TL431Z Datasheet, PDF (9/14 Pages) List of Unclassifed Manufacturers – PROGRAMMABLE PRECISION SHUNT REGULATOR

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
through compensation capacitance, C_P2. The voltage across C_P2 drives the output dependent current source, Go, which
is connected across the device cathode and anode.
Model component values are:
Vref = 1.78 V
Gm = 0.3 + 2.7 exp (–I_C/26 mA)
where I_C is the device cathode current and Gm is in mhos
Go = 1.25 (Vcp²) µmhos.
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