TL1431-EP_15 Datasheet, PDF (13/23 Pages) Texas Instruments – TL1431-EP Precision-Programmable Reference

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TL1431-EP
SLVS529D – APRIL 2004 – REVISED JANUARY 2015
Typical Application (continued)
10.2.1 Design Requirements
To calculate the values for resistors R₁ and R₂, the values of the following parameters must be known: the
feedback current, (I_FB), cathode current, (I_KA), and desired shunt voltage, (V_KA).
Input
I_FB
R1
V_REF
R2
I_REF
V_KA
I_KA
U1 TL431
The Electrical Characteristics table specifies when V_KA = V_REF and I_KA is 10 mA the nominal V_REF, (labeled V_NOM)
is 2.5 V. The reference voltage varies with cathode voltage at two different rates. The reference voltage is –1.1
mV/V from V_REF to 10 V then –1.5 mV/V above 10 V. The reference pin current is 4 µA.
The Z_KA parameter offsets V_REF by (I_KA – I_NOM) × Z_KA .
In addition, the ΔV_REF / ΔV_KA parameter offsets V_REF by either –1.1 mV × (V_KA – 2.5 V) if V_KA ≤ 10 V or –8.25 mV
–1.5 mV/V × (V_KA – 10 V) if V_KA>10 V. The –8.25-mV constant is the V_REF offset as V_KA changes from
V_NOM to 10 V, (10 V – 2.5 V) × –1.1 mV/V.
Therefore:
If V_KA ≤ 10 V then;
V_REF = V_NOM + (I_KA – I_NOM) x Z_KA + (V_KA – V_NOM) × –1.1 mV/V
(1)
If V_KA > 10 then;
V_REF = V_NOM + (I_KA – I_NOM) × Z_KA + (V_KA – 10 V) x –1.5 mV/V –8.25 mV
(2)
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Now that the value of V_REF is calculated, use Equation 1 and Equation 2 to calculate the value of R₁ and R₂.
R₁ = (V_KA – V_REF) / I_FB
(3)
R₂ = V_REF / (I_FB – I_REF)
(4)
R₂ has less current than R₁.
NOTE
10.2.2 Detailed Design Procedure
The goal of the design is: the TL1431 cathode set to 12 V, the cathode current at 2 mA, and a feedback current
of 0.2 mA.
Using the formula derived in the general example for V_KA>10 V.
V_REF = V_NOM + (I_KA – I_NOM) × Z_KA + (V_KA – 10 V) × -1.1 mV – 8.25 mV
(5)
V_REF = 2.500 V + (2 mA – 10 mA) x 2 Ω+ (12 V – 10 V) × –1.1 mV – 8.25 mV
(6)
Using Equation 5 and Equation 6, the value of V_REF = 2.473 V
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R₁ = (V_KA – V_REF) / I_FB
(7)
R₁ = (12 V – 2.473 V) / 0.2 mA
(8)
Using Equation 7 and Equation 8, the value of R₁ = 46.285 kΩ
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