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SN74AHCT244: Logic forum

Yang Wu
Expert 3050 points
Part Number: SN74AHCT244
Other Parts Discussed in Thread: SN74LVC244A,

Hi Team,

When I look the Vo spec in the datasheet of SN74AHCT244 & SN74LVC244A, I found something different. In the DS of SN74LVC244A, the Vo max has different description in power off state and working state (output in high/low state), but in the DS of SN74AHCT244, it doesn't differentiate power off and working states.

we know that SN74AHCT244 has both upper and lower clamp diodes at both input and output, SN74LVC244A only has  lower clamp diodes at both input and output. 

Now, considering the device is not powered ON (ideally, VCC is 0).

1) For SN74AHCT244, does it mean that the output can only tolerate max @0.5V (I guess if higher than that, the clamp diode may be damaged because there has no resistor in series to share the extra voltage, right?)?

2) For SN74LVC244A, does it mean that the output can tolerate max 6.5V (I guess it's max reverse voltage of the lower clamp diode, right?)?

3) In addition, for SN74LVC244A in working state (output in high/low state), why output max is VCC+ 0.5? because there has no upper clamp diode, I don't know how this limit come from.

Could you help answer above questions? thanks a lot.

Best regards,

Yang

  • +1
    TI__Mastermind 25085 points

    Hi Yang,

    Yang Wu said:
    we know that SN74AHCT244 has both upper and lower clamp diodes at both input and output, SN74LVC244A only has  lower clamp diodes at both input and output.

    I would read through this FAQ first: [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc). The AHCT device does not have an upper claim diode at the input. 

    Yang Wu said:
    For SN74AHCT244, does it mean that the output can only tolerate max @0.5V (I guess if higher than that, the clamp diode may be damaged because there has no resistor in series to share the extra voltage, right?)?

    Yes, this is correct (Vcc+0.5). The AHCT does not have an Ioff feature, whereas the LVC device does (see next line)

    Yang Wu said:
    2) For SN74LVC244A, does it mean that the output can tolerate max 6.5V (I guess it's max reverse voltage of the lower clamp diode, right?)?

    The actual max output voltage will be specified in the Ioff specification, seen in the picture below, which is 5.5 V

    Yang Wu said:
    3) In addition, for SN74LVC244A in working state (output in high/low state), why output max is VCC+ 0.5? because there has no upper clamp diode, I don't know how this limit come from.

    Some devices may not have a clamp diode, however, a MOSFET will provide the same functionality due to the body diode within the MOSFET.

  • 0
    Guru 277220 points

    In devices with Ioff, the output P-channel transistor does not have its body shorted to the source:

    When the output is in the working state, the pin is connected to either VCC or GND through a transistor. It might be possible that you exceed the output current even if the voltage is less than 0.5 V beyond the rail.

  • 0 in reply to Albert Xu1
    TI__Expert 3050 points

    Hi Albert,

    very clear, thanks for your help!

    Best regards,

    Yang

  • 0 in reply to Yang Wu
    TI__Expert 3050 points

    Hi Clemens,

    the diagram is a good complementation, thanks for your help!

    Best regards,

    Yang