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Original question:

SN74HC595: queries regarding driving 7 segment display

SN74HC595: Pulse current rating of output pins

Harmeet Singh
Intellectual 1480 points
Part Number: SN74HC595


Hi Team,

Shift register SN74HC595DR , absolute rating specifies the continues output current  per pin is 35mA and total continuous output current is 70mA.

 

What would be the pulse output current rating at  50 % , 70%  and 80% duty cycle?

 

Regards,

Harmeet

 

  • 0
    TI__Guru* 97066 points
    Hi Harmeet,
    The absolute maximum ratings of the device are somewhat misleading when they say "continuous output current" -- especially if you do a lot of power system design.

    In the logic world, that means you can't exceed 35mA per pin, and you can't exceed 70mA total current. These parts are not intended or rated for power delivery - they are intended for signal delivery applications.

    I would recommend keeping each output at 8mA (max) to prevent damage to the device.
  • 0 in reply to Emrys Maier
    TI__Intellectual 1480 points

    Hi Emrys,

    The customer is driving 7 segment display through 74HC595.

    I agree that 8mA is the maximum recommended continuous current .

    Can you specify in case of different duty cycle as mentioned earlier the maximum recommended current for each pin?

    Regards,

    Harmeet

  • 0 in reply to Harmeet Singh
    TI__Guru* 97066 points
    Hi Harmeet,
    Do not exceed the maximum continuous current rating -- ever, for any period of time.
  • 0 in reply to Emrys Maier
    TI__Intellectual 1480 points

    Hi Emrys,

    Thanks.

    Correct me if I am wrong in my understanding.

    You mean to say that the maximum current of each pin should not exceed 8mA irrespective of duty cycle.

    Regards,

    Harmeet

  • 0 in reply to Harmeet Singh
    TI__Guru* 97066 points

    Hi Harmeet,

    No, I said "do not exceed the maximum continuous current rating" 8mA is not the maximum current rating.

    The maximum current rating of the device is 35mA per pin, and 70mA total current through Vcc or GND.  If 8mA is drawn from 8 outputs at the same time, then the total current is 8*8 = 64mA, which puts you pretty close to the 70mA limit. If you are only drawing current from 2 outputs, then 35mA could be drawn from each without violating any maximum specs.