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How to make LSF0102 support same voltage at both sides?

Shawn Han
Genius 9920 points

Hi,

1, Here is a MDIO application to support live insertion. In below circuits (Modified from datasheet Figure 9), there is a 2.5V MDIO Master in left side. For live insertion sub cards at righ side, some sub cards are 3.3V MDIO, others sub card are 2.5V MDIO. Does it work at 15Mbps?

2, If I'd like to support both side 3.3V MDIO, set Vref_A=3.3V, Vref_B & EN 200kOhm to 5V, Vpu=3.3V, does it work?

I saw Voltage-Level Translation With the LSF Family (Rev. B) . Hope get your comfirmation. Thanks.

 

Best Regards,

Shawn

  • 0
    TI__Guru 62180 points
    Hi Shawn ,

    The LSF doesn't work when both sides are at the same voltage level . The speed of operation at 15 Mbps is not a problem here but the translation feature needs to have different pullup voltages at their end .
    The 2.5V to 3.3V should work , but not 2.5V to 2.5V or 3.3V to 3.3V . The VrefB /EN pin although at 5V , will bias the transistor but the input at 3.3V and pullup at 3.3V cannot work .
  • 0 in reply to ShreyasRao
    TI__Genius 9920 points
    Hi Shreys,
    Even though both side has the same voltage level, but any one drive a low pulse, VDS is enough, the low level could be passed to another side. When both side same level (VDS=0) make FET cutoff, it is doesn't matter, pull up resister could hold this high pulse. Does it FET ON/OFF mode affect data rate?

    Regards,
    Shawn
  • 0 in reply to Shawn Han
    TI__Guru 62180 points
    Hi Shawn ,
    The FET on off is related to the data rate . Higher data rate would mean higher FET on off rates .
    The data rates are affected by the output load RC . The higher load would mean slower data rates .
  • 0 in reply to ShreyasRao
    TI__Guru 62180 points
    Hi Shawn ,

    I just did some verification in the lab today .
    Turns out that the LSF can actually run 3.3V signals with 3.3V output at B1 . The Vref A was 3.3V and Vref B was 5V .Signals A1 was 3.3V pktopk and output B1 followed it .It defied my explanation in the previous post .
    It is good to know that though personally .