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PCF8574: PCF8574 I/O output current

Lucas Rosa
Prodigy 110 points
Part Number: PCF8574
Other Parts Discussed in Thread: ULN2003A, , TCA9534, TPL7407LA, TLC59108, PCA9534

Hi team,

I'm facing a strange behavior with the device PCF8574RGTR. I'm using the PCF to drive an ULN2003A that drives a LED. However, when I turn on the PCF's output the LED does not turn on.

PCF8574RGTR's datasheet shows that the maximum I/O output current is 300uA.

When I turn the PCF's output, I measure 0.8V at it. Considering that the ULN has an input impedance of ~13kOhm, 0.8V at its input pin means that the PCF is sourcing only ~60uA. Is that correct? If so, why isn't it sourcing almost 300uA? If I place a external 100kOhm pull up at PCF's output it works, and the output voltage goes to ~1.6V.

When I use the device PCF8574TS/3,188 (from NXP) it works without the external pull up, and both devices have the same specification for output I/O current. Can someone explain what I'm missing?

  • 0 in reply to Lucas Rosa
    TI__Guru 50268 points

    Hi Lucas,

    PCF8574 has 100uA pull-up current source when driving logic high

    How are you writing to the PCF8574 to configure the output? Do we have scope shots to show this? 

    I would expect to see at least 100uA of current being sourced. 

    What is the resistor on the diode, and what is the forward voltage of the diode? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 110 points

    Hi Tyler,

    The LED drop is about ~1.8V and its series resistor is 4.7kOhm.

    Unfortunately I do not have the scope shots. However, I will provide them in the next few of days.

  • 0 in reply to Lucas Rosa
    TI__Guru 50268 points

    Hi Lucas,

    Am I correct when I say the current through the LED is = 

    24V - 1.8V / 4700ohm = 4.723mA? 

    Is the ULN2003 taking care of this current draw? This would be out of spec for both the PCF8574 from TI and NXP if the current is going through the PCF8574. 

    Waiting for the scope shots to double confirm the device is being written to correctly to set the outputs. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 110 points

    Hi Tyler,

    Unfortunately I will not be able to provide the scope shots. Is it possible keep this investigation without it?

  • 0 in reply to Tyler Townsend
    Prodigy 110 points

    Hi Tyler,

    Yes, the ULN is taking care of the LED's current. The PCF is driving only the ULN's input as it shows in the first schematic.

  • +1 in reply to Lucas Rosa
    Guru 317770 points

    Measuring a voltage above zero shows that the output tries to drive high, so the I²C communication should be OK.

    The ULN has a typical off-state input current of 65 µA, and requires more to switch on. Apparently, the NXP device outputs a little bit more than the TI device, but the guaranteed minimum is only 30 µA.

    A pull-up resistor would be able to supply more current. Alternatively, use a device like the PCA9534/TCA9534 that has proper push/pull outputs. Alternatively, use a driver with high-impedance inputs like the TPL7407LA. Alternatively, use an integrated I²C LED driver like the TLC59108(F).