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TCA9546A: recommended VCC value in case VDPUM and VDPU = 1.8V?

AnBer
Mastermind 30210 points
Part Number: TCA9546A

Team,

Can you please help with the below?

In case TCA9546A is just used a multiplexer:
      VDPUM = 1V8 and VDPU also 1V8.
According to the DS section 9.2:
https://www.ti.com/document-viewer/TCA9546A/datasheet#typical-application-scps2047727/SCPS2047727
    "In the event where the master device and all slave devices operate at the same voltage, then the pass voltage, Vpass = VDPUX."

So in our case VPass = 1V8.
Looking at Fig 16 this would result in a VCC of 3V:
https://www.ti.com/document-viewer/TCA9546A/datasheet#tca9546a-application-curves-scps2047901/SCPS2047901



But now looking at section 6.3:
https://www.ti.com/document-viewer/TCA9546A/datasheet#recommended-operating-conditions-scps2059462/SCPS2059462

 
I would have a VIH of 2V1 at 3V, which would be too high for a 1V8 I2C bus.

-Is it my misunderstanding?
-With which VCC do I have to operate the IC if I have VDPUM and VDPU at 1V8?

Thanks in advance,

Anber

  • +1
    Guru 317390 points

    The clamping voltage is somewhat lower than VCC, so to ensure that voltage translation to lower voltages works, VCC must not be too high. (High-level output voltages are generated by the pull-up resistors, so there is no lower limit on VCC.)

    In general, the easiest solution is to use the smallest I²C voltage for VCC; in your case, 1.8 V.

  • +1
    TI__Genius 10552 points

    Hi Anber,

    Apologies for any confusion between the typical application and datasheet specs. 

    Vpass is defined as the smallest pull-up voltage (VDPUM or VDPUx) in the system. Based off this pull-up voltage, the user can determine what is the highest VCC that can be used in the system to ensure correct voltage translation. Basically, it shows that at around 1.8V on Vpass, the Vcc needs to be ~2.7V or less to support 1.8V for level translation.

    So, you can use 1.8V for VCC in this case where VDPUM and VDPU are 1.8V. 

    Regards,

    Alex