I have few doubts regarding TL431.
Please correct if i am wrong..
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TL431 can be used to generate voltages from Vref to 36V. Output voltage is taken across diode in reverse biassed condition. reverse characterstics says Reverse breakdown voltage is 2.5V, then how TL431 can generate different output voltages. Diode breakdown region allows more current(Ika) to be taken but reverse breakdown voltage(Vka) will be almost const.
I didn't understand physically how it will generate different voltages. -
Please explain the operation of the other elements, like When Vref> 2.5V Opamp output will be high, then transistor will be ON and conducts current, but it will not effect diode behavior.
When Ref<2.5 Op-amp output is Low –>Transistor OFF, but it will not affect diode behavior. When the diode will be going to breakdown region?.
- Constant Current source using TL431
How to explain this circuit.?
One explanation says, REF always maintained at VREF->2.5V, so, constant current source Vref/Rs —-> How REF always maintained at 2.5V
Best Answer
Start from scratch imagining the transistor isn't conducting. Voltage across Rs is zero and therefore the TL431 is open circuit - but that's a problem because if it were open circuit (off) there would be decent base current into the transistor and Io would be large.
OK, look at it the other way, TL431 is conducting because Rs has enough voltage across it due to transistor being on - hey but this is wrong too because, if the TL431 were conducting there would be no voltage on the transistor base and therefore it isn't conducting and Rs has no voltage across it.
The goldilocks answer - the transistor is just conducting enough to allow 2.5 v to develop across Rs which in turn is just starting the turn the TL431 on and if it turned on too much, the current would reduce thru Rs and this would counteract the TL431 turning on too much.
It's called negative feedback.