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SN74LVC1G123 / avout K value

Sho Ogane
Genius 5575 points

Hi all

My custimer would like to set the pulse length 2sec,
so please tell me the value of Rext and Cext @Vcc=5V.

And I have one more question.
Please tell me the value of K at the following condition,
1, Vcc=5V,Cext=1uF
2, Vcc=5V,Cext=10uF

I saw the graph of the Output Pulse Duration Constant vs External Timing Capacitance vs Supply Voltage the data sheet P.8
I couldn't get the K's value of 1 and 2 conditions.

Best Regards,
Sho Ogane

  • 0
    TI__Guru 62270 points
    Hi Sho Ogane san,

    I do not have the K value for the Cext either . I would assume that the value would be around 0.9 though going by the datasheet graph.
    With C = 0.1uF and K at 0.925 , I get R as 21.6Mohm , using 21MOhm resistor would result in tw of 1.94sec.
    With C 1uF or 10uF , the R value reduces but k is unknown and should be estimated to be 0.9 -0.85 .
  • 0 in reply to ShreyasRao
    Genius 5575 points

    Hi ShreyasRao san

    Thank you for the quick response!
    I tell your opinion" K is 0.9 -0.85" to my customer.

    Sorry I have one more question.
    I saw the following URL.
    e2e.ti.com/.../3213.what-is-the-largest-timing-resistorcapacitor-that-can-be-used

    This URL is described the below comment.
    "It's generally a good idea to keep your resistance below ~1 Mohm.  This is because the resistance between traces on many board can be in the 1 to 10 Mohm range, and these parasitics can cause unexpected operation."

    You got R as 21.6Mohm,but there is written that the value of resistance is below 1Mohm in this URL.

    Do you know why the resistance value is recommended below 1Mohm?

    Best Regards,
    Sho Ogane

  • 0 in reply to Sho Ogane
    TI__Guru 62270 points
    Hi Sho ,

    as per the suggested K value with 0.1uF , it resulted in 21Mohm resistor .
    The customer would be suggested to test the pulse width to determine the K value and use higher cap values for having realistic and readily available R .
  • 0 in reply to ShreyasRao
    Genius 5575 points

    Hi ShreyasRao san

    I have one more question.

    1.How to treat the A- pin(Pin.1) when we don't use the pin.
    2.Does the CLR- pin need pull up resistance to pull up VCC?
       If it needs, how much value is best?

    Best Regards,
    Sho Ogane

  • 0 in reply to Sho Ogane
    TI__Guru 62270 points

    Hi Sho Ogane San ,

    You would be limiting the functionality of the device when you do not use A and CLR pins as listed per the datasheet function tables .

    You can also refer to the app note for LVC1G123 . I would suggest to have the A low for rising edge triggered application and CLR tied to Vcc through 1Kohm pullup resistor .

  • 0 in reply to ShreyasRao
    Genius 5575 points

    Hi ShreyasRao san

    Thank you for the reply and good application note.
    I checked app note attached, and I understood this input logic.

    My customer set rising Edge,so A bar input is set low.
    My customer use 100Ω pull down resistor, I think it is no problem to use pull down resistor or to tie A bar pin directly to GND. 
    My understanding is correct?
    I concern pull up value. If you know tne best value of pull down resistor, please tell me.

    Best Regards,
    Sho Ogane

  • 0 in reply to Sho Ogane
    TI__Guru 62270 points
    Hi Sho Ogane san ,

    I do not see any concern with 100ohm pull down resistor on the inputs .
  • 0 in reply to ShreyasRao
    Genius 5575 points

    Hi ShreyasRao san

    Sorry again,
    My customer asks me ⊿T/⊿V[ns/V] of CLR pin when my customer uses this device @ 3.3V.

    I saw the tpd value of the datasheet P.7 6.7~6.9 table.
    If my customer uses CL=15pF,min and max value of ⊿T/⊿V(CLR pin) are following.

    Min:2.0/(3.3-0.3)=0.666[ns/V]
    Max:8.0/(3.3+0.3)=2.22[ns/V]

    Is my understanding correct?

    Best Regards,
    Sho Ogane

  • 0 in reply to Sho Ogane
    TI__Guru 62270 points
    Hi Sho Ogane San ,

    The CLR being a input for the LVC part , the faster the better in terms of power consumption.
    Referring to other similar LVC parts , the CLR input transition time to be not greater than 10ns/V .
    This number comes from the design data files for the process ;not related to the tpd.
  • 0 in reply to ShreyasRao
    Genius 5575 points

    Hi ShreyRao san

    Thank you for your great support about two device.

    My customer uses this device @Vcc=3.3V, then is "10ns/V" max value of ⊿T/⊿V[ns/V] ?
    "Referring to other similar LVC parts , the CLR input transition time to be not greater than 10ns/V ."

    And do you know min value?

    Best Regards
    Sho Ogane