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PCF8574: Iol typ for I2C

Hanno Reimer
Intellectual 1520 points
Part Number: PCF8574

for the low level output current of the I2C port pins, the datasheet of PCF8574 only specifies a minimum value. Could you please give an unbinding typical/max value. Thanks

  • 0
    TI__Mastermind 49400 points

    Hello Hanno,

    The I2C pins (SDA/SCL) by I2C standard must generate a VoL of 0.4V at IoL of 3mA or greater. Are you trying to get a value at which the pin breaks?

    -Bobby

  • 0 in reply to BOBBY
    TI__Intellectual 1520 points

    Hi Bobby, 

    my customer is asking, because he wants to source an additional circuitry. 

    ciao Hanno 

  • 0 in reply to Hanno Reimer
    TI__Mastermind 49400 points

    Hanno,

    I don't have characterization data I can provide (device is old) but I was able to dig up some design characterization (simulation results).

    40C 3.3Vcc at 3mA VoL was 0.18V for a nominal case

    40C 5Vcc at 3mA VoL was 0.16V for a nominal case

    Weak model at 100C showed ~300mV and 210mV (2.5Vcc and 6Vcc)

    Strong model at -40C showed ~140mV and 117mV ( 2.5Vcc and 6Vcc)

    You could probably use ~200mV as a typical value for SDA. Keep in mind all these values provided are from simulation results and may differ slightly from real world.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    Hi Bobby/Hanno,

    We are connecting two PCBs which communicate via I2C. The Pull-Ups for the interface are located on the one PCB we are not willingly to change (several approvals and external tests...).

    The Pull-ups are 330R. So the current drain on low level is arount 15mA. The Voltage on low level is ok. We use a PCA9600 on this side so we are fine with more than 1V for low level.

    We just want to be sure that the PCF8574 can handle this current without getting damaged over time.

    BR Nils

  • 0 in reply to user3877429
    TI__Mastermind 49400 points

    Hey Nils,

    "We are connecting two PCBs which communicate via I2C. The Pull-Ups for the interface are located on the one PCB we are not willingly to change"

    Thanks for providing further information here.

    table 6.3 for the IoL spec looks like 25mA is what is suggested you stay below. I peaked at the P-port stages and the SDA output stage and saw we have a little bit more protection on the SDA output stage (we have a series resistor which is double that of the P-ports and the size of the FETs are slightly larger than the P-ports). This tells me the IoL max spec is probably good at 25mA. Going outside of 25mA will likely get into FIT issues (failures in time will be higher). Absolute maximum IoL is 50mA according to 6.1 which will break the device is larger.

    To me, it looks like 15mA will be okay. The series resistance I saw on the SDA line was about 30 ohms which will limit the current a little more so we will probably see around 14mA or lower when our device pulls low.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    Hi Bobby,

    thank you for your fast an precise answer. 

    In this case we will keep our system as is. We don´t see any issues anymore.

    BR Nils