TL1431-EP_15 Datasheet, PDF (12/23 Pages) Texas Instruments – TL1431-EP Precision-Programmable Reference

TL1431-EP
SLVS529D – APRIL 2004 – REVISED JANUARY 2015
10 Application and Implementation
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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, V_KA, to any voltage between V_REF 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 V_REF × (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, Z_KA
• Reference pin current, I_REF
• Ratio of change in reference voltage to the change in cathode voltage, ΔV_REF/ΔV_KA.
The first parameter will cause a V_REF shift for all V_KA values and the last two only apply when V_KA, is set greater
than V_REF.
Z_KA offsets the V_REF in direct proportion to the cathode current. The data sheet generally specifies V_REF at a
specific current. At any other current Z_KA impacts V_REF.
I_REF causes an inequality in the feedback resistor currents which changes the effective DC feedback ratio. This
factor is often included in data sheet formulas.
ΔV_REF/ΔV_KA specifies how much the V_REF voltage changes when the cathode voltage changes. This is a
frequently ignored factor although the effect can be significant.
10.2 Typical Application
R
V_(BATT)
V_O
R1
0.1%
V_I(ref)
TL1431
R2
0.1%
V_O
=
æ
çè¹
+
R1 ö
R2 ^÷ø
V_I(ref)
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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