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SN74HC165: SN74HC165 is pulling significant current compared to what datasheet states.

Craig Rettew
Intellectual 965 points
Part Number: SN74HC165

I'm using the SN74HC165 in my battery operated design. Without powering the SN74HC165, my circuit pulls around 90uA. When I power the SN74HC165, the current jumps around and is not steady. I've seen the current of my design jump up to 500uA but it typically hovers around 200uA. I'm not sure what the SN74HC165 is doing and it's not being clocked, it's in a steady state.

The SHIFT_DATA, CTRL, and CLK are tied to the microcontroller.

  • SHIFT_DATA goes straight into the microcontroller.
  • CTRL has a 10k Pull-up.
  • CLK has a 10k Pull-down.

Why is this device pulling unsteady current at steady-state?

  • 0
    TI__Guru* 96947 points

    Hi Craig,
    ** Correction, got my wires crossed, sorry -- I reposted this quickly to get rid of my ridiculously wrong request. **

    Are the inputs from the MCU being forced to a valid level, or are the MCU outputs off?

    Also, are R26 and R27 populated -- I would assume so, but there's a DNI box around them.

    Primarily I am looking for floating inputs.  If an input is either floating or getting slightly above GND or below Vcc, then the internal circuitry could draw some extra power.  This could also be caused by a faulty connection to one of the pull-up resistors or a pin of the device.

  • 0 in reply to Emrys Maier
    Intellectual 965 points
    The MCU goes in a sleep mode and the GPIO hold their state.

    R26 and R27 are NOT populate but R18 and R19 are, holding those inputs to GND.

    Seems like this part is pretty sensitive to levels at each pin. I'm thinking of shutting the thing down when my MCU is sleeping.
  • 0 in reply to Craig Rettew
    TI__Guru* 96947 points

    The standard CMOS inputs can draw excess current if the voltage on them goes above ~300mV or below Vcc-300mV.

    Most of our devices have a spec labelled as "ΔICC" that shows ~500uA increase at 400mV, however it seems that the HC family didn't get that spec when it was created.

  • 0 in reply to Emrys Maier
    Intellectual 965 points
    So it turns out that leaving SER floating is a bad idea. I grounded SER and the current issue goes away. I'm glad it was an easy fix (after several hours of trying everything else).
  • 0 in reply to Craig Rettew
    TI__Guru* 96947 points
    Thanks for the update Craig. Glad to hear you got it sorted out.