LM431
SNVS020H –MAY 2000–REVISED JANUARY 2016
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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. RSdetermines the current that flows through the load (ILOAD) and
the LM431 (IZ). Since load current and supply voltage may vary, RSmust be small enough to supply at least the
minimum acceptable IZto 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, RSmust be large
enough so that the current flowing through the LM431 is less than 100 mA.
RSmust 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
• VOis 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/ΔVZis found in the Electrical Characteristics and is typically −1.4 mV/V for VZraging from VREF to 10 V
and –1 mV/V for VZraging from 10 V to 36 V. (3)
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