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SN74LVC1G123: LED Drive capabilities

Chetan Karkera
Prodigy 90 points
Part Number: SN74LVC1G123
Other Parts Discussed in Thread: SN74LV123A,

Tool/software:

Hello,

I am using part SN74LVC1G123DCTR to detect the input signals and produce pulse for 1s, 

Since this part can source/sink god amount of current at 3.3V VCC , shall i connect the LED directly to the output ?

Regards,

  • 0
    Guru 317320 points

    Yes. (You still need a current-limiting resistor.)

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Thank you ! I have Few more question regarding the same part , I want to use it as the rising edge detector where the frequency of the signal is around 400hz 

    and the out put pulse should stay for 1s which will drive the LED.

    1.  Is the component suitable for 400Mhz and higher signal rates? ( I tried to simulate similar part in SN74LV123A in LTspice , component is not working signals below 2us cycle. 

    2. In the  SN74LVC1G123 datasheet the output voltage for 3.3V is not given ( for 3V is says 2.4Vmax if we are drawing 24mA of current). could you please suggest the current limiting resistor value LED with 2.4V max Vf and around 8  to 10mA current ?

  • 0 in reply to Chetan Karkera
    Guru 317320 points

    1. The minimum input pulse duration is specified in section 5.6.

    2. 3 V is the worst case of a 3.3 V ±10 % supply. (3.3 V − 2.4 V) / 8 mA = 112.5 Ω. The typical output impedance us about 12.5 Ω, so you would need 100 Ω.

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Thank you again Clemens, 

    1, I have checked the circuit with below condition(for simulation i have used ,SN74LV123A )

    minimum pulse width is 3ns with 3.3VCC. as per the application note slva720a device is edge triggered and will give output soon after rise of positive edge.

    but am not able to use the higher value capacitor to increase output pulse duration. and if i use the very small capacitor for 3ns pulse width i get very small output pulse width. I believe this is because of small pulse input capacitor is not discharging below 0.63V?

    Is there any way where i can get the desired output for 3ns input pulse and keep the output Tw duration for 1s?

    2. May i know why output is the switching around 1.2V?

          

  • +1 in reply to Chetan Karkera
    Guru 317320 points

    The SN74LV123A simulation model is not very useful to determine the behaviour of the SN74LVC1G123.

    The external resistor and capacitor determine the output pulse width. They are not related to the input pulse width.

    When the B input has triggered, the external capacitor is allowed to charge through the external resistor. When the voltage at the Rext_Cext pin reaches a certein threshold, the output goes low again. So the output pulse length is affected by the tolerances of these two components, any by the capacitance and leakage currents of the traces and pins.

    See the datasheet for the ranges of R and C that are typically used. With a very small C, the pin capacitance becomes noticeable. Large electrolytic capacitors can have high leakage currents that make it impossible to use a large R with a small charging current.

    For 1 s, you probably need 10 µF and 100 kΩ. If possible, use a ceramic or foil capacitor.

  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Thank you for your support Clemens! 

    I will buy few samples and test it to confirm the above points.