How to calculate the cathode potential of TL431?

普拉卡图

How to calculate the cathode potential of TL431? [Copy link]

 

As shown in the figure, how is the cathode potential of TL431 calculated?

The simulation result is around 1.2V.

I found a diagram of the internal structure of TL431 and studied it.

Still can't figure out any results,

Does anyone know how this value is calculated?

maychang

[As shown in the figure, how is the cathode potential of TL431 calculated? ]

TL431 cannot be used as shown in the figure. The TL431 in the figure is open loop.

maychang

TL431 is a parallel voltage regulator chip. For the correct usage, please refer to the typical application circuit in the chip datasheet.

bigbat

According to the volt-ampere characteristic, the voltage of the output pin of TL431 can be understood as a voltage source of 2.5V, so the current on R2 is 2.5V/1K=2.5mA

Then the current of R1 is also 2.5mA, that is, the voltage divided by R1 is 2.5V, so 2.5V+2.5V=5V voltage.

davidzhu210

The conduction condition of TL431 is Vref>2.496V. If Ref and K pole are not short-circuited, the cathode potential is clamped at about 1.1V after conduction, just like a voltage-stabilizing diode. If Ref and cathode are short-circuited, the clamping voltage is 2.5V, which is also equivalent to a voltage-stabilizing diode.

gah

The working state of TL431 is related to the control electrode R terminal. If the voltage at the R terminal is greater than 2.5V, through the internal circuit, the amplifier outputs a high level, the internal transistor is turned on, and the voltage at the K terminal is pulled down; if the voltage at the R terminal is less than 2.5V, the internal amplifier outputs a low level, the transistor is cut off, and the K terminal is a high level; 2.5V is its reference voltage.

普拉卡图

davidzhu210 posted on 2022-11-1 21:45 The TL431 conduction condition Vref>2.496V. If Ref and K pole are not short-circuited, the cathode potential is clamped at about 1.1V after conduction, just like a voltage-stabilizing diode...

After the TL431 is turned on, is the cathode voltage of about 1.1V fixed? Just like when a triode is turned on, there will be a 0.3V tube voltage drop? Can we analyze the internal structure of the TL431 to find out why there is this 1.1V voltage?

davidzhu210

1.1V is fixed, just like a voltage regulator. The internal circuit is on the datasheet, and you need an expert to analyze it.

maychang

Placardo published on 2022-11-2 17:26 After TL431 is turned on, is the cathode voltage of about 1.1V fixed? Just like when the triode is turned on, there will be a 0.3V tube voltage drop? This can be measured by T ...

In the first post, you have posted the internal block diagram of TL431. Notice that there is an op amp. Where does the power of this op amp come from? The power of this op amp can obviously only come from pin 1 (i.e., terminal K). The measured voltage of about 1.2V is the power supply voltage that the op amp can output to turn on the internal transistor of TL431.

maychang

Placardo published on 2022-11-2 17:26 After TL431 is turned on, is the cathode voltage of about 1.1V fixed? Just like when the triode is turned on, there will be a 0.3V tube voltage drop? This can be measured by T ...

In fact, the internal voltage reference of TL431 is also powered by pin 1. Now you measure the voltage of pin 1 to be 1.2V, of course the reference voltage cannot be maintained at 2.5V. However, the Ref pin is 2.5V, so the potential of the internal op amp's non-inverting input must be higher than that of the inverting input, and the internal op amp must output a high level, but it is a high level under the 1.2V power supply voltage.

maychang

Placardo published on 2022-11-2 17:26 After TL431 is turned on, is the cathode voltage of about 1.1V fixed? Just like when the triode is turned on, there will be a 0.3V tube voltage drop? This can be measured by T ...

[Can we analyze the internal structure of TL431 to find out why there is a voltage of 1.1V? ]

This is an abnormal working state of TL431, so there is no need to calculate it.

普拉卡图

maychang published on 2022-11-2 21:06 In fact, the internal voltage reference of TL431 is also powered by pin 1. Now the voltage of pin 1 is 1.2V as measured by you. Of course, the reference voltage cannot be maintained at 2.5V...

I changed the connection method and used TL431 as an NPN which can only be turned on if Vbe>2.5V.

It seems that as mentioned on the 8th floor, after the TL431 is turned on, the voltage between the two electrodes will be close to 1.1V.

This is similar to the voltage drop of about 0.3 after the transistor is turned on.

Is this also a feature of TL431?

After being turned on, it is equivalent to a 1.1V voltage regulator?

maychang

"I changed the connection method and used TL431 as an NPN whose conduction condition is Vbe>2.5V. It seems that as mentioned on the 8th post, the voltage between the two electrodes of TL431 will be close to 1.1V after it is turned on. This is similar to the voltage drop of about 0.3 when the transistor is turned on..."

How come I can't see this reply?

普拉卡图

maychang posted on 2022-11-3 08:29 "I changed the connection method and used TL431 as an NPN whose conduction condition is Vbe>2.5V. It seems that as mentioned on the 8th floor, after TL431 is turned on...

Is this picture not showing up?

maychang

Plakatu posted on 2022-11-3 16:29 Is this picture not displayed?

【Isn't this picture displayed? 】

Now it is. But it wasn’t last night. Not only was there no picture last night, there was no text either.

bmahu001

Refer to the instruction manual. . . .