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Original question:

LSF0204: Pull up resistor value limits for LSF devices

LSF0204：A question about LSF family

Echo He
Prodigy 10 points
Part Number: LSF0204

Tool/software:

In this video 2:39, For example,I don't understand the reference in the video

 “on the 1.8-volt data line, if the resistor RB3 were removed, any data sent from the MCU would be translated down to 1.2 volts instead of 1.8 volts.”

Where does 1.2V comes from？VccA？

https://www.ti.com/video/series/understanding-the-lsf-family-of-bidirectional--multi-voltage-lev.html#transcript-tab

  • 0
    Guru 316380 points

    Output voltages are weakly clamped at Vref_A.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks for your answer，but i still don't know the process Vref_A affect 1.8V Data line when MCU send high or low .

    in my understanding，when MCU send high，FET will open until VGS ＜Tn，but 1.8V Data side without pull up RES，how does Vref_A affect 1.8V data line

  • 0 in reply to Echo He
    Guru 316380 points

    This is explained in the videos "Understanding the Bias Circuit for the LSF Family" and "Down Translation with the LSF Family".

  • 0 in reply to Echo He
    TI__Mastermind 39986 points

    Hi Echo, 

    When MCU drives high, VGS (VG= 1.8V, VS = 1.2V) <= VTH. Internal switch will be operating near cutoff region, so the voltage is clamped at A-side voltage. The output side can only be pulled up higher to 1.8V if an external pullup resistor is used. 

    Regards,

    Jack 

  • 0 in reply to Jack Guan
    Prodigy 10 points

    Thanks Jack,

    I have the following questions

    1：VG=Vbias=VccA+Vth is that right？（as red line show）1.8V=1.2+0.6，

    but i didn't see anything in the video about the Vth is 0.6V.

    2：and VS=1.2V just the VS voltag of the first FET , is that right？

    A3 port and B3 port  which side is the source and which is drain？

    in my understanding，A3 port provide 3.3V to B3， until VGS ≤ Vth FET cutoff，and then i don't know where does B3 port voltage come from，how much it is？

    3：the first FET is always in steady state?

    can it affect channel 3？

    Forgive me for asking so many questions XD.

  • 0 in reply to Echo He
    TI__Mastermind 39986 points

    Hi Echo,

    1. Vth can be estimated to be about 0.6V to 0.8V.

    2. Both sides are interchangeable. With no pullup resistor on the B3 side, the voltage will weakly clamp at VREFA. 

    3. Yes the first FET is known as the Bias FET that provides a fixed voltage to the gate of each FET per channel, enabling translation. It is explained in operation in the "Understanding the Bias Circuit for the LSF Family" video.