SNVS020H Data sheet

SNVS020H May 2000 – January 2016 LM431

PRODUCTION DATA.

Package Options

Mechanical Data (Package|Pins)

D|8
- MSOI002K
LP|3
- MSOT002D
DBZ|3
- MPDS108E

Thermal pad, mechanical data (Package|Pins)

Orderable Information

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 (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. LM431 Equation_01_snvs020.gif

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.

Equation 2. V_O = V_REF * (1 + R₁/R₂)

where

V_O is the output voltage (also, cathode voltage, V_Z). The actual value of the internal V_REF is a function of V_Z.

The corrected V_REF is determined by Equation 3:

Equation 3. V_REF = ∆V_Z * (∆V_REF/∆V_Z) + V_Y

where

V_Y = 2.5 V and ∆V_Z = (V_Z– V_Y)
ΔV_REF/ΔV_Z is found in the Electrical Characteristics and is typically −1.4 mV/V for V_Z raging from V_REF to 10 V and –1 mV/V for V_Z raging from 10 V to 36 V.

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.