When does TL431 draw operating current?

[eem2am]

Hi,

I intend to use TL431 in regulation of isolated offline flyback SMPS.

The regulation circuit i use is as in Figure 3, page 2 of the following:-
**broken link removed**


TL431 DATASHEET:
https://www.fairchildsemi.com/ds/TL/TL431A.pdf

On page three of the TL431 datasheet it says "minimum cathode current for regulation" is 450-1000uA.

Does the TL431 draw this current all the time, or only when its REF input is at 2.5V or more?

 

[Genomerics]

Figure 2) on page 5) should demonstrate what the meaning is..



Below the 450-1000uA level the device is not 'stiff' and neither does it attain its specified 2.5V reference level. Above it it does. You therefore need to ensure at least that standing current in the device to guarantee proper operation. It is not so much that it draws that current all the time, it can draw less, but if you want it to work then that is what it needs.

Genome.

 

[eem2am]

the graph in the above thread shows the cathode current when its REF pin is at the CATHODE pin's potential.

i am wondering how much current TL431 draws when V[REF] is not equal to V(CATHODE]?

..for example,
I am wondering if any current is drawn by TL431 if the REF pin is , say 1V, 2V, 2.49V, and
V[CATHODE] is
(a) =2.5V ?
(b) >2.5V ?
(c) <2.5V ?

(Page 3 of the datasheet says that the TL431 only draws 1uA when its cathode is at 36V and
V[REF] = 0V, but it does not state the current draw for other than V[REF]= 0V )

 

[FvM]

I don't know, what you actually want to achieve, but there are two simple points that should basically answer your question:
- the reference pin can't be more than a diode voltage drop above Vcathode
- the current characteristic shown above will be basically identical for all values of Vcathode >= Vref. In other words, it will only depend on Vref.
You can derive this from the internal circuit shown in the TI data sheet.

 

[eem2am]

If an SMPS output voltage is below the intended regulation level, then the regulation circuit should not "brake" the SMPS at all..(i.e. no current required to flow in opto at all) ...as the output voltage is required to rise back up to regulation.........

thus , i believe that at V[REF] <2.49V , there will be no current drawn through the internal BJT thats outside the internal op-amp in the TL431......

What i'm saying is, i believe that there will be some voltage at V[REF] of TL431 , that gives it that there is definetely no current in the BJT thats outside the op-amp inside the TL431..............but i am wondering what this voltage is.........

Certainly if V[REF] of TL431 = 2V then i would expect absolutely no curent flow in the BJT thats at the output of the internal opamp of the TL431?

-do you agree with this?

 

[FvM]

The behaviour is pretty clear from the circuit and the current diagram already posted by Genomerics. Below Vref of 2.5 V, no current is drawn by the output transistor. All Vcathode current is needed to bias the reference circuit. As the diagram clarifies, the bias current is starting at about 1V and monotonically rising to 2.5V/400 uA. The Vref dependence of the bias current is created by the voltage follower transistor powering the reference circuit.