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SN74AUP1G17: Input logic voltage level

CYLow
Prodigy 25 points
Part Number: SN74AUP1G07
Other Parts Discussed in Thread: SN74LVC1G07,

Tool/software:

Hi,

I would like to check can this buffer function with Vcc=3.3V and Vi=1.75V? On the datasheet, it states that the logic high needs to be 2V minimum, however using the PSPICE model 1.75V is able to meet the threshold.

 

CY

  • 0
    Guru 319150 points

    Correct operation is not guaranteed for any voltage below 2 V. Furthermore, lower voltages will result in increased power consumption and maybe oscillations and damage, see [FAQ] How does a slow or floating input affect a CMOS device?

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    HI Clemens, do you have other suggestion for my use case? I want to drive an LED using 1.8V GPIO that has little to no current sourcing/sinking ability. The GPIO is open drain with 100k pull up resistor. The behaviour I want is for LED to light up when GPIO is low, and off when GPIO is high (1.8V). I know that this can be achieved with 2 transistors, but I prefer to minimize the number of components. I found this design from another discussions in e2e.

    CY

  • 0 in reply to CYLow
    Guru 319150 points

    The open-drain outputs of '07 devices are overvoltage tolerant. Just use the SN74LVC1G07 with a 1.8 V supply.

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    Hi thanks for input. The forward voltage of the LED I am using is around 2V. If I connect the LED anode to 3.3V and VCC of SN74LVC1G07 to 1.8V, will it work? The datasheet does not state the output high voltage level, if it is 1.8V, will the circuit function correctly and keep the LED off?

  • 0 in reply to CYLow
    Guru 319150 points

    An open-drain output does not have a high-level voltage. No current is flowing, so the voltage will be pulled up to 3.3 V.

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    Thanks for the clarification. I understand the basic functionality of the buffer with open-drain outputs now. I want to design the LED driver for about 5mA conducting current. The LED is rated at 2V forward voltage at 20mA. At around 5mA, the LED forward voltage is shown to be around 1.8V.

    I will use SN74LVC1G07 because SN74AUP1G07 is not able to sink this current at 1.8V Vcc. I have questions regarding about choosing current limiting resistor in the LED driver circuit in section 9.2 of SN74LVC1G07 datasheet.

    The datasheet states the recommended low-level output current.

    Can the buffer operate correctly at Vcc=1.8V sinking 5mA?

    The datasheet also states the output low voltages at some Vcc and IOL.

     

    Is this the voltage measured at output Y when the buffer sinks the specified current?

    I will need to know the output Y node voltage at 5mA@1.8V Vcc to estimate the current limiting resistor value needed.

  • +1 in reply to CYLow
    Guru 319150 points

    The worst-case LVC output resistance at 1.8 V is 0.45 V / 4 mA = 113 Ω; the typical value is about half that. The LED needs a total current-limiting resistance of about (3.3 V − 1.8 V) / 5 mA = 300 Ω, so your resistor should be about 240 Ω.

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    Thanks a lot. Final questions, for this LED driver, I calculated roughly the power dissipation at SN74LVC1G07 to be (5mA)^2*60=1.5mW. Is this right? Can I use the DRY or DSF package for this?

  • +1 in reply to CYLow
    Guru 319150 points

    Yes. The temperature increase over ambient would be 439 °C/W × 1.5 mW = 0.7 °C; I'd estimate that you do not yet need a headsink for this.

  • 0 in reply to Clemens Ladisch
    Prodigy 25 points

    Many thanks for the superb support! I will proceed with using SN74LVC1G07 for the LED driver design.