TL431, A, B Series, NCV431A
V
CC
R
L
Input
15 k
9.0
mF
Ref
1
500 k
8.25 k
C
L
3
Cathode
R
P2
10 M
+
−
G
M
R
ref
16
V
ref
1.78 V
Go
1.0
mmho
C
P2
0.265 pF
R
GM
1.0 M
C
P1
20 pF
R
Z1
15.9 k
Anode
2
Figure 31. Simplified TL431 Device Model
TL431 OPEN−LOOP VOLTAGE GAIN VERSUS FREQUENCY
60
Av, OPEN−LOOP VOLTAGE GAIN (dB)
50
40
30
20
10
Av, OPEN−LOOP GAIN (dB)
0
−10
−20
10
1
10
2
10
4
10
3
10
5
f, FREQUENCY (Hz)
10
6
10
7
60
40
20
0
−20
10
1
10
2
10
3
10
4
10
5
10
6
f, FREQUENCY (Hz)
TL431 OPEN−LOOP BODE PLOT WITH LOAD CAP
80
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.
Figure 32. Example 1 Circuit Open Loop Gain Plot
Example 2.
I
C
= 7.5 mA, R
L
= 2.2 kW, C
L
= 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 R
GoR
+
M GM
L
(2.323)(1.0 M)(1.25
m)(2200)
+
6389
+
76 dB
Figure 33. Example 2 Circuit Open Loop Gain Plot
The resulting open loop Bode plot is shown in Figure 33.
The asymptotic plot may be expressed as the following
equation:
1
)
Av
+
615
1
)
jf
8.0 kHz
jf
500 kHz
jf
60 kHz
1
)
jf
7.2 kHz
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.
1
)
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