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SN74HCU04: Current Limit Operation

Ryan May
Genius 13700 points
Part Number: SN74HCU04

Hello,

I am wanting to use the SN74HCU04 in one of my designs.

I see that it has max source and sink drive limits of 4mA. Can you help me understand what happens when these limits are hit? Does the device pull the output down for logic high and up for logic low to try and limit this current being sourced or sunk out of reflex?

Thank you for your help.

  • 0
    TI__Guru* 96661 points

    Hi Ryan,

    The SN74HCU04, like most logic parts, does not have a current limit.  The VOH and VOL specs in the datasheet are guaranteed with a load of 4mA, however the device can drive much more current than that.

    It is up to the designer to limit the current out of the device to 4mA if they desire to keep the VOH and VOL within the datasheet defined limits.

    The absolute max table for the device lists the maximum output current from a single channel as 25mA, and the total maximum current as 50mA -- although you could drive 50mA high and 50mA low simultaneously without violating this spec.

    Can you tell me about your planned project with this device?  Using an unbuffered inverter like this is uncommon, so I would love to hear what you are using it for.

  • 0
    Guru 256760 points

    When you exceed the current limit, you indeed get a higher voltage drop.

    As long as you stay below the current limit, you are in the linear (ohmic) region of the output transistors. With higher drain currents, you eventually go into the saturation region.

    See Input and Output Characteristics of Digital Integrated Circuits at 5-V Supply Voltage (SZZA008) for the typical behaviour:

    (Going below GND or above VCC would require a voltage source at the output.)