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Nand gate sn74hc132-q1 - VOH and test conditions

Piero Cambriani
Prodigy 30 points
Other Parts Discussed in Thread: SN74HC132, SN74HC132-Q1, SN74AHC132, SN74LV132A

Hi,

in the data sheet shown below we can see the value VOH referring to different test conditions such as: IOH = -20uA, IOH = -4mA and IOH = -5.2mA to different values of Vcc.

I have to calculate a Pull_down resistor, the power supply is 5V. When the output is high (VOH) the value of the output current is IOH.

Can you explain me the VOH row of the below datasheet?

Why do we have IOH=-20uA and IOH=-4mA at the same Vcc value (4.5V for example), what is the difference?


thank you in advance.

Cheers,

Piero

  • 0
    TI__Mastermind 24915 points
    Hello Piero,

    In short, the output voltage of the device depends on the loading conditions.

    If the load on the output is 20uA (a likely value if driving other Logic gates), then the output voltage will be very close the the VCC provided as there will be minimal voltage drop due to IR losses. If the current is higher (4mA), then the output voltage will start to drop due to IR losses in the device.
    So the spec here shows the typical value and min value expected under these output loading conditions.
    I hope that this helps to explain.

    Additionally, can you explain to me for what purpose you need a pulldown resistor?
    I ask because this device does not need a pulldown or pullup for operation as it can drive the outputs high or low.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 30 points
    Hi Michael,
    I explain you about a pull down resistor.
    In the circuit there are a SVS (tcl7705_ep) and a Nand gate (sn74hc132_q1), the output of SVS (reset signal) is connected to the an input pin of Nand gate and at this input there is a pull down resistor (10 K).
    I believe that the pull down resistor can be removed because as you anwered me the device doesn't need of any pull up/down resistor, moreover the tlc7705 has a totem pole as output stage.
    what do you think?

    Thank you again,
    Piero
  • 0 in reply to Piero Cambriani
    TI__Mastermind 24915 points
    Hello Piero,

    I was referring to a pull-down/pull-up on the output of the device since we were discussing the output drive of the device.

    If you are referring to the input, then the key is to make sure that the input of the logic device is always in a known/defined state. If the TLC7705 device always forces an appropriately high or low level output to drive the input of the logic gate, then you should be fine without a pullup or pulldown. If the output of the TLC7705 is ever in a high-impedance or unknown state, then it would be best to make sure that the inputs to the device are defined, and a pullup or pulldown may be required.

    From what I see in the TLC7705 datasheet, I believe each of the outputs are always in a defined state, so I don't believe a pulldown or pullup should be required as long as the input threshold voltage values for the device are met.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 30 points
    Hi Michael,

    I seen in HCMOS design consideration that the input stage of this family has a clamplig diode.

    I want to know for sure If sn74hc132-q1 device is 5V tolerant when it is powered with 3.3V.

    Thanks again.

    Cheers,

    Piero
  • 0 in reply to Piero Cambriani
    Guru 292510 points
    HC devices have ESD diodes to VCC and therefore are not overvoltage tolerant.

    (5 V tolerance would require the SN74AHC132 or SN74LV132A, which are not available for automotive temperatures.)