VKA

IFB

IKA

U1TL431

IREF

R1

R2

Input

VREF

TL1431-EP

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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, (IFB), cathode current, (IKA), and desired shunt voltage, (VKA).

The Electrical Characteristics table specifies when VKA = VREF and IKA is 10 mA the nominal VREF, (labeled VNOM)

is 2.5 V. The reference voltage varies with cathode voltage at two different rates. The reference voltage is –1.1

mV/V from VREF to 10 V then –1.5 mV/V above 10 V. The reference pin current is 4 µA.

The ZKA parameter offsets VREF by (IKA – INOM) × ZKA .

In addition, the

ΔVREF / ΔVKA parameter offsets VREF by either –1.1 mV × (VKA – 2.5 V) if VKA ≤ 10 V or –8.25 mV

–1.5 mV/V × (VKA – 10 V) if VKA>10 V. The –8.25-mV constant is the VREF offset as VKA changes from

VNOM to 10 V, (10 V – 2.5 V) × –1.1 mV/V.

Therefore:

If VKA ≤ 10 V then;

VREF = VNOM + (IKA – INOM) x ZKA + (VKA – VNOM) × –1.1 mV/V

(1)

If VKA > 10 then;

VREF = VNOM + (IKA – INOM) × ZKA + (VKA – 10 V) x –1.5 mV/V –8.25 mV

(2)

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Now that the value of VREF is calculated, use Equation 1 and Equation 2 to calculate the value of R1 and R2.

R1 = (VKA – VREF) / IFB

(3)

R2 = VREF / (IFB – IREF)

(4)

NOTE

R2 has less current than R1.

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 VKA>10 V.

VREF = VNOM + (IKA – INOM) × ZKA + (VKA – 10 V) × -1.1 mV – 8.25 mV

(5)

VREF = 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 VREF = 2.473 V

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R1 = (VKA – VREF) / IFB

(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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