Datasheet

TL431, A, B Series
12
MOTOROLA ANALOG IC DEVICE DATA
Figure 31. Simplified TL431 Device Model
+
R
L
V
CC
–
C
L
15 k
9.0
F
Input
8.25 k
3
Cathode
500 k
V
ref
1.78 V
R
ref
16
G
M
Anode 2
R
GM
1.0 M
Ref
1
Go
1.0
mho
C
P2
0.265 pF
R
P2
10 M
R
Z1
15.9 k
C
P1
20 pF
f, FREQUENCY (Hz)
10
2
10
1
–20
30
20
60
0
Av, OPEN–LOOP VOLTAGE GAIN (dB)
Figure 32. Example 1
Circuit Open Loop Gain Plot
TL431 OPEN–LOOP VOLTAGE GAIN VERSUS FREQUENCY
40
10
4
10
3
10
7
10
5
10
6
10
–10
50
Example 2.
I
C
= 7.5 mA, R
L
= 2.2 k , C
L
= 0.01 F. 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 )(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)
10
2
10
1
–20
40
20
80
0
Av, OPEN–LOOP GAIN (dB)
Figure 33. Example 2
Circuit Open Loop Gain Plot
TL431 OPEN–LOOP BODE PLOT WITH LOAD CAP
60
10
4
10
3
10
6
10
5
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.