LM431BCM3 datasheet(13/32 Pages) TI1

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_ MIN

S _ MAX

LOAD _ MAX

Z _ MIN

_ MAX

S _ MIN

LOAD _ MIN

Z _ MAX

LM431

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SNVS020H – MAY 2000 – REVISED JANUARY 2016

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+ > VO

•

VO = 5 V

Select RS (a resistor between V+ and VO) such that: 1 mA < IZ < 100 mA

9.2.1.2 Detailed Design Procedure

The resistor RS must be selected such that current IZ 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 RS must be small enough for IZ to remain above 1 mA. The other extreme is V+ at its

maximum, and the load at its minimum, where RS must be large enough to maintain IZ < 100 mA. If unsure, try

using 1 mA

≤ IR ≤ 10 mA as a starting point; just remember the value of IZ varies with input voltage and load.

Use Equation 4 and Equation 5 to set RS between RS_MIN and RS_MAX.

(4)

(5)

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

VO = VREF * (1 + R1/R2)

(6)

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

R1 = 1 kΩ and R2 = 1 kΩ.

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