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LSF0108: LSF0108 translation 1.8V/3.3V@100MHz

Masakazu Yoshida
Prodigy 20 points
Part Number: LSF0108
Other Parts Discussed in Thread: SN74AXC4T774

Hello!!

Please tell me how to connect SDIO at 100MHz (200Mbps) using LSF0108.

The connection is SD HOST (+1.8V) <-> LSF0108 <-> SD Card (+3.3V).

Looking at the URL below, there is the following description regarding the pull-up resistor,
Moreover, it is stated that the load capacitance must be 6pF or less.

"High side pull-up resistors should be 330 ohm, and low side pull-up resistors should be 10 kohm."

e2e.ti.com/.../2269185 # 2269185

Unknown point:
Looking at the data sheet for LSF0108, the load capacitance is described as 10.5pF (typ), so
Isn't it impossible to achieve a load capacity of 6pF?
Should 6pF be considered as the capacity excluding 10.5pF?

  • 0
    TI__Guru* 96977 points

    Hello Yoshida-san, and welcome to the forums!

    Masakazu Yoshida said:

    Looking at the data sheet for LSF0108, the load capacitance is described as 10.5pF (typ), so
    Isn't it impossible to achieve a load capacity of 6pF?

    Should 6pF be considered as the capacity excluding 10.5pF?

    The value of 10.5pF is the typical channel capacitance in the 'on' state -- while in the 'on' state, the capacitance has a reduced impact on the signal due to the fact that the line is being driven actively.

    Masakazu Yoshida said:

    Please tell me how to connect SDIO at 100MHz (200Mbps) using LSF0108.


    The connection is SD HOST (+1.8V) <-> LSF0108 <-> SD Card (+3.3V).

    For the situation described, it is possible to use an LSF translator, but I wouldn't recommend it.  It's better to use an active voltage translator like the SN74AXC4T774.

    Here's an example of operating an LSF translator at 50 MHz under fairly ideal conditions:

    You can see here that even if the pulse width were cut in half, the output would still reach ~3.3V, but the integrity would be less than ideal.

    I don't have a 50 MHz waveform from AXC for a perfect comparison, but you can see the difference in this 2.5 MHz waveform pretty easily:

    The down side to using this device is that it's not auto-bidirectional, so you would have to control direction for the data signal.