V(BATT)

R

R1

0.1%

R2

0.1%

VI(ref)

VO

TL1431

O

I(ref)

R1

V

1

V

R2

æ

ö

=

+

ç

÷

è

ø

TL1431-EP

SLVS529D – APRIL 2004 – REVISED JANUARY 2015

www.ti.com

10 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 should

validate and test their design implementation to confirm system functionality.

10.1 Application Information

The ability to set the shunt voltage, VKA, to any voltage between VREF and the maximum rated voltage for the

shunt regulator provides a lot of flexibility. It takes two resistors to set the shunt voltage. In an ideal common

anode shunt regulator, the shunt voltage would be VREF × (R1/R2 + 1).

Real world shunt regulators have limited gain, non-zero reference input current, and suffer from cathode voltage

modulation. This application report derives comprehensive formulas that accurately represent the relationship

between the shunt voltage and feedback resistors. It also shows a practical example.

10.1.1 Shunt Regulator Limitations

Real world shunt regulators have three parameters that should be considered.

•

Dynamic impedance, ZKA

•

Reference pin current, IREF

•

Ratio of change in reference voltage to the change in cathode voltage,

ΔVREF/ΔVKA.

The first parameter will cause a VREF shift for all VKA values and the last two only apply when VKA, is set greater

than VREF.

ZKA offsets the VREF in direct proportion to the cathode current. The data sheet generally specifies VREF at a

specific current. At any other current ZKA impacts VREF.

IREF causes an inequality in the feedback resistor currents which changes the effective DC feedback ratio. This

factor is often included in data sheet formulas.

ΔVREF/ΔVKA specifies how much the VREF voltage changes when the cathode voltage changes. This is a

frequently ignored factor although the effect can be significant.

10.2 Typical Application

NOTE: R should provide cathode current

≥1 mA to the TL1431-EP at minimum V(BATT).

Figure 21. Shunt Regulator

12

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