2.5V voltage regulator circuit

小魏哥哥

2.5V voltage regulator circuit [Copy link]

 

 

As shown in the figure above, refer to the information on the Internet. If you want a circuit with an input of 2.5V-12V and an output of 2.5V, the circuit implementation principle is: after power-on, Q3 is turned on at a high level, at this time QD1 is turned on, the voltage at point O is the input voltage, and the voltage at point O is connected to pin 1 of TL431 and compared with the reference voltage (about 2.5V). When it is greater than 2.5V, pins 2 and 3 of TL431 are turned on, and the base voltage of Q3 is low and disconnected, and the P-MOS tube is also disconnected. Then the voltage at point O is compared with the reference voltage (about 2.5V). When it is less than 2.5V, pins 2 and 3 of TL431 are disconnected, Q3 is turned on, and QD1 is turned on, realizing the function of outputting 2.5V.

Now the test found that the output is the same as the input, and Q3 cannot be cut off. Can you please tell me if there is any problem with the principle of this circuit?

maychang

"Is there any problem with the principle of this circuit?"

In principle, there seems to be no problem.

"Now the test finds that the output is the same as the input"

This may be caused by leakage current between the drain and source of QD1.

You can connect a 1 kilo-ohm resistor between the output terminal and ground, and then measure the output voltage.

maychang

In addition, please post the datasheet of the P-channel MOS tube you chose.

hungchou

There is a diode between pin 1 and pin 2 inside TL431, so the pull-down voltage of pin 2 can only be as low as 2V. In this way, Q3 will be always turned on and QD1 will not be turned off.

普拉卡图

maychang posted on 2022-3-4 14:20 "Is there any problem with the principle of this circuit?" There seems to be no problem in principle. "Now the test found that the output is the same as the input...

Hello teacher! I would like to ask what is the function of this R4? Can't the TL431 be directly connected to the output? Why do we need to add an extra 3.3k resistor in the middle? Will this extra resistor affect the voltage of pin 1 of TL431?

maychang

Placatto published on 2022-3-6 17:44 Hello teacher! I would like to ask what is the function of this R4? Can't TL431 be directly connected to the output? Why do we need to add an extra 3.3k resistor in the middle? The extra...

The purpose of this resistor R4 is to limit the current flowing into the Ref terminal of TL431. Because the Ref terminal current of TL431 is very small (typical value is 2uA), the voltage drop of R4 is very small (the value in the figure, the voltage drop is a few mV), and the impact is not great.

maychang

What the 4th post said makes sense. There is a PN junction between the Cathode terminal (pin 2) and the Ref terminal of TL431. The Cathode terminal cannot be lower than the zero-point voltage of the Ref terminal, so this circuit is problematic. I was wrong in the 2nd post.

maychang

hungchou posted on 2022-3-5 00:19 There is a diode jumper from pin 1 to pin 2 inside TL431, so the pull-down voltage of pin 2 can only be around 2V, so Q3 will always be turned on, and QD1 will naturally not be turned off.

There is a diode between pin 1 and pin 2 inside TL431, so the pull-down voltage of pin 2 can only be as low as 2V. In this way, Q3 will be always turned on and QD1 will not be turned off.

Makes sense. My reply on the second floor was ill-considered.

maychang

Placatto published on 2022-3-6 17:44 Hello teacher! I would like to ask what is the function of this R4? Can't TL431 be directly connected to the output? Why do we need to add an extra 3.3k resistor in the middle? The extra...

However, as mentioned on the 4th floor, the circuit in the first post has problems and must be redesigned, so the influence of the resistor R4 is no longer important.

maychang

In fact, if the load current is not large (no more than a dozen mA), using only one TL431 can obtain a sufficiently satisfactory 2.5V stable voltage, and Q3 and QD1 can be omitted.

小魏哥哥

hungchou posted on 2022-3-5 00:19 There is a diode jumper from pin 1 to pin 2 inside TL431, so the pull-down voltage of pin 2 can only be around 2V, so Q3 will always be turned on, and QD1 will naturally not be turned off.

I had a misunderstanding about TL431. I thought pins 2 and 3 were equivalent to a voltage regulator. Thanks for clearing up my confusion.

小魏哥哥

maychang posted on 2022-3-6 18:16 In fact, if the load current is not large (no more than a dozen mA), using only one TL431 can already obtain a sufficiently satisfactory 2.5V stable voltage. Both Q3 and QD1 can be used...

The solution to this circuit is to add a resistor between the D pole of the MOS tube and the E pole of the transistor to increase the voltage of the E pole of the transistor.

Gen_X

Directly reduce R3, if that doesn’t work, increase rR2.

小魏哥哥

Gen_X posted on 2022-3-10 09:54 Directly reduce R3, if that doesn’t work, increase rR2.

This is the principle block diagram of TL431. When the voltage at the K pole is lower than that at the R pole, the internal diode of TL431 is turned on, so the voltage at the K pole will not be lower than 2V.

chipsimi003

Directly use a high-voltage LDO to stabilize the voltage to 2.5V, making the periphery simpler.

胡杨1

When the input and output voltages are almost the same, it is normal for Q3 not to be cut off.