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# TL431 cathode voltage

#### cupoftea

Hi,
If you have a 6k2 resistor feeding the cathode of a TL431 (as attached),
then supposing the voltage rises up from zero steadily.....obviously the
cathode voltage cannot become 2.5V until the rising voltage has reached 6.2V + 2.5V = 9V
But when this rising voltage is below 9V, do you agree, that the v(cathode) of the
TL431 will actually plateau at around 1.9-2.1V , and will only snap up to 2.5V
when the rising voltage reaches 9V.

In other words, the graph of v(cathode) vs I(cathode) on pg 15 of the TL431 datasheet
is only correct for the case of an actual current source feeding the TL431 cathode?

This must be considered because if you want a 2.5V reference, then we must appreciate
that the TL431's cathode current will not always allow us to have a 2.5V reference.

#### Attachments

• TL431.png
37.1 KB · Views: 34
• TL431.zip
1 KB · Views: 29
• LTspice _TL431 MODEL FROM HELMUT.zip
1.2 KB · Views: 30

Sim shows (for 6.2K-dashed line and 1K solid line) :

Key is min cathode current to meet reference specs, of 1 mA. So your series R wants to
drop.

Regards, Dana.

#### Attachments

• 1716742039495.png
164.8 KB · Views: 28
But when this rising voltage is below 9V, do you agree, that the v(cathode) of the
TL431 will actually plateau at around 1.9-2.1V , and will only snap up to 2.5V
when the rising voltage reaches 9V.
Do you see any indications for snap behaviour in the datasheet? I don't.
Monotonous I/V behaviour is clearly shown in a diagram. Imin is typically 0.4 mA but can be as high as 1 mA. I presume the I/V curve shape isn't different, just scaled.

--- Updated ---

There are shunt references with considerable lower Imin on the market.