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TCA9546A: Whether 3p3V bus to 3p3V and 1p8V bus switching can work with 3p3V Vcc supply

Mitsuhiro Hanabe
Prodigy 30 points

Part Number: TCA9546A

Tool/software:

(reposted, as the original post seems disappeared.

I want to use TCA9546A on my system with Vcc=3.3V supply.
The main i2c bus to be connected is running at 3.3V (pulled-up to 3.3V line) and the two downstream buses to be connected are 3.3V/1.8V (pulled-up to 3.3V/1.8V power line respectively).

Thinking based on the description below, "Vpass" will have to be equal to or below 1.8V (the lowest bus voltage).
According to Figure 17, Vpass(max) is 1.8V when TC9546A supply voltage is 3V or lower...

Does this mean I cannot use 3.3V but need to prepare another supply line such as 3V?
Could you please advise? (maybe I don't understand a role of "Vpass" well...)

  • 0
    TI__Mastermind 35648 points

    Hey Mitsuhiro,

    I think you've understood it correctly. Vpass will be the lowest voltage on the i2c buss whether it's the main or secondary. So from there you'll look at figure 17 and see where the intersects. 1.8V looks like it will be around 3V so you're supply will need to be at 3V (maybe slightly lower) to support 1.8V as your lowest I2C bus

    Thanks,
    Rami

  • 0 in reply to Rami Mooti1
    Prodigy 30 points

    Hi Rami, thanks for your prompt reply.

    Let me double check just in case...even if we have pull-up resistors on the downstream bus (TCA9546 doesn't output any voltage, just toggling between 0V and Hi-Z), still do we need to take care of this Vcc range constraint?

    If that's the case...our system only has 3.3V power supply line today.
    Do you have any alternative device that can be used in between 3.3V bus and 3.3/1.8V bus with 3.3V Vcc supply other than TCA9546?

  • +1 in reply to Mitsuhiro Hanabe
    Guru 327760 points

    Vpass is the voltage that higher-voltage signals will be clamped to. With a 3.3 V supply, the 1.8 V devices will see more than 1.8 V.

    All level shifters need a low voltage.

    You have to change your board to get the 1.8 V supply from somewhere, or to generate it (or 2.5 V or 3 V) locally with an LDO.