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LSF0108PWR translates 3.3V not to 1.8V as expected but to 2.5V

Phillip Schuster
Prodigy 30 points
Other Parts Discussed in Thread: LSF0108, SN74LV1T125, SN74LV1T34

Hi,

I have to use a level shifter for my parallel camera interface. I am using an MCU with 3.3V level and the OV5640 camera sensor uses 1.8V. I have setup a small prototype. In this prototype my 3.3V MCU sets a GPIO HIGH and LOW every second. This is connected to Pin 11 of LSF0108. I pulled pin 20 up on 3.3V with a 200k resistor. I connected pin 19 to 3.3V. On the other side I connected pin 1 to GND and pin 2 to 1.8V (which is created from a voltage divider from the 3.3V).

Which a scope I measure pin 11 which shows the signal oscillating from 0V to 3.3V and back every second. Measuring pin 10, which should give me a 1.8V HIGH signal (as I understand it) just gives me 2.5V.

I don't understand why that happens? I have tried with a Dual Power Supply, delivering 3.3V and 1.8V sharing GND. The same result, I don't think it's the voltage divider. I also pulled up pin 11 and 10 with different resistor values (47, 100, 220, 330, 1K, 10K, etc) - the voltage level changes between 2.3V and 2.5V.

I have screenshots from my scope and made a picture of my breadboard, but I am not able to post them here in this post as far as I know?

Any help would be appreciated.

Thanks,

Phillip

  • 0
    TI__Genius 9860 points

    Hi Phillip, you need to tie the EN and Vref_B pins together before pulling up to 3.3 V through the 200k resistor. You can find more design guidelines on the LSF010X application report: http://www.ti.com/lit/pdf/slva675

    If you are only doing single-direction translation from 3.3 V to 1.8 V on one channel, the SN74LV1T34 or SN74LV1T125 (w/enable) will save you board space and reduce the need for pullups.

    -Ryan

  • 0 in reply to C. Ryan Land
    Prodigy 30 points

    Thanks for your help and the application description. That helped a lot. I have now tied 3.3V and Vref_B together and pulled up to 3.3V using a 200K resistor. This brings the level down to 1.96V on the low side, but should be 1.8V. I added a pull up resistor of 47k and now the signal is very fast and is exactly at 1.84V. That sounds good I think.

    Parallel camera interface is in one direction, but it's a lot of bits (8) and there are various signals that are both directions. I think I will stick with the LSF0108 devices but I will have to add a lot of pull-up resistors. In this area board space is not a big problem.

    Thank you very much for your help.

    Phillip