Author Topic: What happens to a TL431 with disconnected cathode?  (Read 1856 times)

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Offline rr100

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What happens to a TL431 with disconnected cathode?
« on: September 21, 2015, 09:06:01 am »
simple question - it looks like the Ref is sinking some current if going about certain voltage. Looking at the detailed schematic from datasheet it looks there's a path for the current to go from Ref to Anode via some resistor(s) in KOhm range and transistor junctions. This is it? Any idea how much voltage can take and/or current can sink?
 

Offline ralphd

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Re: What happens to a TL431 with disconnected cathode?
« Reply #1 on: September 21, 2015, 09:53:20 am »
At around 3.2V you'll reverse bias the ref input transistor, giving a low-resistance path to ground.  I mistakenly thought they have an internal fet-based comparator for the ref input, but it is transistor-based.
Unthinking respect for authority is the greatest enemy of truth. Einstein
 

Offline ralphd

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Re: What happens to a TL431 with disconnected cathode?
« Reply #2 on: September 21, 2015, 06:28:08 pm »
I measured the ref-cathode junction voltage drop on a few TL431s, and they were all very close to 0.73V @ ~0.5mA.
http://nerdralph.blogspot.ca/2015/09/diodes-diodes-everywhere.html
Unthinking respect for authority is the greatest enemy of truth. Einstein
 

Offline nuno

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Re: What happens to a TL431 with disconnected cathode?
« Reply #3 on: September 21, 2015, 07:37:56 pm »
ralphd, I've never seen this value mentioned in an AVR datasheet, but an AppNote (I think it's the one on the mains zero cross detector) mentions that current through an AVR pin protection diode should not exceed 1mA.
 

Offline ralphd

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Re: What happens to a TL431 with disconnected cathode?
« Reply #4 on: September 21, 2015, 10:56:15 pm »
ralphd, I've never seen this value mentioned in an AVR datasheet, but an AppNote (I think it's the one on the mains zero cross detector) mentions that current through an AVR pin protection diode should not exceed 1mA.
Probably not a bad idea to keep the current low, but based on my measurements I'd guess that 10mA would not harm the AVR.
Unthinking respect for authority is the greatest enemy of truth. Einstein
 

Offline rr100

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Re: What happens to a TL431 with disconnected cathode?
« Reply #5 on: September 23, 2015, 10:37:17 am »
Yea, I was thinking there's a high impedance comparator too (and actually connected the Ref directly to the voltage I was "measuring" (lucky it was just a LiIon cell, the circuit still works...).
 


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