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LSF0108: LSF0108 sometimes won't reach 3.3V

Kevin Lin
Expert 1445 points
Part Number: LSF0108

Hi there,

I am having trouble with LSF0108.  LSF0108 sometimes can't reach 3.3V for a 25MHz system.  I have tried with 220 ohms, 330 ohms, and 1k ohms for pull-up resistors on the B side, but the results are the same results, and the high on the B side can be as low as 1.8V.   Alos, there is some offset, approximately 400V,  on both A and B sides,  when voltage is supposed to be 0V.   Any suggestion for my issues?  

By the way, I don't have pull-up on the 1.8V side

thank you  

regards,

Kevin

  • 0
    TI__Guru* 97046 points

    Hi Kevin,

    Short answer: reduce capacitance on the 3.3V side of the LSF0108 by moving it as close as physically possible to the device(s) on that side.  You can also cut-out any ground plane under those traces to reduce capacitance.

    Long answer: The LSF0108 is a passive translator, which means that the output isn't driven by the LSF0108 but by either the pull-up resistor or the driving device (whatever is inputting to the LSF0108).  Note the RC curve on the leading edge of your 3.3V waveforms -- when up-translating, the LSF0108 goes into a high impedance state, and the output signal is determined by the RC circuit created between the line capacitance (including the input capacitance of the 3.3V device) and the pull-up resistor.

    There are two ways to improve the edge rate - reduce the capacitance between the LSF0108 and the 3.3V device, or reduce the pull-up resistor value.

    Unfortunately, there is a limit on reducing the pull-up resistor value which is related to the amount of current your driving device can sink -- say that device can only sink 10 mA, then you would have to keep your pull-up resistor to a minimum of 330 ohms (3.3V / 330 ohms = 10mA).

    As for the VOL issue, again since this is a passive device, it will never drive the output below the input voltage (in the low state).  You are seeing the input voltage plus a small drop across the LSF0108 based on how much current is going through the device, which becomes the low-level output voltage of the LSF0108.