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LSF0204D: Is the right pick for my application?

d_zero
Genius 4670 points
Part Number: LSF0204D
Other Parts Discussed in Thread: PCA9306, LSF0204, SN74LVC2G07, SN74AUP2G07, SN74LVC2G34, SN74LV1T34, TCA9406, SN74LV1T126

Dear *,

i have a application where i have to shift signal from 3.3V to 1.8V. I have to shift 4 signals.

1) two of them are I2C, that have pull-up resistor i think 1k8 and the speed is not more than 400kHz

2) and other two are GPIO pins ( I assume that are push-pull) with no pull-up or pull-down. The GPIO are output from the host side (3.3V side) and input to my board (1.8V side)  

Questions:

1) Is the LSF0204D a right device to translate those 4 signals?

2) What is the proper way to size the pull-up resistor on the 1.8V side of the I2C if the 3.3V already has pull-up resistors?

3) The GPIO signal i want to use as an reset and trigger pin, so i want to have a pull-down on those lines, so that only when the host (3.3V side) drives those pins high is true, can i have pull-down on the LSF0204D device?

And will this translation on GPIO work if i don't have pull-up resistors ? ( the max speed of trigger is 1us -> 1MHz)

Best Regards.

  • 0
    TI__Guru* 96742 points

    Hi,

    I would highly recommend watching this short video series to become familiar with how the LSF family of translators work:

    To answer your questions directly:

    1) Is the LSF0204D a right device to translate those 4 signals?

    Based on your question (3) I would say probably not.  You can use the PCA9306 for translating the I2C signals only, and then use a different device for the other translation.

    2) What is the proper way to size the pull-up resistor on the 1.8V side of the I2C if the 3.3V already has pull-up resistors?

    I would recommend using a 10kohm resistor on the 1.8V side. The size limits depend on your I2C master/slave device's current drive capabilities, but 10k is typically a safe value.

    3) The GPIO signal i want to use as an reset and trigger pin, so i want to have a pull-down on those lines, so that only when the host (3.3V side) drives those pins high is true, can i have pull-down on the LSF0204D device?

    You can't put pull-downs on the LSF0204's channels due to how it works (see the above mentioned video series for details).

    And will this translation on GPIO work if i don't have pull-up resistors ? ( the max speed of trigger is 1us -> 1MHz)

    It depends on the directionality of the signals - but since the pull-downs can't work, there's no point in explaining this as well.

    Are your GPIO's always going to be sending signals the same way, or will they be changing directions during operation?

  • 0 in reply to Emrys Maier
    Genius 4670 points
    Dear Emrys,

    thank you for the answers.

    About the GPIO , yes the gpio signals are always the same way the host side is output nad my board is input.

    If the GPIO on the host side is push-pull then in general i don't need the pull down resistor is this correct?

    I wanted to add the pull-down resistor to be shore that external conditions will not affect the state of the GPIO, and only when the host drives the pin high is true high state on the line -> to avoid miss-triggering and to have full control with reset signal of the device.

    Best Regards.
  • 0 in reply to d_zero
    TI__Guru* 96742 points
    From the description, I'm not certain if the signal is being translated down from 3.3V to 1.8V or up from 1.8V to 3.3V.

    You can do either with an open-drain buffer such as SN74LVC2G07 or SN74AUP2G07.

    You can also use a buffer/translator like SN74LV1T34 for those channels (for up-translation) or a regular old buffer for down-translation like the SN74LVC2G34.

    Any unidirectional buffer will be OK with a pull-down resistor on either side.
  • 0 in reply to Emrys Maier
    Genius 4670 points
    Yes you are right the GPIO signal is always translated from 3.3V to 1.8V.

    So there is no case for LSF devices to work without pull-up resistors? If there is a case where the LSF0204D can work without pull-up resistor what are the requirement in-order to work ?

    I was thinking to Translate all four different signals (I2C and the GPIO signals) with one device.
  • 0 in reply to d_zero
    Genius 4670 points

    Dear Emrys,

    i think i will use PCA9306 for I2C because it is designed for I2C or TCA9406 if I need 1MHz I2C.

    and for unidirectional GPIO down translation from 3.3V to 1.8 i will use SN74LV1T126 or SN74LV1T34 in this configuration :

    Will the down translation work with SN74LV1T126 or SN74LV1T34 with pull-down on gpio signal?

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 96742 points

    Hi,

    To answer your questions directly:

    So there is no case for LSF devices to work without pull-up resistors?

    That's not true -- there are cases when the LSF will work without pull-up resistors.  It's difficult to list all of these in text, which is why I recommend watching the video series (from my first post) which will explain all of those (specifically in the 'up translation' video).

    If there is a case where the LSF0204D can work without pull-up resistor what are the requirement in-order to work ?

    Again - hard to explain here, please watch the videos and return with questions.  They really aren't very long and contain a great deal of information.

    I was thinking to Translate all four different signals (I2C and the GPIO signals) with one device.

    This is possible - as long as you don't have pull-down resistors on the GPIO lines.  Since there have to be pull-up resistors anyways, why not use logic HIGH as your default state instead?

    Will the down translation work with SN74LV1T126 or SN74LV1T34 with pull-down on gpio signal?

    Yes - either of those will work with a pull-down resistor on the input or output.

  • 0 in reply to Emrys Maier
    Genius 4670 points
    I can't use logic HIGH as default state because the reset and trigger pin on my device are active in high state ( can't be changed) and i want in my initial state that the device is turned off so i will put pull-down resistor to be one the safe side. And when i want to turn on or trigger my device the host will drive the GPIO pin high .

    Thank you for the help and Best Regards.