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LSF0108 application question

Nanfang Liao
Intellectual 2085 points
Other Parts Discussed in Thread: LSF0108

Hi team,

My customer will use our LSF0108, they use this part for 1.8V to 3.3V SPI clock signal shift. The datarate is 100MHz, is this application ok? it is any special design for the pull up resistor, which is related to the datarate? thank

Regards,

Nanfang

  • 0
    TI__Guru* 97046 points
    The pull-up resistor should be as small as possible to allow for fast operation (consider the amount of current that the driving device must sink from the pull-up). The RC constant on the 3.3 V line will greatly affect the maximum achievable speed.
  • 0 in reply to Emrys Maier
    TI__Intellectual 2085 points
    Thanks, for the 100MHz application, what's your suggestion?
  • 0 in reply to Nanfang Liao
    TI__Guru* 97046 points

    Keep the traces as short as possible, especially on the high side.  We recommend placing the LSF device as close to the high side device as possible.  

    I cannot make a specific pull-up resistor recommendation without knowing more about the circuit.  The pull-up resistor value can be calculated based on the amount of current that the input device can sink.  If there is only a pull-up on the high side, this value is simply V_refb/Imax -- for example, if the device is operating at 3.3 V (high side) and the maximum sink current is 32 mA, then the pull-up resistor must be at least 3.3/0.032 = 103.125 ohms.

    Once the lowest resistor value has been selected, you can use the output capacitance to estimate the maximum data rate:

    fmax ~= 1/(6*RC)  -- using 3 time constants as a full charge (~95%)

    Assuming 10 pF on the output, fmax for my example would be ~= 1/(104 ohms * 10 pF) = 160 MHz