9.1 Application Information

The LM431 is an adjustable precision shunt voltage regulator with ensured temperature stability over the entire temperature range. For space critical applications, the LM431 is available in space saving SOIC-8, SOT-23 and TO-92 packages. The minimum operating current is 1 mA while the maximum operating current is 100 mA.

The typical thermal hysteresis specification is defined as the change in 25°C voltage measured after thermal cycling. The device is thermal cycled to temperature 0°C and then measured at 25°C. Next the device is thermal cycled to temperature 70°C and again measured at 25°C. The resulting VOUT delta shift between the 25°C measurements is thermal hysteresis. Thermal hysteresis is common in precision references and is induced by thermal-mechanical package stress. Changes in environmental storage temperature, operating temperature and board mounting temperature are all factors that can contribute to thermal hysteresis.

In a conventional shunt regulator application (Figure 12), an external series resistor (R_S) is connected between the supply voltage and the LM431 cathode pin. R_S determines the current that flows through the load (I_LOAD) and the LM431 (I_Z). Since load current and supply voltage may vary, R_S must be small enough to supply at least the minimum acceptable I_Z to the LM431 even when the supply voltage is at its minimum and the load current is at its maximum value. When the supply voltage is at its maximum and I_LOAD is at its minimum, R_S must be large enough so that the current flowing through the LM431 is less than 100 mA.

R_S must be selected based on the supply voltage, (V+), the desired load and operating current, (I_LOAD and I_Z), and the output voltage, see Equation 1.

Equation 1.

The LM431 output voltage can be adjusted to any value in the range of 2.5 V through 37 V. It is a function of the internal reference voltage (VREF) and the ratio of the external feedback resistors as shown in Figure 12. The output voltage is found using Equation 2.

The corrected V_REF is determined by Equation 3:

9.2 Typical Applications

9.2.1 Shunt Regulator

Figure 12. Shunt Regulator

9.2.1.1 Design Requirements

Design a shunt regulator with the following requirements:

• V₊ > V_O
• V_O = 5 V

Select R_S (a resistor between V₊ and V_O) such that: 1 mA < I_Z < 100 mA

9.2.1.2 Detailed Design Procedure

The resistor R_S must be selected such that current I_Z remains in the operational region of the part for the entire V₊ range and load current range. The two extremes to consider are V₊ at its minimum, and the load at its maximum, where R_S must be small enough for I_Z to remain above 1 mA. The other extreme is V₊ at its maximum, and the load at its minimum, where R_S must be large enough to maintain I_Z < 100 mA. If unsure, try using 1 mA ≤ I_R ≤ 10 mA as a starting point; just remember the value of I_Z varies with input voltage and load.

Use Equation 4 and Equation 5 to set R_S between R_{S_MIN} and R_{S_MAX}.

Equation 4.

Equation 5.

Set feedback resistors R₁ and R₂ for a resistor divider based on Equation 2 and reproduced in Equation 6

Equation 6. V_O = V_REF * (1 + R₁/R₂)

So, for a 5-V output voltage, V_O, and V_REF of 2.5 V, simple calculation yields R₁/R₂ = 1. Based on this, select
R₁ = 1 kΩ and R2 = 1 kΩ.

9.2.1.3 Application Curves

Figure 13. Input Current vs V_Z

Figure 15. Input Current vs V_Z

Figure 14. Thermal Information

9.2.2 Other Applications

Figure 16. Single Supply Comparator With Temperature Compensated Threshold

Figure 17. Series Regulator

Figure 18. Output Control of a Three Terminal Fixed Regulator

Figure 19. Higher Current Shunt Regulator

Figure 20. Crow Bar

Figure 21. Over Voltage and Under Voltage Protection Circuit

Figure 22. Voltage Monitor

Figure 23. Delay Timer

Figure 24. Current Limiter or Current Source

Figure 25. Constant Current Sink