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TCA9534: 5V and 3.3V combined port levels for the port expander

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Replies: 5

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Part Number: TCA9534

Hi,

    I have looked at TCA9534 specs and have a question about it.

My design intent is to use the port expander with Vcc at 3.3V with two of the P0-P7 ports would be set as outputs tied to 5V logic circuits and the remaining 6 would be set as outputs tied to 3.3V logic. The I2C_SCL and SDA are communicating with a Master which also operates at 3.3V.

The two ports tied to 5V logic have 10K pull-up resistor to 5V to limit current into the ports.

1. Is it acceptable to use this I2C port expander with this design intent?

2. Can the two 5V logic I/O ports be safely driven Low or High with Vcc at 3.3V?

I would appreciate if any of TI Experts can chime into this question.

Thanks,

Ram.

  • Hey Ram,

    Are you saying that you two of the ports will be used as an input for a 5V logic and the remaining 6 would be for an output (Vcc is 3.3V so the output logic high would be 3.3V minus some change due to parasitics). Is my understanding correct? Otherwise I can also interpret what you are asking as you want 2 ports on our device to output a 5V during an output high logic which is not possible (Vcc of our device is is 3.3V so it cannot output 5V).

    I am assuming the first assumption from above, if so then the answers to your questions are:

    "1. Is it acceptable to use this I2C port expander with this design intent?"

    Our device is 5V tolerant meaning if the port is designed as an input then it can handle it even at 3.3V as Vcc. You would not want to set the port to an output high logic though because you would have a low impedance path between 3.3V and 5V.

    If you were asking assuming the second assumption then it is not possible to have a 5V output with Vcc at 3.3V with this device.

    "2. Can the two 5V logic I/O ports be safely driven Low or High with Vcc at 3.3V?"

    Can I get a block diagram of what you are suggesting? Our device is an I/O but you probably do not want to set the device to be an output at 3.3V into a 5V without some kind of resistance between the two voltage rails. Otherwise you would get a short circuit. The device does not use diodes to prevent the device from sinking current.

    Thanks,

    -Bobby

  • In reply to BOBBY:

    Hi Bobby,

                    Sorry for the confusion.

    I was referring to your first assumption. The port expander will be driven by Vcc at 3.3V. All 8 ports will be set as outputs from the port expander with logic high at 3.3V.

    Two of the outputs at logic High (3.3V) would be controlling logic for 5V circuits. There's a 10K pull-up from the output of the port expander to 5V rail to prevent short circuit between the rails like you mentioned.

    Based on this scenario, is the port expander safe to use?

    Thanks,

    Ram.

  • In reply to Ram Vijayakumar:

    Hey Ram,

    "Two of the outputs at logic High (3.3V) would be controlling logic for 5V circuits. There's a 10K pull-up from the output of the port expander to 5V rail to prevent short circuit between the rails like you mentioned.

    Based on this scenario, is the port expander safe to use?"

    Thank you for the additional information. Based on what you have provided, you should be fine. The 10k is more than enough to prevent a short circuit. When the I/O is an input or even output low our device is 5V tolerant so this will not be a problem.

    Thanks,

    -Bobby

  • In reply to BOBBY:

    Hi Bobby,

                     Thanks for the confirmation on the part.

    What's the difference between TCA9534 and TCA9534A? Looks identical to me.

    Thanks,


    Ram.

  • In reply to Ram Vijayakumar:

    Hey Ram,

    The only difference is the internal static address is different.

    Everything else is the same.

    Thanks,

    -Bobby

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