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TL431 View Datasheet(PDF) - Motorola => Freescale

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Description
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TL431
Motorola
Motorola => Freescale Motorola
TL431 Datasheet PDF : 16 Pages
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TL431, A, B Series
Figure 31. Simplified TL431 Device Model
VCC
RL
Input
CL
15 k
9.0 mF
8.25 k
Ref
Vref
1
1.78 V
+ Rref
GM
500 k
16
3
Cathode
RGM
1.0 M
RP2
10 M
CP1
20 pF
RZ1
15.9 k
Go
1.0 mmho
CP2
0.265 pF
Anode 2
Figure 32. Example 1
Circuit Open Loop Gain Plot
TL431 OPEN–LOOP VOLTAGE GAIN VERSUS FREQUENCY
60
50
40
30
20
10
0
–10
–20
101 102
103
104
105
106
107
f, FREQUENCY (Hz)
Example 2.
W m IC = 7.5 mA, RL = 2.2 k , CL = 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 GMRGMGoRL
+ + m (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:
)1 jf
ǒ Ǔ + ) ) ) 500 kHz
Av 615
ǒ Ǔǒ Ǔǒ Ǔ 1 jf 1 jf 1 jf
8.0 kHz 60 kHz 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.
Figure 33. Example 2
Circuit Open Loop Gain Plot
TL431 OPEN–LOOP BODE PLOT WITH LOAD CAP
80
60
40
20
0
–20
101
102
103
104
105
106
f, FREQUENCY (Hz)
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.
12
MOTOROLA ANALOG IC DEVICE DATA
 

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