TL431Z

APPLICATIONS INFORMATION

The TL431Z is a programmable precision reference which is used in a variety of ways. It serves as a reference

voltage in circuits where a non–standard reference voltage is needed. Other uses include feedback control for driving an

optocoupler in power supplies, voltage monitor, constant current source, constant current sink and series pass regulator.

In each of these applications, it is critical to maintain stability of the device at various operating currents and load

capacitances. In some cases the circuit designer can estimate the stabilization capacitance from the stability boundary

conditions curve provided in Figure 15. However, these typical curves only provide stability information at specific

cathode voltages and at a specific load condition.

Additional information is needed to determine the capacitance needed to optimize phase margin or allow for

process variation. A simplified model of the TL431Z is shown in Figure 31. When tested for stability boundaries, the load

resistance is 150 Ω. The model reference input consists of an input transistor and a dc emitter resistance connected to

the device anode. A dependent current source, Gm, develops a current whose amplidute is determined by the difference

between the 1.78 V internal reference voltage source and the input transistor emitter voltage. A portion of Gm flows

is connected across the device cathode and anode.

Model component values are:

Vref = 1.78 V

Go = 1.25 (Vcp

Resistor and capacitor typical values are shown on the model. Process tolerances are

±20% for resistors, ±10%

for capacitors, and

±40% for transconductances.

An examination of the device model reveals the location of circuit poles and zeroes:

In addition, there is an external circuit pole defined by the load:

Also, the transfer dc voltage gain of the TL431Z is:

Example 1:

The resulting transfer function Bode plot is shown in Figure 32. The asymptotic plot may be expressed as the

following equation:

The Bode plot shows a unity gain crossover frequency of approximately 600 kHz. The phase margin, calculated

from the equation, would be 55.9 degrees. This model matches the Open–Loop Bode Plot of Figure 12. The total loop

would have a unity gain frequency of about 300 kHz with a phase margin of about 44 degrees.

Rev. 01