TL431, A, B Series, NCV431A, B

http://onsemi.com

12

Figure 31. Simplified TL431 Device Model

+

−

15 k

9.0

mF

Input

8.25 k

3

Cathode

500 k

1.78 V

16

Anode

2

1.0 M

Ref

1

Go

1.0

mmho

0.265 pF

10 M

15.9 k

20 pF

f, FREQUENCY (Hz)

−20

30

20

60

0

Figure 32. Example 1 Circuit Open Loop Gain Plot

TL431 OPEN−LOOP VOLTAGE GAIN VERSUS FREQUENCY

40

10

−10

50

Example 2.

reference input pin. An examination of the data sheet

stability boundary curve (Figure 15) shows that this value of

load capacitance and cathode current is on the boundary.

Define the transfer gain.

The DC gain is:

G

+ G

M

R

GM

GoR

L +

(2.323)(1.0 M)(1.25

m)(2200) + 6389 + 76 dB

The resulting open loop Bode plot is shown in Figure 33.

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

1

)

jf

7.2 kHz

Note that the transfer function now has an extra pole

formed by the load capacitance and load resistance.

Note that the crossover frequency in this case is about

250 kHz, having a phase margin of about −46 degrees.

Therefore, instability of this circuit is likely.

f, FREQUENCY (Hz)

−20

40

20

80

0

Figure 33. Example 2 Circuit Open Loop Gain Plot

TL431 OPEN−LOOP BODE PLOT WITH LOAD CAP

60

With three poles, this system is unstable. The only hope

for stabilizing this circuit is to add a zero. However, that can

only be done by adding a series resistance to the output

capacitance, which will reduce its effectiveness as a noise

filter.

Therefore,

practically,

in

reference

voltage

applications, the best solution appears to be to use a smaller

value of capacitance in low noise applications or a very

large value to provide noise filtering and a dominant pole

rolloff of the system.