TL431Z
Figure 31. Simplified TL431Z Device Model
Figure 32. Example 1
Circuit Open Loop Gain Plot
Figure 33. Example 2
Circuit Open Loop Gain Plot
Example 2.
I
C
= 7.5 mA, R
L
= 2.2 kΩ, C
L
= 0.01 µF.
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 µ)(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:
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.
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.
Rev. 01
IKSEMICON [ IK SEMICON CO., LTD ]
