can cause deviation on the output V KA."
$V_{ref}(V_{KA}) = V_{ref} + \frac{\Delta V_{ref}}{\Delta V_{KA}}\,(V_{KA} - V_{ref})$: Derived from the ΔV_ref/ΔV_KA rows of §6.5–§6.13: −1.4 mV/V typical, −2.7 mV/V maximum for ΔV_KA = 10 V − V_ref, and −1 and −2 mV/V for 36 V − 10 V, applied piecewise and solved with Figure 7-2 as a fixed point. The datasheet lists the term among the errors of §9.2.2.2.2 but does not combine it with Figure 7-2.

Assumptions

The typical output uses V_ref = 2495 mV and I_ref = 2 µA. The 25 °C band takes the grade's V_ref limits with I_ref from 0 to 4 µA and the resistors at the tolerance that moves the output the same way; the datasheet gives no minimum for I_ref, so zero is a bound.
The full band widens the 25 °C V_ref limits by V_I(dev) each way and I_ref by I_I(dev), since those are spreads "between the maximum and minimum values obtained over the rated temperature range" and the 25 °C value can sit anywhere inside them. It adds the maximum ΔV_ref/ΔV_KA at the low end (zero at the high end, the tables printing no minimum) and |z'| maximum times the distance of the cathode-current extremes from 10 mA. A conservative bound, not a prediction.
The cathode resistor is sized at the typical V_KA and for DC. Load steps, ripple faster than the TL431 responds, and the start-up delay of §9.2.2.2.4 are not modelled.
No output capacitor is modelled. Stability with one depends on Figure 6-16 or 6-18, which this page names but does not read values from.
The TL431 dissipation is V_KA × I_KA; the V_ref × I_ref the REF pin adds is microwatts and is left out.

What sets the TL431 output voltage

The TL431 is a shunt regulator: it sits across the output like a Zener diode and sinks whatever current it must to hold the voltage. TI's datasheet (SLVS543S) describes it as a three-terminal "adjustable shunt regulator" whose "output voltage can be set to any value between Vref (approximately 2.5V) and 36V, with two external resistors" (p. 1). Inside is a reference and an amplifier: the part "outputs a sink current base on the difference between the reference pin and the virtual internal pin" (p. 22). Tie REF to the midpoint of a divider from cathode to anode and it sinks current until the midpoint sits at V_ref.

With R1 from CATHODE to REF and R2 from REF to ANODE, Figure 7-2 (p. 19) gives the cathode voltage as V_KA = V_ref(1 + R1/R2) + I_ref × R1. V_ref is 2495 mV typical in every grade. The second term is the current the REF pin draws, 2 µA typical and 4 µA maximum, flowing through R1 on its way in. With 10 kΩ for both resistors the first term is 4.990 V and the second adds 20 mV, for 5.010 V.

The datasheet prints the same equation twice, with different signs. Page 26 gives "Vo=(1+R1/R2)*VREF-IREF*R1", with a minus. Figure 7-2's plus is the one that follows from the circuit: I_ref leaves the divider's midpoint into the REF pin, so R1 carries the R2 current plus I_ref, and the extra drop across R1 raises the cathode. For the 10 kΩ pair the two forms differ by 40 mV, twice the term. The calculator uses Figure 7-2's sign, and its tests derive the node equation independently to check it.

The other number that matters is the current. The TL431 only regulates with enough current through it: the electrical characteristics give a "Minimum cathode current for regulation", I_min, of 0.4 mA typical and, at most, 1 mA for the standard grade and 0.6 or 0.7 mA for the A and B grades (pp. 6–14). The recommended operating conditions (p. 5) run the cathode from 1 mA to 100 mA, and the overview is blunt: "In order for this device to behave as a shunt regulator or error amplifier, >1mA (I min(max)) must be supplied in to the cathode pin" (p. 21).

The cathode resistor: keeping I_KA between its limits

In a shunt regulator, one resistor from the input, R_S (R_SUP in the datasheet's Figure 9-3), feeds everything: the load, the divider and the TL431. Kirchhoff's current law at the cathode node is the whole design: (V_IN − V_KA)/R_S = I_KA + I_LOAD + I_R1, where I_R1 = V_ref/R2 + I_ref is the divider's share. The input and the load both move, and the TL431 takes up the difference. Its current is smallest at the lowest input with the heaviest load, and largest at the highest input with the lightest load.

That gives the resistor two limits. It must be small enough to leave the minimum cathode current at the low corner, R_S ≤ (V_IN(min) − V_KA) / (I_KA(min) + I_LOAD(max) + I_R1), which is what the datasheet's notes ask of every figure that has one: "R should provide cathode current ≥1mA to the TL431 at minimum V(BATT)" (Figure 9-5, p. 28). And it must be large enough to keep the cathode current within 100 mA at the high corner, R_S ≥ (V_IN(max) − V_KA) / (100 mA + I_LOAD(min) + I_R1). The calculator gives both, the largest E24 value under the first, and the cathode current the resistor you enter actually runs at, drawn as the bar under the schematic against the grade's I_min and the 1 mA to 100 mA window.

If the first limit comes out below the second, no resistor works: the input range is too wide for the load range, because a resistor sized to feed the load at the lowest input dumps too much into the TL431 at the highest. And since R_S passes the same current whatever the load draws, a shunt regulator burns its full design load current even when the load is light.

Power follows from the same corner. R_S dissipates (V_IN(max) − V_KA)²/R_S whatever the load does. The TL431 dissipates V_KA × I_KA, worst at the highest input with the load removed, and the thermal table (p. 5) gives the junction-to-ambient resistance to turn that into a temperature, with the note "PD = (TJ(max) – TA)/θJA" and a 150 °C maximum junction. The 100 mA limit is not one you can use in every package: 5 V at 100 mA is 501 mW, and a SOT-23 (206 °C/W) at 70 °C may dissipate 388 mW, which already puts the junction at the 150 °C limit: about 78 mA at 5 V. A TO-92 (140 °C/W) allows 571 mW and a SOT-89 (52 °C/W) 1538 mW under the same conditions.

TL431 resistor values for common outputs

The table solves Figure 7-2 for R1 with R2 = 10 kΩ, the value of the datasheet's Table 9-2, at the typical 2495 mV and 2 µA, then rounds R1 to the E24 and E96 neighbour whose output lands closest and recomputes the output with that real resistor. With 10 kΩ the divider draws 0.252 mA, which the cathode resistor must supply on top of the load.

V_KA	R1 exact	E24 R1	gives	E96 R1	gives
3.3 V	3.201 kΩ	3.3 kΩ	3.325 V (+0.76 %)	3.24 kΩ	3.310 V (+0.30 %)
5 V	9.960 kΩ	10 kΩ	5.010 V (+0.20 %)	10 kΩ	5.010 V (+0.20 %)
6 V	13.94 kΩ	13 kΩ	5.764 V (−3.93 %)	14 kΩ	6.016 V (+0.27 %)
9 V	25.86 kΩ	27 kΩ	9.286 V (+3.17 %)	26.1 kΩ	9.059 V (+0.66 %)
12 V	37.79 kΩ	39 kΩ	12.304 V (+2.53 %)	37.4 kΩ	11.901 V (−0.82 %)
15 V	49.72 kΩ	51 kΩ	15.322 V (+2.14 %)	49.9 kΩ	15.045 V (+0.30 %)
18 V	61.65 kΩ	62 kΩ	18.088 V (+0.49 %)	61.9 kΩ	18.063 V (+0.35 %)
24 V	85.51 kΩ	82 kΩ	23.118 V (−3.67 %)	84.5 kΩ	23.747 V (−1.06 %)

The rounding error is fixed by the series, and against a 1 % or 2 % reference it is often the smaller error. A 3.3 V output, the lowest row, needs R1 = 3.201 kΩ exactly; E96 gives 3.24 kΩ and +0.30 %. The calculator's divider mode does the same for any target and any R2, and also searches a free R2 within a factor of √10 of the one you enter, which often finds a closer pair.

Worked example: the datasheet's 24 V shunt regulator

Section 9.2.2 works a shunt regulator through Figure 9-3, with the design parameters of Table 9-2 (p. 26): "Reference Initial Accuracy 1.0 %", "Supply Voltage 24V", "Cathode Current (Ik) 5mA", "Load Capacitance 10μF" and "Feedback Resistor Values and Accuracy (R1 & R2) 10kΩ"; Figure 9-3 marks both resistors 0.1 %. A 1 % reference is the A grade. The datasheet stops at the parameters: it prints no R_SUP. Working it with the calculator, with no load:

V_KA = 2.495 × (1 + 10/10) + 2 µA × 10 kΩ = 5.010 V.
The divider draws 0.2515 mA.
For 5 mA in the cathode, R_SUP = (24 − 5.010) / (5 mA + 0.2515 mA) = 3.616 kΩ. The nearest E24 value below is 3.6 kΩ, which gives 5.024 mA.
R_SUP dissipates 100 mW; the TL431 25.2 mW, which in SOT-23 at 70 °C puts the junction at 75.2 °C.

That is the calculator's default, with an illustrative ±10 % on the supply and a 0 to 1 mA load added to give it something to regulate. With the input at 21.6 V and the load at 1 mA the cathode current falls to 3.36 mA; at 26.4 V with the load gone it rises to 5.69 mA. The same 3.6 kΩ resistor could feed up to 3.36 mA of load at 21.6 V before the cathode current reached 1 mA. A heavier load needs a smaller R_S, and every milliamp it adds at the low corner returns as a milliamp in the TL431 when the load is removed.

The output with 0.1 % resistors and the A grade, at 5 mA, comes to 4.935 V to 5.085 V at 25 °C and 4.885 V to 5.129 V over 0 to 70 °C, the second being the calculator's conservative bound with every error term at its worst. The circuit impedance is |z'| ≈ |Z_KA|(1 + R1/R2) = 0.4 Ω typical and 1.0 Ω maximum (§7.2, p. 20), so a 1 mA load step moves the output by at most 1.0 mV.

Worked example: Figure 9-11's 5 V reference from 12 V

Figure 9-11 (p. 29), "PWM Converter With Reference", uses the TL431 as the reference for a TL598: 12 V through 6.8 kΩ to the cathode, two 10 kΩ 0.1 % resistors as the divider, and the node labelled 5 V. The divider gives 5.010 V. The 6.8 kΩ resistor passes (12 − 5.010) / 6.8 kΩ = 1.028 mA, the divider takes 0.252 mA, and 0.776 mA is left for the cathode, before anything the 10 kΩ into the TL598's error amplifier draws, which the figure does not state.

0.78 mA is above the 0.4 mA typical I_min, so a typical part regulates. It is below the 1 mA maximum the standard grade guarantees and below the 1 mA the recommended operating conditions start at; the figure names no grade. Holding 1 mA from exactly 12 V takes R_S ≤ 5.585 kΩ, and the nearest E24 value under that, 5.1 kΩ, gives 1.12 mA. Enter 12 V, 6.8 kΩ and a zero load in the calculator to see the bar fall short of the I_min line.

Worked example: a 12 V output, and the datasheet's 5 V dividers

The start-up curves of Figure 9-4 (p. 27) include a TL431 with "R1=38kΩ & R2=10kΩ". Figure 7-2 puts that at 12.052 V, a 12 V setting, though the figure prints no output voltage. 38 kΩ is in neither E24 nor E96. Solving for exactly 12 V with R2 = 10 kΩ gives 37.79 kΩ (38.10 kΩ if the I_ref term is left out, a +0.8 % difference in R1). E96's nearest is 37.4 kΩ, for 11.901 V (−0.82 %); E24's is 39 kΩ, for 12.304 V (+2.53 %). Letting R2 move as well, E96 has 75 kΩ over 20 kΩ, for 12.001 V (+0.01 %).

At 12 V the cathode-voltage coefficient starts to show. The tables give ΔV_ref/ΔV_KA as −1.4 mV/V typical and −2.7 mV/V maximum from V_ref to 10 V, and −1 and −2 mV/V from 10 V to 36 V. V_ref is specified with the cathode at V_ref, so at 12 V it has fallen, and the divider multiplies the fall by 4.74: the E96 divider gives 11.843 V with the typical coefficient and 11.788 V with the maximum, against 11.901 V from Figure 7-2 alone. At 5 V the typical shift is only −7.0 mV. This is derived from the two table rows; the datasheet lists the coefficient among the errors but does not combine it with Figure 7-2.

The datasheet's own 5 V regulators use 1:1 dividers. Figure 9-10 (p. 29), "Efficient 5V Precision Regulator", uses 27.4 kΩ for both, and labels the output "VO ≈5 V"; Figure 7-2 gives 5.045 V. The I_ref term is 55 mV of that typical and up to 110 mV at 4 µA, which is why a pair this large is not the most accurate choice even with 0.1 % resistors. Figure 9-9's LM317 regulator uses 243 Ω for both, "VO ≈5 V, 1.5 A", for 4.990 V; there the I_ref term is under a millivolt.

How accurate the output is: the four error terms

Section 9.2.2.2.2 (p. 27) lists what "may effect the overall accuracy beyond VREF" once the output is above V_ref: "R1 and R2 accuracies", "VI(dev) - Change in reference voltage over temperature", "ΔVREF / ΔVKA - Change in reference voltage to the change in cathode voltage" and "|zKA| - Dynamic impedance, causing a change in cathode voltage with cathode current". The calculator adds them into a worst-case band. Each is in the tables:

Initial accuracy. V_ref at 25 °C is 2483 to 2507 mV for the B grade, 2470 to 2520 mV for A and 2440 to 2550 mV for the standard grade. The divider multiplies the error along with the voltage, so it is the same percentage at every output.
Temperature. V_I(dev) is a spread, not a coefficient: "the differences between the maximum and minimum values obtained over the rated temperature range" (p. 6). In the C range it is 16 mV maximum for SOT23-3 parts, the TL432 and the B grade, and 25 mV for the standard and A grades in other packages; in the I range the same split is 34 and 50 mV; every Q-range part is 34 mV. Since the 25 °C value can sit anywhere inside the spread, the calculator widens the 25 °C limits by V_I(dev) both ways. That is a bound, not a prediction. §7.1 turns the same number into an average coefficient in ppm/°C.
Cathode voltage. ΔV_ref/ΔV_KA, above. It only ever lowers the output; the tables print no minimum, so the high end of the band takes it as zero.
Cathode current. V_ref is specified at I_KA = 10 mA, and "Any deviation from I test can cause deviation on the output V KA" (§7.2, p. 20). With |Z_KA| = 0.5 Ω maximum, the band moves by |z'| times the distance from 10 mA at each end of the cathode-current range.

For the 12 V E96 divider with 0.1 % resistors over 0 to 70 °C and a cathode current between 1 and 20 mA, the band comes out as follows.

Grade	25 °C	0 to 70 °C, all terms
B	11.751 V to 12.052 V	11.543 V to 12.196 V
A	11.689 V to 12.113 V	11.439 V to 12.301 V
standard	11.547 V to 12.256 V	11.298 V to 12.443 V

Past the B grade, the resistors and the cathode-voltage coefficient set what is left. The datasheet refers the fuller treatment to its application note SLVA445, "Setting the Shunt Voltage on an Adjustable Shunt Regulator". One inconsistency is worth knowing: the comparator section (p. 24) puts the internal reference at "2.5V±(0.5%, 1.0% or 1.5%) depending on which version is being used", where page 1 and the tables give 2 % for the standard grade. The calculator uses the tables.

Stability: a capacitor across the cathode

A TL431 does not need an output capacitor: "Unlike many linear regulators, TL43xx is internally compensated to be stable without an output capacitor between the cathode and anode. However, if it is desired to use an output capacitor Figure 6-18 can be used as a guide to assist in choosing the correct capacitor to maintain stability" (p. 22). The datasheet has two such charts, Figure 6-16 "for All TL431 and TL431A Devices (Except for SOT23-3, SC-70, and Q-Temp Devices)" and Figure 6-18 "for All TL431B, TL432, SOT-23, SC-70, and Q-Temp Devices" (pp. 17–18). The calculator names the one your part and package fall under.

Each chart plots cathode current against load capacitance on a log axis from 0.001 µF to 10 µF, with four boundary curves for V_KA = V_ref, 5 V, 10 V and 15 V, and the note: "The areas under the curves represent conditions that can cause the device to oscillate." The region marked "Stable" lies on both sides: at small capacitance and at large. The unstable region is in the middle of the axis, and how far it reaches depends on the cathode voltage and current. This page does not read boundary values off the curves; check your capacitor, cathode voltage and current against the chart for your part, as section 9.2.2.2.3 asks: "When using additional capacitance between Cathode and Anode, refer to Figure 6-16 and Figure 6-18." The datasheet points to SLVA482, "Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet", for the detail. The crowbar of Figure 9-8 carries the same instruction for its capacitor C.

More current: the TL431 driving a transistor

The 100 mA ceiling and the dissipation above are the TL431's own. The datasheet's system examples (pp. 28–30) get past them by letting the TL431 control something larger. Figure 9-7, "High-Current Shunt Regulator", hangs a transistor across the output, driven from the cathode, so the transistor sinks the bulk of the current. Figure 9-5, "Precision High-Current Series Regulator", turns it into a series regulator with a pair of 2N222 transistors as the pass element, and the note under it is the cathode-resistor rule again: "R should provide cathode current ≥1mA to the TL431 at minimum V(BATT)." Figure 9-10 does the same with one transistor, and its R_b carries the same note.

The TL431 can also sit in the feedback of another regulator. Figure 9-6 puts it in the common leg of a uA7805, with "Minimum VO = Vref + 5 V", and Figure 9-9 sets an LM317's ADJUST pin for a precision 5 V at 1.5 A. Figure 9-15, the "Precision Constant-Current Sink", holds V_ref across a sense resistor for I_O = V_ref/R_S, the same idea as the constant current source calculator. In each of these the divider equation is unchanged; only the cathode current comes from somewhere else.

One behaviour shows up at power-up: "TL43xx has a fast response up to ~2Vand then slowly charges to its programmed value. This is due to the compensation capacitance" (§9.2.2.2.4, p. 27), which Figure 9-4 shows for V_KA = V_ref, 10 kΩ over 10 kΩ, and 38 kΩ over 10 kΩ.

Where the model stops being valid

The equation assumes regulation. Page 26: "In order for this equation to be valid, TL43xx must be fully biased so that it has enough open loop gain to mitigate any gain error. This can be done by meeting the Imin spec". Below I_min the cathode voltage is whatever the input, R_S and the load make it, and the calculator's V_KA no longer describes the circuit; it marks the cathode current in red when that happens.

The limits are at 25 °C and 10 mA. V_ref, I_ref and I_min are tabulated at T_A = 25 °C; the temperature terms are spreads over the rated range, which the band uses as bounds. |Z_KA| is specified for f ≤ 1 kHz and I_KA from 1 to 100 mA; at higher frequencies Figure 6-12 shows the impedance rising.

The applications are not specification. Section 9 opens: "Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness" (p. 23). The examples worked on this page come from that section, and the two signs of the I_ref term are an example of why the tables, not section 9, are the place to take a limit from.

Steady state, one resistor. The calculator sizes R_S for DC. A load that steps faster than the TL431 responds, a capacitor that puts the part in an unstable region of Figure 6-16 or 6-18, and a supply that is not current-limited are outside it. On the last, §9.4: "In order to not exceed the maximum cathode current, be sure that the supply voltage is current limited."

Common TL431 mistakes

Sizing R_S at the nominal input. The cathode current is smallest at the lowest input with the heaviest load; size for that corner and check the other.
Forgetting the divider current. 10 kΩ for R2 takes 0.25 mA out of R_S before the TL431 sees any, which in Figure 9-11's circuit is a quarter of the current through 6.8 kΩ.
Designing to the typical 0.4 mA I_min. The maximum is 1 mA for the standard grade, and the recommended operating conditions begin at 1 mA.
Dropping the I_ref × R1 term with large resistors, or taking page 26's minus sign. With 27.4 kΩ it is 55 mV typical.
Adding a capacitor across the cathode "for stability". The part is stable without one; with one, it can land under a boundary curve. Check the chart.
Taking 100 mA as available in any package. At 5 V that is 501 mW, past a SOT-23's limit at 70 °C.
Confusing the TL431 and TL432. They are electrically the same, but the TL432 "has different pinouts for the DBV, DBZ, and PK packages" (p. 1), and the nomenclature notes "Cathode and REF pins are switched" (p. 32).
Setting the output above 36 V. That is the recommended maximum, and 37 V the absolute one (p. 5).