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SN74AHCT14: If the datasheet doesn't indicate delta ICC parameter, do I need to worry about the current spike?

Prodigy 30 points

Replies: 4

Views: 67

Part Number: SN74AHCT14

Lets say if it is a line receiver, in the case that all the inputs are not in the rail in some situation. 

here is the paragraph from ahct14 datasheet

7.9 Typical Characteristics One common misconception is that the current consumption will be less when switching a slow signal into a Schmitt trigger. This is partly true because the Schmitt trigger prevents oscillation which can draw a lot of current; however, you will see higher ICC current due to the amount of time the input is not at the rail. This is Delta ICC. Delta ICC is where the inputs are not at the rails and upper or lower drive transistors are partially on. Figure 1 shows ICC across the input voltage sweep.

thank you so much in advance!!

  • Hi,

    You still need to worry about higher current draws if your input is not near a rail. Please read the following FAQ on slow or floating inputs.

    What Schmitt trigger devices do is they add hysteresis to the inputs. However, if your line is floating at the voltage between Vt+ and Vt-, you will have a large amount of current draw. The current draw just will be much lower than that of a standard CMOS input device. The Icc plot provided in the datasheet explains this. So its alright to have a slow input signal but not okay to let it float.

    Secondly, the device you mention does have a delta Icc spec in the electrical characteristics table.

    Thanks!

    -Karan

  • In reply to Karan Kotadia:

    for AM26C32

    if all Vt+=Vt-=VCC/2 draw much current than tie them to the VCC or GND?

    it seems if I disable the output, the ICC won't effected by the input level,

    Thanks!! 

  • In reply to Chen Hong:

    Hi,

    There are fets at the input of the device which will be the ones drawing current. Tying the outputs high or low does not change current draw. You need to tie the inputs to either Vcc or GND.

    Thanks!

    -Karan

  • In reply to Karan Kotadia:

    Got it, thank you !!