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LM431

SNVS020H – MAY 2000 – REVISED JANUARY 2016

www.ti.com

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers must

validate and test their design implementation to confirm system functionality.

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 (RS) is connected between

the supply voltage and the LM431 cathode pin. RS determines the current that flows through the load (ILOAD) and

the LM431 (IZ). Since load current and supply voltage may vary, RS must be small enough to supply at least the

minimum acceptable IZ 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 ILOAD is at its minimum, RS must be large

enough so that the current flowing through the LM431 is less than 100 mA.

RS must be selected based on the supply voltage, (V+), the desired load and operating current, (ILOAD and IZ),

and the output voltage, see Equation 1.

(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.

VO = VREF * (1 + R1/R2)

where

•

VO is the output voltage (also, cathode voltage, VZ). The actual value of the internal VREF is a function of VZ. (2)

The corrected VREF is determined by Equation 3:

VREF = ∆VZ * (∆VREF/∆VZ) + VY

where

•

VY = 2.5 V and ∆VZ = (VZ– VY)

•

ΔVREF/ΔVZ is found in the Electrical Characteristics and is typically −1.4 mV/V for VZ raging from VREF to 10 V

and –1 mV/V for VZ raging from 10 V to 36 V.

(3)

12

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