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LSF0102: 1.8V to 2.5V

Allan Fan
Expert 7100 points
Part Number: LSF0102

Hi Expert,

Customer used LSF0102 for MDIO/MDC 1.8V to 2.5V but failed communication. Vh minimum need to > 2V.

MDC

Yellow:  A port 1.8V

Blue: B port 2.5V

Communication fail:

Fig.1

We see DS recommend Vref_B and Ven to be 1V higher than Vref_A so we change Vref_B to 3.3V(still keep port B pullup to 2.5V), see fig.2. The communication is established. Is it correct way to do for Vref_A, Vref_B delta < 1V? Cause we are wondering what if the system don't have extra higher voltage(Vref_A + 1V) for the case? The two waveforms looks similar, do you have any idea why fig.2 works? Both Vh < 2V.

The port B high level looks not right cause they pullup to 2.5V, but it's only about 1.68V at rising edge though the communication works.

Communication pass.

Fig.2

We changed port B pullup resistors from 10Kohm to 150ohm and it looks pretty good. 

Fig.3

 

Regards,

Allan

  • 0
    Guru 317370 points

    The datasheet mentions the 1 V difference for a reason. The reason is that the threshold voltage of the internal MOSFETs is near this voltage, and that this difference is needed to switch the pass transistors on.

    If you do not have another power supply that is high enough, you can run the LSF in an alternate mode: drop the 200 kΩ resistor, connect EN to the lowest supply (here: 1.8 V), and leave Vref_A and Vref_B open. (Actually, Vref_A and Vref_B can be used as an additional I/O channel.)

    The LSF is a passive switch; the time needed for the signal to rise from the lower voltage to the higher voltage is determined by the current through the pull-up resistors. The only way to get faster edges is to reduce the value of the pull-up resistors. (The actual value needed depends on the parasitic capacitances on the line.)

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 46045 points

    Hey Allen,

    Here are training videos that elaborate more on what Clemens explained.