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PCA9306: PCA9306 Enable issue

Yang Wu
Expert 3330 points
Part Number: PCA9306

Hi Team,

I got a question, from the datasheet PCA9306, in 6.5 section. it shows that it's on state when EN is 0.6V higher than VI (VI=2.4V, EN=3V & VI=1.7V, EN=2.3V)  

While in 6.8 section. The test condition is EN=3.3V, VIH = 2.3V, which means EN is 1volt higher than VIH. my question is why VIH is 2.3V (according to the figure above, it's on-state when EN is 0.6V higher than VI, I guess VIH should at least be 2.7V)? 

thanks a lot.

yang

  • 0
    TI__Guru 62920 points
    Hi Yang,

    This device is a FET-based switch, so it wouldn't have traditional digital logic input voltage thresholds (VIH, VIL). Instead, it has a more "analog" behavior in which the series resistance becomes low as the difference between the SDA/SCL voltages and the EN pin voltage increases. Figures 1 and 2 in the datasheet help to illustrate this. So, the input voltage level is given in Table 6.8 as a reference (since it has an impact on the switch resistance, which will in turn influence the timing specifications) rather than as a minimum required level (like the VIH specifications for most other devices).

    Does this make since, or did I mis-understand your question?

    Regards,
    Max
  • 0
    TI__Guru 73871 points

    Hey Yang,

    Interesting observation. Section 6.5 points out the voltage at which the PCA9306 will begin to turn on it's switch between side 1 and side 2 which is about 0.6V (in translation mode) as you have noticed.

    Section 6.8 however is a little different. The ViH which you are looking at is not the point at which the device turns off its switch like you are thinking (this section discusses test conditions rather than the device's thresholds), instead it is the voltage used to drive high. (take a look at figure 5) You will see ViH=VoH. This seems misleading to me but this looks to be how the device was characterized (Yes.....while the switch is always conducting).

    Thanks,

    -Bobby

  • 0 in reply to Miguel Robertson
    TI__Expert 3330 points

    Hi Max,

    Thanks a lot. I guess Bobby's answer is what I meant. 

    Yang

  • 0 in reply to BOBBY
    TI__Expert 3330 points

    Hi Bobby,

    thanks  a lot. That helps.

    Yang