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SN74HC125: Logic state related query

NIBIN S
Intellectual 550 points
Part Number: SN74HC125
Other Parts Discussed in Thread: SN74LVCH244A, SN74LVC1G125, SN74LVC2G125, SN74LVC244A

Dear Team,

We are using the below buffers in our design.

1) SN74HC125PWR

2) SN74LVCH244APWRG4

We have below queries on the buffers,

* If the power to the buffer is OFF and the input of the buffer is in high logic state (OE is enabled always), Is there any chance of buffer getting damaged?

* If the power to the buffer is OFF and the output of the buffer is in high logic state (OE is enabled always), Is there any chance of buffer getting damaged?

* In the above two case, what is the input and output logic states of the buffers?

Please let us know the updates ASAP.

Regards,

Nibin

  • 0
    TI__Guru* 97046 points

    Hello Nibin,

    There are multiple definitions of "power to the buffer is off" and "in high logic state" - so I'm going to define those for you.  For my answers I will assume that Vcc = 0V

    * If the power to the buffer is OFF [Vcc = 0V] and the input of the buffer is in high logic state [Vin = 5.5V] (OE is enabled always), Is there any chance of buffer getting damaged?

    For the SN74HC125, if the input voltage is greater than Vcc, then the positive input clamp diode will activate (indicated by the I_IK spec in the absolute maximum ratings table of the datasheet). As long as the current is kept below 20mA, this is not a problem, however if the clamp diode current exceeds 20mA, the device will be damaged.

    There are other issues associated with this -- since current will be flowing from the input to the supply pin, and the supply is off, the current may back-power other device on the same "off" voltage rail.  This is generally not recommended.

    For the SN74LVCH244A, the inputs are over-voltage tolerant and the device has partial power down protection (Ioff) specified in the electrical characteristics, so the device will not have any problems with the input being at 5.5V while the supply is at 0V.

    * If the power to the buffer is OFF [Vcc = 0V] and the output of the buffer is in high logic state [Vo = 5.5V (HC device), Vo = 3.3V (LVC device)] (OE is enabled always), Is there any chance of buffer getting damaged?

    How is the output to the buffer in the 'high logic state' when the device is turned off? 

    Are you forcing the output to a voltage from an external source?  If so, you can see the effects from the absolute maximum ratings table. (ie look at the output clamp current and   If you are asking what the output will do when the supply is at 0V, then I'd say it's pretty obvious that it won't be doing anything... with no power supplied, the device can neither source nor sink current through the internal MOSFETs.  There are some diodes that will be activated if an external voltage is applied, as indicated in the absolute maximum ratings table.

    * In the above two case, what is the input and output logic states of the buffers?

    Since the device does not have a supply rail, it cannot supply a voltage to the output, nor can it sink current at the output.  There is no 'logic state' on the output since the device is not active. There will be a voltage on the input and output, but that will depend on your system design.

    If you tell me what you are trying to accomplish with this device, I might be able to help.  From the questions, I really have no idea what your circuit looks like and can't help.

  • 0 in reply to Emrys Maier
    Intellectual 550 points
    Dear Emrys Maier,

    Thank you for the quick reply.

    As per the details mentioned, the SN74HC125 buffer is not suitable, since it can create issues in the design.

    Our actual requirement is to interface two modules, out of which, any of the module can be in ON (VCC present)or OFF (VCC absent) state.
    At this condition, the power to the buffer (VCC) can be ON or OFF. So for all the above cases, the module or the buffer should not get damaged due to the partial operation of the circuit.

    Regards,
    Nibin
  • 0 in reply to NIBIN S
    TI__Guru* 97046 points
    Hi Nibin,
    My recommendation would be to use the SN74LVC2G125 (dual channel) or the SN74LVC1G125 (single channel). These devices have Ioff protection like the SN74LVCH125A, and also operate up to 5V like the HC125. The reduced channel count is a benefit in many systems since you can more easily place/route.

    The SN74LVCH244A is a good choice if you need your outputs to be at 3.3V with inputs at 5V and sometimes have the I/O's set to a high impedance state and don't want to add pull-up or pull-down resistors. If you aren't concerned about having floating I/Os, I would recommend the SN74LVC244A instead (no H = no bus-hold circuits).