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TCA9548A-Q1: IOL in application

Piotr F.
Intellectual 2920 points
Part Number: TCA9548A-Q1
Other Parts Discussed in Thread: TCA9517-Q1

Team,

Customer plans to use TCA9548A-Q1 in their application, and they consider some worst case scenarios in termls of Controller MCU SDA/SCL load current.

Scenarios: 

--> 1 output channel active in fast mode, 3.3V pullup

--> all 8 output channels active in fast mode, 3.3V pullup

There are two questons:

1. how to calculate pull resistors on controller and output channels sides, to makes sure both cases work correctly? (I tried to use I2C designer tool, but I cant find TCA9548 there..)

2. TCA9548A-Q1 datasheet say: 

What does it mean to the MCU Controller SDA/SCL pins current capability?

Can you help to calculate currents that MCU Controller needs to support? 

What if this current is outside of MCU Controller capabilities... can you recomend any I2C automotive buffer IC to help? 

  • 0
    TI__Mastermind 41266 points

    Hi Piotr,

    I think your concern is not so much when 1 channel is active (and pulls the data line low), but when all 8 output channels are active. When this is the case, we need to take consider the fact that the pullup resistors will act in parallel. If the controller pulls the line low for example, it will need to be able to sink in all the current coming from the peripherals that are active, and also have a low enough VOL for downstream devices to recognize as an input low. To resolve this, we need to use the value for the pullup resistors to be as weak (maximum values) as possible. 

    Here is an app note that shows how to calculate the pullup resistor values (note that the calculation can be treated individually per channel). Also see section 9.2.1, 9.2.2 of the datasheet for more information.

    Regards,

    Jack 

  • +1
    Guru 317460 points

    To estimate the bus capacitance, you could assume roughly 1 pF per cm of trace, and 10 pF per device. But this is anything but accurate, and the I²C specification requires all devices to support 3 mA, so you should just aim for a total pull-up current of 3 mA, and ignore VOL.

    With all 9 channels active, all pull-up resistors in parallel should be at least 3.3 V / 3 mA = 1.1 kΩ. So if you make all pull-ups the same, this would be at least 9.9 kΩ per channel; round it up to 10 kΩ.

    If certain channels have a higher capacitance than others (i.e., if the traces are much longer, and/or they have many more devices), then it would make sense for them to have a higher pull-current than the other channels. But I do not know how the actual boards look like.

    Does the scenario with all channels active actually happen? What is the purpose of the TCA9548A-Q1 in that circuit? (Usually, the switch is used to allow multiple I²C devices with the same address, so the maximum number of active channels would be lower than 8.)

    (The only automotive I²C buffer is the TCA9517-Q1. It makes sense if the total capacitance is too high.)