Low drop out voltage regulator with TL431 Problem!

[Shahriar]

Hi
I want to build a Low Drop out voltage regulator(LDO)
I tried the schematic Below but the output Changes when I change the input voltage.
I wrote the voltages I measured at Vi=8V.
I put that 8.2K and 2.2K to create a voltage devider at the base of NPN tarnsistor because the lowest voltage i could get of TL431 was around 1.8V So I affrade maybe the NPN do not gets Off.
I put the other 2.2K to creat Enough current for TL431.
the output of this circuit should be 5V. The reference voltage of TL431 is 2.5V

ThanX in advance
Shahriar

[Staigen]

Hey Shahriar, where is the base-emitter resistor for the 2N4403 transistor, it must be one for cirquits like this ;D

//Staigen

[Shahriar]

Dear Staigen

I put Different resistors there to ensure 2N4403 gets off.
when I put a low resistor(2.2K even 330ohm) the output goes to 3.8V
when I put a bigger resistor the output goes higher 4.8
But in both cases, the output changes when I change the Vi

ThanX
Shahriar

[Staigen]

Oscillating? Check with your scope! A cap on the output wouldn't hurt!

//Staigen

[ante]

Hi Guys,

I would also try a lower value resistor from (Vi) to pin3.

[Guest Alun]

The open loop gain is far too high.

Ante,
The potential divider between pin 3 of TL431 and the base of Q11 reduces the gain if you reduce the 8k2 resistor on pin 3 then the gain will increase thus making the problem worse!

I think you should increase its value and connect a 1nF compensation capacitor across the TL431 and 100nF followed by 100uf on the output of the whole regulator.

By the way, there are better ways of doing this using a MOSFET, if you're interested I'll post a schematic.

[audioguru]

Horray, this site is back "on the air". ;D

I fixed the circuit so its voltage regulation should be superb.
I added a couple of diodes to raise the base voltage of the NPN transistor to closer to the absolute minimum 2.5V required at the cathode of the TL431. I also calculated the minimum of 1mA (a little more when using standard resistance values) for it.
When the input voltage is high, the TL431 "zener diode" current is also high since zeners and shunt regulators waste a lot of current.

If the circuit is built with proper close parts layout on a pcb then maybe it won't oscillate since the TL431 has compensation for its own gain built-in. If it does oscillate then a 0.01uF cap can be added between the cathode to adj pins as in the datasheet. ;D

[Guest Alun]

audioguru,
A 741 is compensated but if you added a transistor amplifier with enough gain in its feedback loop it would oscillate because there would still be gain when it's output is 180^o.

[Shahriar]

ThanX all for your replies and your Pictures.


At last I made this circuit working. I was sure about my circuit wirings and I was really disgusted. Do you know How I solved it. Look at the picture below:

I don’t know much about Zero and Poles in Feedback circuits. But I think this cap will do something with Poles of the circuit. Before this I thought these poles are not very important…..
At last I connected the output of circuit to Oscilloscope, Wow, without that cap that output was oscillating very neatly and as soon as I connected the Cap the output was a Clean +5V DC Voltage. I have attached the output signal without a cap.

Dear Staigen
ThanX vey much, I think you solved the problem. the time difference between your replies and when I read them, causes I work a whole day on circuit. when you type the replies I am sleeping and when I write questions, you are sleeping.
at last If I had checked the ouptput with an Oscilloscoe then I would take result a 12hours earlier. 8)

Dear Audioguru,
are the values which you have written, from a circuit or you have witten them from simulating. anyway, I am going to test the circuit you have drawn. (I build the circuit on a BreadBoard!) :D

By the way, when I just Inserted a 1nF cap between Pin3 and Pin1,(while there was no output cap) the output was still oscillating but the amplitude was smaller.

ThanX All again
Shahriar

[audioguru]

Hi Shahriar,
Great, you fixed it! ;D
It won't be low dropout until you reduce the value of the 2.2k resistor low enough that it can supply base current to the NPN transistor when the circuit is fully loaded plus at least 1mA to the cathode (its power supply) of the TL431 at 2.5V or more.

[Guest Alun]

The best way to picture a TL431 is to see it as a transistor with a 2.5V saturation voltage and a base voltage of exactly 2.1V and a very high gain and base impedance, in fact you can actually use the TL431 as an amplifier in pretty much the same way you would with a transistor.

Phase shift and oscillation in amplifiers
Voltage regulators are basically DC amplifiers and are just as susceptible to instability as any audio amplifier.

Phase shifts caused by parasitic capacitances in amplifiers can lead to oscillation when a large amount of negitive feedback is used, here's an extract from a college assignment I did a couple of years ago that covers this and here's the datasheet for the 741 op-amp discussed

[audioguru]

Typo? The threshold voltage of a TL431 at its adj terminal is 2.5V, not 2.1V.
I haven't tried it but the datasheet shows that it makes a pretty good low power class-A audio amp, much better than a single transistor. It has only about 11 transistors inside. ;D