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LSF0102: Can LSF0102 support series application?

Ruixin Feng
Intellectual 2375 points
Part Number: LSF0102
Other Parts Discussed in Thread: TCA9617A,

Tool/software:

Hi team,

can LSF0102 support series application like TCA9617A? If can, how to connect it? Thanks!

Rayna

  • 0
    TI__Mastermind 39946 points

    Hi Rayna,

    It is possible to put the LSF0102 in series with two devices at a time. However please note that this device is passive in nature and will not be able to buffer your signals like the TCA9617A. The VOL level after each series connection will also be increased due to the switch resistance between the I/O of each LSF0102 and as a result of the additional external pullups used at the outputs of the 2nd LSF0102:

    Regards,

    Jack

  • 0
    TI__Intellectual 2375 points

    Hi Jack,

    thanks for reply! 

    Customer circuit is like below diagram, chip1 is used to translate 3.3V to 5V and chip 2 is used to translate 5V to 3.3V. All pull up resistors are 4.7kohm. And we found that at the A1and A2 of chip1, 3.3V is pulled to 3.7V. I was wondering the reason. And I want to check with you if the design is correct or not. Thanks!

    Rayna

  • 0 in reply to Ruixin Feng
    Guru 316440 points

    Please check the voltage at the Vref_A pin.

    (There appears to be no I²C device on the 5 V bus segment. Do you need the 5 V for anything? If there is no 5 V device, then you can remove the 5 V pull-ups, or for that matter, both translators.)

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2375 points

    Hi Ladisch,

    VERFA is 3.3V. Customer want to use this circuit to expand I2C so I want to check this series application is okay for LSF0102. 

    And we found there is around 30mA current flow from A1 and A2 of chip1. Can you pls help check the reason? 

    Thanks!

    Rayna

  • 0 in reply to Ruixin Feng
    TI__Mastermind 39946 points

    Hey Rayna,

    One possible reason is due to the use of the pullups used at the LSF input and outputs. When A1/A2 is being driven LOW, the internal switch of the LSF is on and all current must be sunk back into the driver side. It can be seen here: https://www.ti.com/video/series/understanding-the-lsf-family-of-bidirectional--multi-voltage-lev.html

    Regards,

    Jack 

  • 0 in reply to Jack Guan
    TI__Intellectual 2375 points

    Hi Jack,

    understood, Can I say LSF0102 is not recommended in series application? or do you have any other design suggestion to solve this problem? Thanks!

    Rayna

  • 0 in reply to Ruixin Feng
    TI__Mastermind 39946 points

    Hi Ruixin,

    LSF0102 is OK to be used in series application, though it requires additional tinkering from system designer side. 30mA is still acceptable to flow through our device (recommended operating conditions is below <64mA). To reduce the current sunk back into the driver side, you will need to use weaker pullups.

    If application is I2C, customer may see I2C buffers that do not perform translation via passFET architecture. This will reduce the total current sink into driver side as the bus segments at the I/Os will be seperated and redriven via the buffering circuitry. www.ti.com/.../scpa054.pdf

    Regards,

    Jack