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LSF0108: is translation between Vccb = 5V and Vcca= 5V or 3.3V possible in the configuration shown

Craig Javid
Prodigy 50 points
Part Number: LSF0108

In the circuit below, Vcc_b is 5V and Vref_b is supplied from 5V.  However, the desire is that Vcc_a can either be 3.3V or 5V.  I believe both cases can be handled by making sure that Vref_a is derived from Vcc_b and is set to support the lowest translation voltage in the system which can be on the A side at 3.3V.

Is this a valid configuration?

  • 0
    Guru 316600 points

    During normal operation, a current flows into VREF_B and out of VREF_A, so the voltage divider at VREF_A must have a low enough impedance that it is not much affected by that current.

    The pull-up voltages do not need to be the same as the VREF voltages (the only restriction is that VREF_A is the lowest voltage). So you can simply connect VREF_A to 3.3 V.

  • 0 in reply to Clemens Ladisch
    Prodigy 50 points

    I selected R1/R2 voltage divider using the guidance associated with Figure 9-5 in the data sheet.  Are you saying this approach will not operate correctly because the voltage divider resistance is too high?  I could simply tie Vref_A to a DC 3.3V rail but will that damage the pin?

  • 0 in reply to Craig Javid
    TI__Mastermind 40006 points

    Hey Craig,

    If your application only allows for a single power supply, then yes, figure 9-5 can be used as a reference. Note that VREF_A can also be tied directly to 3.3V without any damage to the pin (as long as this is the lowest voltage you are translating to/ from.

    Note that this device is also passive in nature so it will not be able to provide any drive strength on its own. If drive strength is needed and the auto bi-directional functionality is not needed, please refer to fixed direction/ direction controlled translators instead (such as the LVC8T245/ LXC8T245). 

    Regards,
    Jack