Hello,
I am having troubles with the NVT2002DP voltage translator. I have a 5V TTL signal, that needs to be translated to 3.3V. The schematic is attached below.
I tried a 1.75K Ohm resistor on R144 (pull up on the B-side, 5V side). Found that the 3.3V side still measures 0V and 5V (low and high value respectively). I also tried a 350 ohm pull up resistor and found that the A side signal measured about 1.5V and 5V (low and high value respectively).
Thanks for the help!
Hello Aaron,
please let me know your required communication speed. The Rpu(A) and (B) values are recommended for 3mA driver sink current for I2C Fast-mode and Standard-mode communication speed up to 400kHz.
I see, that you tried to test both 1.74k and 348Ohm values, for 3mA and 15mA driver sink current, but please note, that you need to populate both pull-up resistors on the B side. The R144 and also R145. Please remove the U12 just for the test, populate the R144, R145 with 1.74kOhm pull-ups if the communication speed is up to 400kHz, or with 348Ohm pull-ups if communication speed is up to 1000kHz.
With Best Regards,
Jozef
Hi Jozef,
It does not look like populating both pull up resistors on the 5V side seem to work.
i am working at a pretty slow speed (~5KHz). I populated both R144 and R145 with 1.5K Ohm resistors.
A 5V signal on the B side, is converted to a slighly lower voltage ~4.7V on the A side. I need the A side to be 3.3V. Attached is a scope image. The purple line is the input 5V signal and the yellow is the output.
Hi Aaron,
thank you for confirming the communication speed.
The 1.5kOhm pull-up resistors for the 5V and communication speed of 5kHz are too low.
Please use at least the 1.75kOhm, or even higher. For such a low communication speed you have a large pull-up resistors margin. Please use 2.5kOhm pull-up resistors for R144 and R145. I have calculated maximum allowed pull-up resistor for standard-mode communication speed (up to 100kHz).
tr=1000ns
Cb=400pF
Rp(max)=1000ns/(0.8473*400pF)=1000ns/0.00000000033892
Rp(max)=2.95kOhm.
Please confirm:
1. Have you removed the U12?
2. Have you purchased the components from one of our authorized distributors? Please see this link for a list of our authorized distributors.
3. Please let me know the SCL and SDA PCB traces and/or cable lengths, so I can estimate the bus capacitance.
With Best Regards,
Jozef
Thanks for taking the time to look into this Jozef.
Increasing the pullups to 2K Ohms has no effect. It does not seem to be speed related. If I just leave the input signal high (5V), I measure ~4.7V on the output side when I expect 3.3V.
By the way these are not I2C (SDA/SCL) signals, these are just some miscellaneous TTL signals, so the speed varys, thats why I was trying to keep the pullup small. I want to have a clean edge to trigger on.
1. I did remove U12, although there is no real connections here, the only connection is from power, there are many other devices on these power busses right now.
2. Yes, the parts come from Digikey.
3. As I said these are not I2C signals, they are just TTL signals. But the trace and cable lengths are pretty short. I am not sure what the circuit board inside the device that I am communicating with looks like, but I have a 6 inch DSUB cable, then about an inch of trace width between the connector and the NVT2002 level shifter. On the 3.3V side of the level shifter about 8 inches of pcb trace to the output connector. I am measuring the voltage right at the level shifter with nothing plugged in. Eventually I will have another cable to a MCU that is about 24 inches long, and then ~4 inches of trace length. I added all this to an image attached.
So ~7inches on the 5V side and ~36 inches on the 3.3V side, however I am testing currently with only 8 inches on the 3.3V side.
Hello Aaron,
thank you for the answers. I have received an advice from my colleague. He advised to add a 200kOhm pull-up resistor on the EN pin to the VREF. I have missed that. Please add the 200k pull-up.
The please test the 1.75k or 2k pull-up resistors on the B-side and no pull-up resistors on the A-side. Please let me know if this helped.
With Best Regards,
Jozef
