TL431, A, B Series

11

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

The TL431 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 TL431 is shown in Figure 31.

When tested for stability boundaries, the load resistance is

150

W. 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, CP2. The voltage across CP2

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 (–IC/26 mA)

where IC 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:

P1

+

1

2

p R

GM

C

P1

+

1

2

p * 1.0 M * 20 pF +

7.96 kHz

P2

+

1

2

p R

P2

C

P2

+

1

2

p * 10 M * 0.265 pF +

60 kHz

Z1

+

1

2

p R

Z1

C

P1

+

1

2

p * 15.9k * 20pF +

500 kHz

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

load:

P

L +

1

2

p R

L

C

L

Also, the transfer dc voltage gain of the TL431 is:

G

+ G

M

R

GM

GoR

L

Example 1:

I

C +

10 mA, R

L+

230

W,C

L+

0. Define the transfer gain .

The DC gain is:

G

+ G

M

R

GM

GoR

L +

(2.138)(1.0 M)(1.25

m)(230) + 615 + 56 dB

Loop gain

+ G

8.25 k

8.25 k

) 15 k +

218

+ 47 dB

The resulting transfer function Bode plot is shown in

Figure 32. The asymptotic plot may be expressed as the

following equation:

Av

+ 615

1

) jf

500 kHz

1

) jf

8.0 kHz

1

) jf

60 kHz

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.