TL431, A, B Series

12

MOTOROLA ANALOG IC DEVICE DATA

Figure 31. Simplified TL431 Device Model

+

RL

VCC

–

CL

15 k

9.0

mF

Input

8.25 k

3

Cathode

500 k

Vref

1.78 V

Rref

16

GM

Anode

2

RGM

1.0 M

Ref

1

Go

1.0

mmho

CP2

0.265 pF

RP2

10 M

RZ1

15.9 k

CP1

20 pF

f, FREQUENCY (Hz)

102

101

–20

30

20

60

0

Figure 32. Example 1

Circuit Open Loop Gain Plot

TL431 OPEN–LOOP VOLTAGE GAIN VERSUS FREQUENCY

40

104

103

107

105

106

10

–10

50

Example 2.

IC = 7.5 mA, RL = 2.2 kW, CL = 0.01 mF. Cathode tied to

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)

102

101

–20

40

20

80

0

Figure 33. Example 2

Circuit Open Loop Gain Plot

TL431 OPEN–LOOP BODE PLOT WITH LOAD CAP

60

104

103

106

105

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.