What is the function of the capacitor in the TL431 reference circuit?

kal9623287

What is the function of the capacitor in the TL431 reference circuit? [Copy link]

I saw a question about the TL431 benchmark in other articles. If you don't understand something, please ask the teachers.

The circuit below

When no capacitor is added:
1. In this conventional connection method, the output voltage of 10V is measured alone, which is very accurate;

2. If you measure the 2.5V voltage alone, the 2.5V voltage may fluctuate between 2.0V and 2.4V, which is very inaccurate.

3. We measure the above two voltages at the same time. The 2.5V reference is inaccurate, and the 10V theoretical output also decreases synchronously (inaccurate), but the proportional relationship still exists.

The reference voltage becomes stable after adding the capacitor.

What is the function of this added capacitor?

呜呼哀哉

My friend, your description is a little unclear. Is it true that when there is no capacitor, the 10V and 2.5V positions are accurate, but after adding the capacitor, they are inaccurate, or vice versa? In addition, the C pole output of TL431 is 10V. Is there any capacitor at that potential point?

fjdeepblue

This capacitor is for loop compensation. If the 431 does not have this capacitor, oscillation will occur. You can see it by looking at the waveform with an oscilloscope.

fjdeepblue

This capacitor is generally added across the 10V output, that is, between the lower end of 3.9K and ground.

Gen_X

Generally, adding it to R2 can improve the stability of the output by improving the stability of the reference;

Added across R1 to increase the response speed of output change adjustment (i.e. compensation).

If the requirements are not high, only R3 to the ground is enough.

kal9623287

Alas, published on 2024-4-24 14:28 Friend, your description is a bit unclear. Is it that when there is no capacitor, the 10V and 2.5V positions are accurate, but after adding the capacitor, they are inaccurate, or vice versa? In addition, TL431 ...

The reference voltage is stable only after adding capacitors.

kal9623287

Gen_X posted on 2024-4-25 13:59 Generally, adding it to R2 can improve the stability of the output by improving the stability of the benchmark; adding it to both ends of R1 is used to improve the response speed of the output change adjustment...

So in this circuit, no matter how much the output is, the output should have output capacitance, right?

Gen_X

kal9623287 posted on 2024-4-26 12:39 Then in this circuit, no matter how much the output is, the output should have an output capacitor

Yes! According to the output current, if the current is large, increase it; if the current is small, decrease it.

Generally, these are mostly used as references, the current is very small, so the capacitance is not large.

wzc85

Is it a decoupling capacitor?

Gen_X

I did an experiment and found that a capacitor that is too large (I used 0.1uF) cannot be connected in parallel with R2, otherwise it will self-excite at low frequency.

This shows that the gain of 431 is very high, so the accuracy is also very high.