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SN74LVC244A: Total Output current larger than 70mA

user5083091
Prodigy 80 points
Part Number: SN74LVC244A

Hi logic team,

We are using our device SN74LVC244 to drive LED array. Schematic is shown below. 

D3 is the LED array. When the output of SN74LVC244 is low, the current through each output pin is about 18mA.

I can see from the datasheet that the total current through Vcc or GND should be less than 70mA.

If all the output turns low, then the total current through GND should be 8*18mA = 144mA, which exceeds the limit of 70mA. 

However, time when all LED turns on are 20% of the total time. That is to say, the average current is only 144*20%=28.8mA, which is much lower than 70mA.

I just wonder if this could damage our device, or have any potential risk for this device. 

Thanks a lot for your help.

Best regards,

Vio.

  • 0
    TI__Guru* 97046 points
    Hi Vio,
    The values given in the Absolute Maximum Ratings table are absolute - operating beyond 70 mA, even for a nanosecond, is not supported by TI, and the reliability of that part cannot be guaranteed.

    That being said, operating at 20% duty cycle with an on-period of less than 1 ms, the device will work as though the current is 28.8mA through Vcc or ground.

    To summarize - physically, this will not damage the device, however it will void the warranty.
  • 0 in reply to Emrys Maier
    Prodigy 80 points
    Hi Emrys,

    Thanks a lot for your detailed explanation. I'd like to ask you another question.
    What's the over current behavior for SN74LVC244 ? Will the current be limited, clamped or enter protection mode?

    Thanks.
    Vio.
  • 0 in reply to user5083091
    Guru 317000 points

    For each individual output, the current will eventually be clamped, but that happens only at more than 70 mA through one pin (see figure 17 of SZZA010).

    As far as I know, the VCC/GND limit is a thermal limit, i.e., exceeding it will let the magic smoke escape.