# Issue with setting TVL431A output voltage

#### pager48

Joined Nov 25, 2018
161
Trying to have a low battery cutoff for a 9V alkaline which is typically 5.4v. The TLV431A was set to 5.8V using 2 resistors

Vout = Vref x (R1/R2) = 1.24 X ( 4.7k/1004) = 5.8V regulator has a typicall 1.24 Vref https://www.onsemi.com/pub/Collateral/TLV431A-D.PDF but simulation shows incorect output voltage.

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#### Jony130

Joined Feb 17, 2009
5,419
Vout = Vref x (R1/R2)
Wrong.

Vout = Vref x (1 + R1/R2) = 1.24V*(1 + 4.7kΩ/1kΩ) = 1.24V*5.7 ≈ 7V

You circuit should look like this:

And in sim file, the R5 resistor was added only for simulation purpose.

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#### crutschow

Joined Mar 14, 2008
30,818
Here's your circuit modified to operate properly.
R5 adds about 200mV of hysteresis so the circuit won't oscillate around the trip point.

#### crutschow

Joined Mar 14, 2008
30,818
Is this a voltage monitor circuit?
Yes.

To get better resolution in your simulation you need to reduce the value of the voltage steps in your .step command.
A step of 0.4V means the resolution of the circuit trip point is no more than that.

For this circuit, that has a switch point with hysteresis, it's probably better to vary the voltage with the pulse command, rather than use the .step command.
Another better alternate is the use the DC sweep command.

#### crutschow

Joined Mar 14, 2008
30,818
Datasheet says minimum cathode current is 80uA is the 47k resistor too much?
Not according to the spec, but I would use a somewhat smaller resistance (say about 30kΩ) for a better design margin.

#### CharlesWMcDonald

Joined May 16, 2019
233
Your voltage divider calculation is incorrect. The correct voltage across R2 would be:

Given a value of 1K for R2 the value of R1 would be:

Nearest standard value for R1 is 3600 ohms.

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#### crutschow

Joined Mar 14, 2008
30,818
Your voltage divider calculation is incorrect.
Whose calculation for what?

#### crutschow

Joined Mar 14, 2008
30,818
The current needs to stay above 80uA until the set regulator voltage is reached? The cathode current drops rapidly right when the set regulator voltage is reached.
When it reaches and drops below the set voltage, yes.
At that point the cathode current drops to ≤0.1μA.