In the datasheet for TL431, page 26, figure 32, is an example schematic of a crowbar circuit. I've been toying around with this one, since this is the first time I use a TL431. Instead of a TRIAC I use an opamp to catch the trigger (Vref to opamp = ~2.5V).
For Vcc = 5V, setting R1 = 2k4 and R2 = 2k, the "trip" point for TL431 will be at 5.5V. If Vcc goes >5.5V, the opamp will trip and switch its output. Works nice on my breadboard.
But, I just found something that I cannot find an answer/explanation for in the datasheet; if the trip-point is set at 5.5V and the total Vcc load is 0.5A, the actual trip will not occur until Vcc >= 6.1V. Even worse; if the load is 1A, I managed to turn Vcc up to 8V before the current limiter in my lab supply started to choke, and the opamp did not even react.
Why does TL431 not "trip" when there's a fair amount of load? The load current isn't even going through TL431, so what's going on here?
Where in the datasheet can I read about this?
EDIT The original post was very vague. Rewrote it for clarification.