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SN74AUP2G02: Driving femtofet with logic device.

Part Number: SN74AUP2G02
Other Parts Discussed in Thread: CSD17484F4,

Hello,

I would like to drive a femtofet, CSD17484F4 for example, with the output of a nor gate such as the low power SN74AUP2G02 and have some questions:

1.) What is the maximum amount of output current(of the NOR gate) that would be expected if I were to short the output at Vcc = 0.8V, 1.1V,,,,,3.3V? Ignoring for the moment device destruction.

2.) Related to question 1, what sort of internal resistances exist within the device which may help in the effort to not exceed maximum currents? I'm looking to find out how much current limiting must be added after accounting for any intrinsic impedances.

3.) All things considered, at around Vcc=1.1V, is there a setup which would allow the NOR output to safely and repeatedly switch the CSD17484F4 femtofet with propogation delays of less than about 200ns? Assume for the moment that Vth of the femtofet is less than 1.1V. What about two fets being driven by the same output? 

4.) In the absolute maximum section of the NOR gate's datasheet there are two parameters listed named, " Continuous output current " and "Continuous current through VCC or GND". I'm failing to see the difference based on name alone since, at least for the cases I'm imagining, any output current would necessarily flow through either Vcc or ground. Would you please explain the difference between these two parameters.

5.) If the NOR gate in question is not suitable, can you recommend a different part?

Thank you.

  • Hi Pongo,
    We don't specify a short circuit current on any of our logic devices, but at 0.8V I would only expect to see ~1 to 2mA out of this device. with a 0.8V supply. At 3.3V this device can probably drive 15+mA.

    The output impedance of any logic device is basically VOH/IOH or VOL/IOL, but at 0.8V you are at the absolute worst case of the device (FET impedance increases as supply decreases) and the device is only specified for a high impedance operation (20 uA) - ie I would not use those numbers to calculate an output impedance. Only use the values that have mA - ie at 3V operation VOH is 2.6V @4mA --> R~=650 ohms

    At 0.8V, I seriously doubt you could get the device to drive enough current to damage itself, so no external limiting is required.

    The AUP device is specifically designed for low power operation, which means that it has internal circuitry to slow down edges and improve switching efficiency (via proprietary circuits).

    The propagation delay of the AUP1G02 is 25.5ns max at 1.1 V operation. Taking that time plus the charge time of the gate (4* 195 pF * 700 ohms ~= 550ns) gives about 600ns of total delay... which is much too slow.

    You might consider asking on the femtofet forum (open a new post and use the femtofet part number) and see how they usually drive those FETs at 1.1V.
  • Thanks Emrys,

    While were on it, do you know what the startup time is for the SN74AUP2G02? I'm looking for the time to operate within spec after having transitioned from a sustained zero power period(days), to some operable Vcc.
  • Hi Pongo,
    This is a simple analog device - as soon as it has power, it is operating.