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TCA9546A: TCA9546A spec confirmation

Part Number: TCA9546A

Dear TI members,

There are some spec questions need to confirm with you.

1. The definition of Vpass is "Switch output voltage" in the spec. This will confuse our customer that we need to reference Vpass to decide the output voltage we should use.

Could  you help to explain the detail definition of Vpass for us to reply our customer?

2. Are the MOSFET in Functional Block Diagram NMOS?              (Question2~4, please refer to Functional Block Diagram)

3. If Vcc=3.3V, is the output of Switch Control Logic 3.3V, too?

4. What is the value of the Vf of the internal diode of MOSFET?

5. There is no definition in the spec that which side voltage should be higher than the other side.

If we use the higher voltage to output side(such as 5V) and the lower voltage to host side(such as 3.3V), Is there anything problem with this?

Thanks.

  • Hi Howard,

    1. Could you help to explain the detail definition of Vpass for us to reply our customer?
    Vpass is an internally generated value based on the device supply voltage Vcc. The correct supply voltage must be chosen such that Vpass is lower than the smallest bus voltage. Vpass <= Min(Vpux). See the last paragraph in Section 9.2.1 Design Requirements for further description and an example.

    2. Are the MOSFET in Functional Block Diagram NMOS? 
    Yes. When there is no voltage supplied by the Switch Control Logic (disabled or unpowered) the pass-FET will be open and the bus lines will be disconnected.

    3. If Vcc=3.3V, is the output of Switch Control Logic 3.3V, too?
    The Switch Control Logic drives the pass-FET gates with the Vpass voltage. This is generated based on the supply voltage Vcc. In the case where Vcc = 3.3V, the Switch Control Logic would drive a Vpass of 1.9V typical.

    4. What is the value of the Vf of the internal diode of MOSFET?
    There is no effective body diode in the pass-FETs. This prevents either side of the device from propagating LOWs when the pass-FET is disabled. When the FET is disabled, it will act as an open circuit to prevent signals from propagating across the device on the corresponding channel. 

    5. There is no definition in the spec that which side voltage should be higher than the other side.
    There is no such requirement for this device. Note that SCL and SDA High- and Low-level input voltages are dependent on the device supply voltage Vcc. Vcc is also limited by the minimum bus voltage by Vpass requirements, so not every combination of supported voltages is possible. Let me know if you have a question on a particular implementation.

    Regards,
    Eric

    [Edit]: Formatting