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Low freq OSC 74lvc1G14

Eknath P.R.
Prodigy 40 points
Other Parts Discussed in Thread: SN74LVC1G14, SN74AUP1G14, SN74LVC1G123

Using SN74LVC1G14 Schmitt inverter to generate a low frequency  square wave with a feed back  resistor 680K from output to input and a 0.1 microfarad capacitor at input. It works but Icc is 2 mA at 3 V. The data sheet claims 10 micro Amp Icc.

Fail to understand. Anyone can help me out?

P.R.Eknath

  • 0
    TI__Guru 56220 points
    Hi Eknath ,

    What is your Vcc and what is your input level at ?
  • 0 in reply to ShreyasRao
    Prodigy 20 points
    Hi Eknath

    I ran into the same problem I believe. Even though it is a schmitt trigger, I believe that it's intended for noise reduction or reshape of signals. Not for slow transitions. I did a characterization of ICC versus Input voltage. I saw that the current raises fast from GND level towards approx. 1/2 supply and then again drops towards a few uAmps. At half supply the current is several mA. If you look at the datasheet, right below Icc < 10uA, you will see another data (delta) Icc <500uA. This is with input close to GND or VCC (0.6V away). They do not specify any other levels. This is normal for old CMOS, but I did not expect it to be this bad for a Schmitt trigger device.

    Claus
  • 0 in reply to Claus Petersen
    TI__Guru 56220 points

    lvc1G14-04_dcchar.pdfHi Eknath /Claus ,

    The behavior of CMOS logic is such that it minimizes the Icc by not having a direct path from Vcc to ground at any point of normal operating conditions . However , there is a time when the input voltage being slowly ramped up from 0 to Vcc that both NMOS and PMOS can be turned on therby drawing huge supply current connecting directly to ground .As the input crosses the threshold turning off either of NMOS or PMOS , the Icc returns to uA of current . The Icct rating is precisely for that but has been shown only for Vin =Vcc-0.6V .

    I have attached DC char of LVC1G14 for your reference . Let me know if anything else .

  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Hi Shreyas,

    Thanks for the precise explanation.

    1. Now coming to the second part, will sn74aup1g14 will be any different or it will also be same. If so, should I assume that simple RC feedback oscillator with a large time constant using schmitt gate will never work. Any alternative suggestions?

    2. Basic question, how to handle very slow rise time inputs in a digital system?

    Eknath

  • 0 in reply to Eknath P.R.
    TI__Guru 56220 points

    Hi Eknath ,

    Schmitt trigger devices are recommended for slow rising inputs and SN74AUP1G14 is Schmitt trigger as well. It should be suitable for inputs from RC network circuits .The only thing to watch out for ,since there is slow rising inputs is the Icc current . I hope the attached files are good reads and will be useful for your knowhow.

    schmitt_trigger.pdfscba004c_slownfloatingCMOS.pdf

  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Hi Shreyas,

    Extremely thankful to you for educating me on fundamentals and the pains you took to do that.

    But can you please give me a lead on how to solve my problem of generating low frequency in shoestring power budget and cost sensitive design.

    Thanks

    PRE

  • 0 in reply to Eknath P.R.
    TI__Guru 56220 points
    Hi Eknath,
    It seems that your oscillator is working but your inputs are switching around the threshold voltages where the Icc shoots through .
    There are also oscillator solutions from TI you can look into at www.ti.com at in clocks and timing page .
    My colleague will also suggest more on this for your design .
  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Hi Shreyas,

    The purpose of the osc is to generate interrupt to wake up the rest of the circuits from deep sleep mode. The accuracy of timing is not important. The interrupt are expected to be generated around 2 to 3 times in a second. This is to minimize the power consumption  so as to extend the button cell battery life which is powering the circuit. Unfortunately, I  was horribly wrong in assuming the schmitt RC Oscillator for this purpose.

    Thanks

    PRE

  • 0 in reply to Eknath P.R.
    TI__Guru* 96591 points
    Do you think you could do this all with a single falling-edge interrupt? The SN74LVC1G123 monostable multivibrator (aka one-shot) can have a set pulse width that could be activated prior to the MCU going to sleep.