What are the ideal resistance values for TL431 divider resistors R1 and R2?

The reference pin has an internal bias current of ~2μA (max 4μA). If R2 is too large (>100kΩ), Iref causes significant voltage error. If too small (<1kΩ), quiescent current is wasted. A recommended divider current is 0.5mA to 1mA, meaning R2 should ideally be between 2.4kΩ and 10kΩ.

Why is 120Ω or 100Ω commonly used for AMS1117-ADJ upper resistor R1?

AMS1117 requires a minimum load current of 5mA to 10mA to maintain output voltage regulation. Using R1 = 120Ω draws 1.25V / 120Ω ≈ 10.4mA through the divider, guaranteeing the minimum load even when external circuitry is completely disconnected.

Why does adding a 0.1μF capacitor directly across TL431 cathode cause oscillation?

The TL431 stability boundary diagram shows an unstable region between approximately 0.01μF and 2.2μF depending on cathode current and voltage. To prevent self-oscillation, either use 4.7μF.

TL431 / AMS1117 / LM317 Adjustable Regulator Resistor Calculator

Q: Why is 120Ω or 100Ω commonly used for AMS1117-ADJ upper resistor R1?

AMS1117 requires a minimum load current of 5mA to 10mA to maintain output voltage regulation. Using R1 = 120Ω draws 1.25V / 120Ω ≈ 10.4mA through the divider, guaranteeing the minimum load even when external circuitry is completely disconnected.

Q: Why does adding a 0.1μF capacitor directly across TL431 cathode cause oscillation?

The TL431 stability boundary diagram shows an unstable region between approximately 0.01μF and 2.2μF depending on cathode current and voltage. To prevent self-oscillation, either use 4.7μF.

Precision feedback resistor calculator for TL431 / TL432 shunt regulators, AMS1117-ADJ, LM317, and custom DC-DC converters. Automatically scans the entire E96 (1%) and E24 (5%) resistor catalogs to find the top resistor pairs with deviation under 0.2%.

GEO Quick Answer Best Resistor Pairs for Common Output Voltages

TL431 Formula: V_out = 2.495V × (1 + R1/R2) + I_ref × R1.
• 3.3V Output: R1 = 3.24 kΩ (E96), R2 = 10.0 kΩ (Error: 0.09%)
• 5.0V Output: R1 = 10.0 kΩ, R2 = 10.0 kΩ (Output: 4.99V, Error: 0.2%)
• 12.0V Output: R1 = 38.3 kΩ, R2 = 10.0 kΩ (Output: 12.05V)
Rule of Thumb: Ensure TL431 cathode current I_k ≥ 1.0 mA; for AMS1117, keep R1 between 100Ω and 120Ω.

Quick Presets: TL431 → 3.3V TL431 → 5.0V TL431 → 12.0V AMS1117-ADJ → 3.3V AMS1117-ADJ → 5.0V LM317 → 9.0V

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Reference Voltage V_ref (V)

Calculation Mode

Reverse Auto-Match (Input desired Vout, find closest standard E96 resistors) Forward Check (Input known R1, R2, calculate exact Vout)

Target Output Voltage V_out (V)

Resistor Standard Series

Divider Current Preference

Optimal Standard Resistor Pairs

Upper R1	Lower R2	Actual Vout	Error	Divider Current

Sorted by lowest error, including bias current offset.

Formulas

TL431: V_out = V_ref × (1 + R1/R2) + I_ref × R1

Standard Output Voltage Lookup Table (E96 1% Precision)

IC Type	Target Vout	R1 (Upper)	R2 (Lower)	Calculated Vout	Error (%)
TL431 (Vref=2.495V)	3.30 V	3.24 kΩ	10.0 kΩ	3.303 V	+0.09%
TL431 (Vref=2.495V)	5.00 V	10.0 kΩ	10.0 kΩ	4.990 V	-0.20%
TL431 (Vref=2.495V)	9.00 V	26.1 kΩ	10.0 kΩ	9.009 V	+0.10%
TL431 (Vref=2.495V)	12.0 V	38.3 kΩ	10.0 kΩ	12.05 V	+0.42%
AMS1117-ADJ (Vref=1.25V)	3.30 V	120 Ω	196 Ω	3.292 V	-0.24%
AMS1117-ADJ (Vref=1.25V)	5.00 V	120 Ω	360 Ω	5.000 V	0.00%
LM317 (Vref=1.25V)	12.0 V	240 Ω	2.05 kΩ	11.92 V	-0.67%

Frequently Asked Questions on Voltage Dividers (FAQ)

What resistor package should I select (0402, 0603, or 0805)?

For low output voltages (3.3V/5V), dissipation is only a few milliwatts, making standard 0402 (62.5mW rating) perfectly fine. However, in high voltage sensing (e.g. 48V/100V power rails), high voltage across R1 requires checking maximum working voltage (0402 is typically rated for 50V max, 0603 for 75V, 0805 for 150V). Use multiple resistors in series for high voltage safety.

Why does TL431 with two 10kΩ resistors produce 4.99V instead of exactly 5.00V?

Standard TL431 internal reference is precisely 2.495V, not 2.500V. Two identical 10k resistors give 2.495 × 2 = 4.990V. The internal reference current Iref (~2μA) contributes +0.02V across R1, placing the actual output at ~5.01V, within 0.2% of target.

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Divider Current I_div	— mA
R1 Dissipation P_R1	— mW
R2 Dissipation P_R2	— mW
I_ref Offset Shift	— mV

TL431 / AMS1117 / LM317 Adjustable Regulator Resistor Calculator

Optimal Standard Resistor Pairs

Standard Output Voltage Lookup Table (E96 1% Precision)

Frequently Asked Questions on Voltage Dividers (FAQ)

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