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Original question:

LSF0108: LSF0108 werid waveforms for A->B and B->A

LSF0108: Delay during the LOW transition

Ella KIM
Expert 3862 points
Part Number: LSF0108
Other Parts Discussed in Thread: TINA-TI

Hi Team,

While I look into how LSF device works, I ran into an e2e thread (please find the original thread) and I have a question.

In the original thread, there is a waveform as below - B side (yellow) input to A side (purple) output.

To my understanding so far, the output should drop immediately following the input signal, because LSF acts just like a resistor with almost zero delay. But in the waveform, there shows a delay, t_d. Could you help me to understand under what circumstance the delay can occur? 

As  mentioned, the B side is connected to something that can discharge faster than A side in the customer board. Is the t_d also occurring depending on the driver/receiver driving capability or something?

Thanks,

  • 0
    TI__Guru* 96591 points

    Hello,

    This is just how the circuit solves -- using a basic simulation you can show that the delay is just the RC components interacting with the switch normally:

    Rearranging my markers a bit, you can see when the MOSFET starts to conduct:

    As soon as the input signal starts to drop, the MOSFET starts conducting (with zero delay the output starts changing), however the output voltage cannot change instantly (capacitors resist instantaneous change in voltage) and the MOSFET won't be in the fully ON state instantly either because the input signal has a slew rate less than infinity, so current will be reduced for a short time also.

    In other words, it takes time to reach the 'fully on' state and you see a small amount of delay (~ 1ns) in the output. If 1ns is a concern in your system, I would highly recommend _not_ using the LSF as a translator. In fact - you should redesign the system to not use a translator at all.

    Additionally, by modifying the slew rate of the input (making it slower), you can see that the delay changes, and it reduces as the input/output voltage approaches zero:

    Please find the Tina-TI simulation attached if you would like to try variations yourself:

    LSF0101_Timing_Example.tsc