# Finding resistor values in a voltage divider circuit involving a zener diode

I want to use the TLC59212 latched driver. Recommended voltage supply is 4V min, 5.5V max. I understand that I can't use it on 3V. High level input is Vcc*0.7 Min. So I plan to use a 5V supply. 5V*0.7 = 3.5V. To make sure, I would like it to be +- 4V or more. Low level input is Vcc*0.3 max. 5V*0.3=1.5V. So I would like it to be +-1.2V or less.

The problem is that it will be connected with a touch sensor device which supports only 3V, including for its open drain, active low, outputs. Its outputs have the same effect as SW1 on the schematic.

So I decided to add 3V Zener diodes to power the open drain outputs.

Could you help me calculate the resistor values so that the TLC59212 inputs are not lower than 4V when the sensor switch is not sinking, but the Zener diodes are drawing current and that the inputs are not higher than 1.2V when it's sinking, but the resistors in series with the Zener impedes the sinking at the input.

I know that the usual formula is Vout = R2/(R1+R2)*Vin. But I'm lost with the presence of the Zener...

simulate this circuit – Schematic created using CircuitLab

EDITUM:

I did some math. Please correct me if I'm wrong with the concept. Vlow should be 1V Vhigh should be 4V This gives me 0.5V of safety margin both sides. To find R2, assuming R1 = 10K:

5*(R2/R2+10000) = 1
R2/R2+10000 = 1/5
R2*5 = R2+10000
R2 = 2500


Vhigh will be

Vzenner + Vlow
3 + 1 = 4V


Is it correct that Vhigh will be Vzenner + Vlow?

You need a voltage difference of 2.8 V at the TLC input between low and high level and the touch sensor output provides only 3.3 V difference maximum.

This is too close to find a solution using a voltage divider. One option is to replace R2 in your circuit with two diodes connected in series.

I recommend a circuit with a NPN BJT, where the base resistor acts a pull up resistor for the sensor output. There are so called "digital transistors", that contain the base resistor if board space is critical.

It is also possible to use a logic level NMOSFET here.

simulate this circuit – Schematic created using CircuitLab

• Thanks for the suggestion. I think it can work because the difference between the 5V supply and the Zener voltage is only 2V. R2 is not pulling to zero. But to +3V. Commented Mar 20 at 10:47

Your idea isn't going to work. What I would do in, in order of preference:

• Get a 3.3V compatible Latch. This way, everything runs on 3.3V. No additional circuitry necessary.
• Get a level-shifter IC (preferably an 8-channel version to reduce needed board real-estate)
• Or, roll your own level-shifter:

simulate this circuit – Schematic created using CircuitLab

Ideally, you would get 8 transistors in a single package, again, to save real-estate. And same for the resistors. If there is no space requirement, the whole circuit could be built with discretes on a breadboard with parts in your junk bin. You may also need a highish resistor (47k?) from base to emitter depending on how "open" your open drain outputs are. I'm talking about bleeding the leakage.

• Thanks for the answer. I'm not convinced that it's not going to work because there is only 2V of difference between the Zener and the input pins. In case you would be right, a 3V latch is possible (74VHCT573) with the addition of second ic because it doesn't have the same function as the TLC59212. I could also add Schmidtt Triggers before the inputs.... Alternative solutions are not a problem. The goal is to get the circuit as simple as possible. the TLC59212 does exactly what I want in a single ic. Commented Mar 20 at 10:57
• One of the requirements was, that the open drain voltage stays below 3.3 V. In this circuit it is 4.4 V.
– Jens
Commented Mar 20 at 13:52
• @Jens, I seemed to have overlooked that requirement; kind of undermines using open-drain output logic if you can't raise the rail. Anyway, The 74VHCT573 seems promising. If not, I would use SN74AHCT541 to interface the logic. Commented Mar 20 at 15:39
• I think I will use the 74VHCT573 + a MOSFET array ic to copy the functionality of the TLC59212. Too bad the TLC59212 is not 3V compatible (at least from the datasheet). Moreover it's an old ic and the its other variants are already discontinued or at "end of life". I'm afraid this one too may not be available in the future. Thanks again for your replies. Commented Mar 22 at 0:21

After testing in reality, the idea didn't work as Jens and MOSFET said. The reason is that the Zener diode acts exactly like a 25K resistor when R1 is 10K and 5V is applied. Vlow is 1V as expected when SW is closed. But Vhigh is only 3.4V because the voltage is divided as if the Zener diode was a resistor. It's not like the Zener "gives" 3V. It sinks current to regulate as the resistor would.