TL431Z

2011, January, rev. 03

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.

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:

(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:

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.

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.