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SN74AXC4T774-Q1: About tpd when A-Port: 3.3V, B-Port: 1.2V

Part Number: SN74AXC4T774-Q1

Hi team,

In the SN74AXC4T774QPWRQ1 data sheet, 6.13 states "Switching Characteristics, VCCA = 3.3 +/- 0.3V",
in the table above, "tpd" when "B-Port Supply Voltage (VCCB)" is "1.2 +/-0.1V" is indicated as "min.0.5ns, MAX.8ns".


Could you tell us about this delay time and provide us with the following ① to ⑤?


By the way, a-Port 3.3V and B-Port 1.2V signals with a period of 25 MHz output from a-Port to B-Port is my customer's target.
They are trying to examine how much duty will collapse due to the difference between the rise delay time and the fall delay time.

① They imagined that "min.0.5ns, MAX.8ns" would vary depending on the temperature, is it correct? Is there any other factor that effects this?
②If the temperature is constant (assuming 25℃), is it correct to assume that the rise delay time and fall delay time are the same?
③If the delay times are different, how many ns will the rise delay time and the fall delay time be respectively?
④Could you tell us the rise delay time and fall delay time at "-20°C", "25°C", "55°C", "70°C".?
⑤ If you have a characteristic graph that shows the relationship between temperature and rise fall delay time, could you provide it?

Regards,

Ohashi

  • Hi Ohashi,

    Please see SN74AXC4T774-Q1: Switching Characteristics, thanks.

    Best Regards,

    Michael.

  • Hi Michael2,

    Thanks for your support.

    We have additional questions from the customer about the delay time calculation:

    (1) When VCCA=3.3V, VCCB=1.2V,  A port as input and B port as output.
    Is it correct to calculate the ROL from VCCA 1.1 V, VCCB 1.1 V" in "6.5 Electrical Characteristics" in the datasheet

    VOL/IOL = 0.25 V/3 mA = 83 Ω?


    (2) it is said that the delay time is determined by "td = 2RC (td: Delay time, R: Maximum output load, C: Maximum output capacitance)".
    Is this the "delay time" here the time from VCCI/2 of Input A&B to VCCI/2 of Output B,A  in Figure 7-2 of "7.1 Load Circuit and Voltage Waveforms" ? Same as the tPD in the datasheet?


    (3) in relation to (2) above, is the delay time obtained by "td=2RC" the sum of "delay time of the IC itself" and "rise/fall time"?


    (4) it is said that the delay time can be estimated with "td=2RC", but could you tell us why doubling the RC results in the delay time of the IC?

    Regards,

    Ohashi

  • Hey Ryoma,

    While Michael is currently out, I will do my best to answer your questions. 

    1. Yes this is correct. Also see this FAQ for more information.

    2. I believe 2RC calculates the rise time, not the propagation delay (which is generally specified in the d/s and not calculated). Your understanding of the Figure 7-2 is correct and is referencing to the tpd spec in the datasheet. 

    Regards,

    Jack 

  • Hi Guan,

    Thanks for your reply.

    To summarize:

    • transient time can be calculated by t_t=2RC and it differs with rising/falling
    • "propagation delay" is the delay for the IC  and it can't be calculated. It is speced in the datasheet.

    Right?

    Just one more question from the customer.

    Q: If the temparature and the load condition is not changing, will the tpd be same value when rising up and falling down?

    Regards,

    Ohashi

  • Hi Ohashi,

    To summarize:

    • transient time can be calculated by t_t=2RC and it differs with rising/falling
    • "propagation delay" is the delay for the IC  and it can't be calculated. It is speced in the datasheet.

    Right?

    They both have independent values but similar reasoning as the worst case (max) is given, if a value is needed for timing.

    Q: If the temparature and the load condition is not changing, will the tpd be same value when rising up and falling down?

    Yes, Tpd is as spec'd across temperature and rise/fall times is as spec'd for the same loading as shown in table 7-1 per the datasheet, thanks.

    Best Regards,

    Michael.

  • Hi Michael,

    Few more things to confirm.

    Yes, Tpd is as spec'd across temperature and rise/fall times is as spec'd for the same loading as shown in table 7-1 per the datasheet

    When you look up to figure 9.2 in the datasheet, I can see that tpd for rise time and fall time differs respectively. So, tpd for rise time and fall time might differs in the real condition, right?

    Questions:

    1. The tpd for rise time and fall time differs. But all the tpd for rise time will be the same. Same as fall time. Is this correct?
    2. tpd is spec'ed「MIN.0.5ns, MAX.8ns」in the datasheet. Will there be a possibilty that (A) is 0.5ns and (B) is 8ns? Or will the difference between (A) and (B) be almost the same for all of the condition?

    Asked Q1 since my customer is worried about the possibility of alternative tpd that causes unstable duty.

    Regards,

    Ohashi

  • Hi Ohashi,

    When you look up to figure 9.2 in the datasheet, I can see that tpd for rise time and fall time differs respectively. So, tpd for rise time and fall time might differs in the real condition, right?

    Questions:

    1. The tpd for rise time and fall time differs. But all the tpd for rise time will be the same. Same as fall time. Is this correct?

    Tpd and rise/fall times are independent. Please see Figures 7-2 and 7-3 as tpd is the delay or time shift propagating from the input to output or the output to input i.e A to B or B to A while the rise/fall times can be observed on the input signal.

    tpd is spec'ed「MIN.0.5ns, MAX.8ns」in the datasheet. Will there be a possibilty that (A) is 0.5ns and (B) is 8ns? Or will the difference between (A) and (B) be almost the same for all of the condition?

    Similar to Q1, Figure 7-2 indicates how propagation delay is measured and will be the same measured using either the rising or the falling edges of the input and output.

    Asked Q1 since my customer is worried about the possibility of alternative tpd that causes unstable duty.

    The time shift from the input to the output will be the same as tpd, thanks. 

    Best Regards,

    Michael.