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SN74HCT08: Pulldown resistor value for 'TTL Compatible' inputs

Oscar Walsh
Prodigy 150 points
Part Number: SN74HCT08

I am using 2 SN74HCT08 ICs to control some MOSFETS, with some of the AND-gate inputs controlled by 3.3V GPIO pins on a microcontroller. When the microcontroller is being programmed, the GPIO pins go to 'analog mode,' leaving the corresponding inputs on the SN74HCT08s floating.

To avoid unpredictable behavior, I want to add pulldown resistors on these logic inputs. But, the device is battery powered so I don't want these resistors to be too low.

I know that for CMOS logic, a weak pulldown (e.g. 100k) is sufficient to prevent floating state, while for TTL the resistance needs to be much lower to achieve the same thing, due to the lower input impedance.

Since these inputs are TTL-Voltage compatible, but on a CMOS family device, I'm not sure what pulldown resistance to use.

What's the highest pull-down resistor value I can use, that will still prevent floating inputs? 

Thanks,

Oscar

  • +1
    Guru 316860 points

    These input have TTL-compatible input switching thresholds, but their input current is the same as for other CMOS inputs.

    The specified worst-case input leakage current is 1 µA. The voltage drop over the pull-down resistor must not exceed VIL = 0.8 V, so R < 0.8 V / 1 µA = 800 kΩ.