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SN74AC244: IO voltage limits

Cedrick
Expert 8270 points
Part Number: SN74AC244

Hi,

Good day.

If the device rails are +3.3V and GND, can the inputs and outputs be any voltage just as long as the current is limited? The note indicates that the I/O voltages may be exceeded, Is there no limit on the voltage as long as the current rating is not exceeded?


Regards,

Cedrick

  • 0
    TI__Guru* 96681 points

    Hi Cedrick,

    This table indicates that the device includes both positive and negative clamp diodes on all I/O pins, so those will clamp the I/O pins to Vcc + 0.5V if a voltage larger than VCC is applied, and to -0.5V is a negative voltage is applied.

    Our recommendation is not to exceed the absolute maximum rating of the device (7V) for the total voltage applied to the device, however technically this can be exceeded so long as the device never "sees" a larger voltage.

    To give you an example, here's a circuit with +/- 100V as an input signal to the AC family buffer:

    You can see that the voltage at VG1 is 100V, but the actual voltage at A (the input of the device) is limited to 5.71V. The device never "sees" more than 7V, so this is a perfectly valid way to operate the device.

    The key factors to ensure that this type of operation doesn't cause issues are:

    (1) Make sure the supply (V1 in my simulation) can support the current flowing into it from the 100V source.

    (2) Make sure the voltage doesn't ramp too fast - if the input gets above 7V even for a short time, the gates of the input MOSFETs can be permanently damaged.

    (3) Make sure the current through the clamp diode never exceeds the abs max rating (clamp current) in the datasheet