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TCA9406: Vgs(th) of TCA9406's Internal Passgate

Part Number: TCA9406

Hi Supporter,

According to I2C base spec., the maximum VOL value of I2C devices is specified as 0.4V. If the 0.15V you pointed out is the maximum acceptable low level that can make TCA9406's internal passgate turned on, it means TCA9406 cannot be applied to most of I2C devices, right? Additionally, the footnote under the table you extracted shows that "The maximum VIL value is provided to ensure that a valid VOL is maintained". So, my understanding is this VIL(MAX)=0.15V is not the necessary condition to let TCA9406's internal passgate turned-on. 
Therefore, can you let me know the VGS(th) of the internal passgate? I am considering the case that both sides have the same voltage level and TCA9406 is applied as a bus switch.

Regards,
S.P. Lin

  • Hi S.P.,

    It isn't that 0.15 V is needed for the switch to activate, it's just the switch itself introduces some series resistance which will cause the output low level to be a little higher than the input low level. The voltage difference is just the product of the switch impedance and the pull-down current flowing through it (which is a function of the external pull-up resistances used and the pull-down strength of the I2C devices driving the bus). The 0.15-V spec in the TCA9406 is what is needed to ensure a 0.4-V output low level under worst-case conditions assuming a pull-down current (IOL) of 1 mA. Note that for typical conditions the VOL will be much lower - you can reference Figures 1 through 3 in the datasheet to get a better idea what to expect.

    I hope this addresses your concerns, and please let us know if you have further questions on this device.

    Regards,
    Max
  • Hi Max,

    Thanks for your reply. I know the passgate's working principle, and I know for most cases VCCB greater than VCCA the gate voltage of the I/O passgate would be the lower voltage level plus the threshold voltage as well. That is why any side lower than VCCA turns on the I/O passgate. However, to use TCA9406 as a bus switch for both sides with the same level, I think the gate voltage of the I/O passgates would be the voltage level of I/O's pull-up. So, I'd like to know what is the maximum acceptable low level. Or, can you tell me what is the threshold voltage of the I/O passgates?
    Your kindly help will be appreciated.
  • Hi S.P.

    The switch begins to conduct (which is how the low level propagate from one side to another) when the input voltage is about 700 mV below the VCCA voltage. The switch resistance then reduces further the lower the input voltage becomes.

    Max
  • Hi Max,

    Thank you very much. May I know is there any concern to list it into TCA9406's spec.?

    I think this parameter should be taken into account for logic-low's evaluation.

    Regards,

    S.P. Lin

  • S.P.,

    I don't see any concern. We'll look into how this information could be better incorporated into the datasheet so that customers have a better idea what to expect.

    Regards,
    Max