Problem with TL431 regulation, plse help me.

[commstech]

Problem with TL431 regulation, plse help me. Post #1


I had recently tried to add shunt regulation to my existing LM317 and LM337 regulated supply for my TDA1541 DAC. However, i encountered the problems below:

1. using the formula Vout= (1+R1/R2)*Vref , i worked out the R1/R2 for -5V & +5V to be 1K and -15V to be 1K/5K. And i adjust the Vo at the LM317 and LM337 to be +6V, -6V and -18V and connect directly to the shunt regulator directly. However, the Vout at the shunt regulators do not give the Vout i need. They simply give out whatever input the LM317 and LM337 feed them. Therefore to obtain the correct voltages after the shunt regulator, i need to adjust the LM317 and LM337.

2. the LM317 and LM337 runs hot! the same goes for the TL431!


Can anyone please enlighten?

[janneman]

Schematic please!

Jan Didden

[Werner]

Read the datasheet

http://www.onsemi.com/pub/Collateral/TL431-D.PDF

Figure 18: a shunt needs a series resistive element between raw voltage supply and regulated output. The sum of shunt current and load current flow through that resistor and so make up the difference voltage between raw supply and regulated output.

In your case you connect a near zero-Ohm 6V supply right to the shunt. The TL431 tries to lower the output voltage by ever passing more current. As this current is happily passed on by the LM317, without voltage sagging, the two get into a deadly, erm, deadlock.

How to calculate the series resistor?

Suppose the load consumes 15mA. We add to this a shunt current of 10mA, for a total of 25mA. Vraw is 6V, Vreg is 5V:

R = (6V-5V)/55mA = 40.

But a better solution, BTW, might be to use the 317/337s as current sources, in which case they indeed act as voltage dropping elements for the 431s.





The Naked Truth article on the use of TL431s for audio.

[sparkle]

Hi!

I will post You today some Excell spreadsheet from (i belive) Texas Instruments with some aditional data . It should be easy to calculate the required voltage and series resistot. I am on work and I have that spreadsheet at my home pc - don't worry I won't forget.

Don't worry - in 10 hours I will post it to You.
regards
daniel

[peranders]

I don't have the URL but it's on my hardisk

[Bricolo]

What minimum current does TI's file use for the TL431?

I think it is 1mA, but I read on this forum that 10mA as a minimum current is recommended

[sparkle]

Originally posted by peranders
I don't have the URL but it's on my hardisk

That's the file I ment. You can also use a little lower value (for Rser) than calculated to increase thecurent through TL431 (that is for obtaining 10mA or so through TL431- check the temperature of the TL after You do so).
regards
daniel

[commstech]

hi Werner, your information is certainly very useful. Thanks a million!!.

[commstech]

checked out the Texas spreadsheet. very useful and convenient tool indeed.

what is the Idiv/Iref? i realise that changing this value will affect the value of R1/R2. What is the link here? how to determine this value to put into the table for calculation?

[sparkle]

Originally posted by commstech
checked out the Texas spreadsheet. very useful and convenient tool indeed.

what is the Idiv/Iref? how to determine this value to put into the table for calculation?

That is the ratio between the currents of the resistor divider and the current throught the TL431 if I remember it right.