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PCF8575C: PCF8575C

ARJUN P.BHASKAR
Prodigy 245 points
Part Number: PCF8575C

Hi sir,

         In the respective application of this IC,pull up resistor values are given as 10K. If we calculate the pull up value according to the equation Rp(max) = tr/(0.8473*Cb) = (300ns/(0.8473*400pf) gives 885ohms,which implies maximum value of pull up is 885. them why 10k is used. The vcc supply for my design is 5V. shall I stick on to 10K?

  • +1
    TI__Mastermind 49040 points

    Hi Arjun,

    I don't see where in the datasheet that state a 10k pull up resistor should be used. I only see figure 15 in the datasheet which shows an example schematic with the PCF8575C and an I2C controller. This would make bus cap only 20pF in this example if we assume max cap based on I2C spec and no parasitic cap from the PCB. This would give a max of 17.7k ohm resistor according to a 300ns rise time which would make a 10k pull up resistor sufficient.

    ARJUN P.BHASKAR said:
    shall I stick on to 10K?

    It will depend on your I2C bus load. If you really have 400pF that would imply you have a lot of I2C devices on the bus, you have really long traces on your PCB, or are transmitting through a heavily loaded/long cable. Most I2C applications don't get close to 400pF.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 245 points

    Thank you sir, So how to calculate the bus capacitance based on the i2c load? I thought 400pF  should be used to calculate the pull up resistor value.

  • 0 in reply to ARJUN P.BHASKAR
    Guru 243120 points

    Add up the capacitances of all devices and the traces. Effect of Parasitic Capacitance in Op Amp Circuits says:

    • Circuit traces on a PCB with a ground and power plane will be about 1−3 pF/in.
    • Low capacitance cables are about 20−30 pF/ft conductor to shield.
  • 0 in reply to ARJUN P.BHASKAR
    TI__Mastermind 49040 points

    Clemen's is correct. Additionally, you may also assume max capacitance of any I2C device/Controller to be about 10pF (exceptions are with switches/muxes). Adding this up should give you an approximation of the value Cbus you will need to use in that formula to calculate the max pull up resistance.

    -Bobby