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INA230: SDA/SCL

Darren (FAE)
Expert 5483 points
Part Number: INA230

I have a customer that wants to pullup the I2C SDA/SCL lines to the supply (3.3V) via a 1kΩ resistor.

This means the pin will need to sink up to 3.3mA.

Is this setup possible with this device?

  • 0
    TI__Mastermind 26335 points

    Hello Darren,

    Thanks for reaching out on the forum.  According to the absolute maximum ratings table found in the datasheet, the device can handle an input current up to 5mA into any pin. I understand your concern about the power consumption, your customer may have just copied what he has seen others do, or he may have alot of parasitic capacitance on his i2c lines and therefore used a lower resistance to increase the high to low transition speed.

  • 0 in reply to Patrick Simmons
    TI__Expert 5483 points

    Sorry, my concern wasn't regarding the power consumption.

    The I2C specification (from NXP) states:
    SMBus High Power supports Vol < 0.4V up to 4mA of Isink (through DATA / CLK)
    I2C-bus supports Vol < 0.4V up to 3mA of Isink (through SDL/SCL)
    SMBus ‘high power’ devices and I2C-bus devices will work together if the pull-up resistor is sized for 3 mA

    So my question:
    If the INA230 is used for communication with other SMBus/PMBus devices, and the pullup resistor is 1kΩ to 3.3V, will the Vol < 0.4V?

    From my understanding, there is only 1-pullup resistor to Vdd...so for example, if the SDL/SMB_DATA was pulled to 3.3V (VDD) via a 1kΩ resistor, and there were three I2C devices on the rail, then would each pins sink current be as shown in the below image? i.g. 3.3mA would flow from Vdd through 1kΩ to the SDL rail, but each device would sink 1/3, or 1.1mA each?

    If so, it seems it should be possible to keep the 3mA maximum (for Vol < 0.4) as long as there is more than 1 device on the lines with 1kΩ to 3.3V...is this understanding correct?

  • 0 in reply to Darren (FAE)
    TI__Mastermind 26335 points

    Hello Darren,

    You raise an interesting point. While the INA230 may handle over 3mA, other SMBus devices on the line may not.  From what I can tell, SMBus is very similar to I2C.  My understanding is that there is some sort of arbitration and only one device communicates at a time. Therefore only one device should be pulling low at a time.  When that particular device is pulling low, then that will be the dominant current path. In order for there to only be only 3mA flowing into that device's closed drain, Rs and RDSon would need need to sum up to 100Ω (3.3V/(1000Ω+100Ω)=0.003A).  Otherwise, when all of the devices are open drain, I expect the current into all devices to be roughly equal to (3.3V - >2.1V)/1kΩ/(number of devices).