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Driving problem with PCF8575

Hossam Alzomor
Prodigy 140 points
Other Parts Discussed in Thread: PCF8575, TCA9535

Hi

I am using PCF8575 for driving MOC3603 OPTOCOUPLER .

When use the PCF8575 without driving the optocoupler, the output on the IO's of PCF8575 is 4.5V when driving the outputs high.

When I connect the PCF8575 outputs to the optocoupler through a 390 ohm resistor and drive the output high the output of the PCF8575 is 1V.

Vf of the input led of the MOC3606 optocoupler is 1.2V , so the current sinked from the port is (5V-1.2)/390=9.74mA

May you kindly tell me what is wrong with my calculations and design and how to solve the problem
Attached you can find schematic for my circuit

Regards
Hossam Alzomor

Schematic Prints.pdf
  • 0
    TI__Mastermind 30310 points

    Hossam,

    It appears that your topology for driving the diode, or opto-coupler, is the opposite of the ideal topology for this part.

    The way it is connected in your circuit (the outputs connected to the anode of the diodes), the PCF8575 needs to source current, which is represented by the IOH spec. The PCF8575 can only source ~300uA DC. On the other hand, the PCF8575 can sink ~15-25mA of DC current (IOL)

    Depending on the package you used, the TCA9535 is pin-2-pin compatible with the PCF8575  and the TCA9535 can source ~10mA of current (IOH).

     

    This leaves you with 2 options:

    1) re-design the PCB for the PCF8575 in the opposite topology (with the outputs connected to the cathode of the diodes)

    2) implement the TCA9535 , which will require some software changes

     

    Thanks,

    Brian

  • 0 in reply to Brian Berner
    Prodigy 140 points

    Thanks Brian,

    IS the IOH and IOL you mentioned in your reply per pin or per device?

    and if it's per pin what is the max per device?

    Regards

    Hossam Alzomor

  • 0 in reply to Hossam Alzomor
    TI__Mastermind 30310 points

    Hossam,

    The IOH and IOL specs in the "Electrical Characteristics" section of the datasheet are on a per pin (i.e. per channel) basis. These are the specs that I was referring to in my response.

    However, there is also a per device spec for IOH (a.k.a ICC through VCC) and IOL (a.k.a. ICC through GND) in the "Absolute Maximum Ratings" section of the datasheet. At the device level, the sum of all of the current in each channel should not exceed this spec.

    If this answers your question fully, please click "Verify Answer". If not, please follow up with clarification questions.

    Best Regards,

    Brian

  • 0 in reply to Brian Berner
    Prodigy 140 points

    Thanks Brian,

    It's done regarding this issues, but I have another problem regarding reading from IC,
    We can read the 1st byte correctly but the 2nd one is not red correctly we always read it 0xff regardless of what is on the port.

    you can find more details in the following post by my colleague Sherif
    http://e2e.ti.com/support/interface/etc_interface/f/390/t/318813.aspx


    , may you kindly