TL431 in SMPS feedback replaced with LT1009?

[eem2am]

Why can't the LT1009 be used in place of TL431 as an opto-coupler based feedback regulator in SMPS's?

i.e. in the following type of feedback circuit.....
http://i51.tinypic.com/30skly9.jpg

..after all, they are both Shunt Voltage Regulators.



LT1009 DATASHEET
http://cds.linear.com/docs/Datasheet/1009ff.pdf


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

 

[svhb]

Why are you sure it can not be used? Did you try it or is it just an assumption?

Maybe the difference is mainly the small signal frequency response of the regulator (just based on the two pulse response charts in the data sheets), and you need to adapt the control loop compensation.

Stefaan

 

[eem2am]

Here are the internals of an LM431 (equivalent to TL431)
http://i55.tinypic.com/2zp124p.jpg


Here are the internals of an LT1009
http://i52.tinypic.com/dy0i0h.jpg


I am wondering if they are similar enough to allow both to be used in SMPS feedback circuit as in top post?

LM431 DATASHEET
**broken link removed**

 

[svhb]

So I assume you didn't try it yet. If you have a working system with the TL431, it should be not too difficult to test a LT1009.

If you assume there is virtual no current in the "ref" input, and the cathode-current versus cathode voltage (in the case the cathode is connected directly to the reference) behaves the same, you should be able to swap them.

On a larger scale , it doesn't matter how the component is made internally, if the behavior on the pins is more-or-less the same.

I've put the characteristics next to each other in following pdf. Be careful, the scale of the LT1009 is logarithmic.

 

[FvM]

I think, it's pretty obvious, that LT1009 can't be used in a similar circuit as TL431, because the low impedance internal voltage divider and other internal circuit details don't allow the "cathode" terminal to act independant of the reference voltage. You need a shunt regulator with high impedance output terminal and suffcient output voltage range.

 

[svhb]

FvM,

Indeed, I overlooked the input impedance, you can not do the voltage divider at the input to set the desired output voltage of your power supply if using the LT1009.

Stefaan