That's true, but it's easy to forget when the chip has no explicit supply pin
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
Also, something like LM317 will take a few volts on its sense input before protection kicks in. TL431 could do the same by getting rid of the follower, but then its input bias current would be more like LM317's.
Some vendors are more honest in their datasheets
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
This is Wing Shing ("WS"), popular on auction sites. ON Semi also shows the trap diode.
![](https://www.eevblog.com/forum/projects/trap-for-young-players-tl431-reference-input-current-and-how-to-cook-your-tl431/?action=dlattach;attach=1380529;image)
I tested a few chips today:
- 1431T - 1990s, Asian, maybe Toshiba(?)
- AZ431 - 2000s, BCD Semiconductor (?)
- TL431CLP - modern, ON Semi
- HA17431 - 1990s, Hitachi (?)
- TL431ACLP - fairly modern, Fairchild, likely ex-Samsung
The test: cathode is open circuit, reference is pulled with 100Ω to 5V.
The 1431T clamped to 1.7V, sinking 33mA.
The next pair clamped to 3V, sinking 20mA.
The final pair clamped to 4.2V, sinking 8mA.
Then the two modern chips were subjected to the same with 12V.
ON Semi clamped to 4V, sinking 80mA.
Fairchild clamped to 8.5V, sinking 35mA.
In all cases, the cathode was 0.8~1.2V below the reference. Shorting the cathode to the reference restored normal operation, with 25mA/95mA sink current.
Conclusions: everything as predicted
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/xcheesy.gif.pagespeed.ic.KPzuH4J-TG.png)
They don't sink as much as they would want to, but enough to cause trouble. High variability between vendors.
The chip which I abused previously was National, BTW, but it's still soldered into its circuit (and working normally) so I left it alone. And unfortunately I ran out of Wing Shing too.