[SOLVED] Comparator in a TL431 battery voltage circuit

[fendertunes]

I am trying to use TL431 as a comparator (kind of like this article):

**broken link removed**

The article says - "When the battery voltage is higher than the threshold voltage, the cathode voltage of the TL431 is at its low level of approximately 2V".

My question is = Why does the cathode voltage does not go to 0V (instead of 2V)? I think this has to do with the 2.5V reference, but can anyone explain it a bit more?

 

[steveelliott]

Not much to do with reference. The TL431 turns on an internal transistor from cathode to anode if the voltage at the R terminal is > 2.5V and turns it off when the R terminal is < 2.5V. Anode terminal is also power supply positive lead for IC. Below 2V, the chip starts shutting down and it can't drive the anode any lower or it would turn itself off (which would raise the voltage at the anode and turn it back on which would make it pull the anode low which would turn it off which would make the anode go high...) . The 2V is just the balance point between turning on and off.

 

[fendertunes]

Hmm I kind of understand it. This 2V cathode voltage when R<2.5, where is that in the datasheet? or how would you tell that from the datasheet?

 

[steveelliott]

Fig 7 of Datasheet, plus if you look at the equivalent schematic you can see the "longest" non-saturating path from + to - is through the shorted CB transistor (800 ohm emitter resistor), then the two transistors at ends of 4k ohm resistor. 3 PN junctions = 2V to keep them turned on.