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TMUX1574: What is the maximum tpd @ 3.3V?

Part Number: TMUX1574

Hi,

TMUX1574 datasheet is only specified typical tpd value @3.3V 25degC.

Customer would like to know whether TMUX1574 maximum tpd @3.3V is less than 150pS or not?

Please advise!

Thank you!

Tiger

  • Hi Tiger,

    Which package is the customer using? 

    Also there isn't a max tpd because the load greatly affects this spec.  Can you give me the loading condition, i.e. load resistance and capacitance?  That will help me give you a more solid answer if you can expect the tpd @3.3V to be lower than 150ps. If you also know the operating temperature of the application it will also be helpful!

    Best

    Parker Dodson

  • Hi Parker , 

    Sorry to interrupt your communication with Tiger , I`m Jerry working for MSI in charge of this question 

    We are going to use the TMUX1574 for SPI signal switch by NIST controller chip

    Just get the draft schematic by NIST vendor , no capacitance but stuff 33ohm between SPI ROM and TMUX1574 

    Sorry for insufficient information , i know that maybe the speed will be impacted by the distance also

    But if using TSSOP package with 25 degrees Celsius , should you reference those information to solid answer of tPD ?

    Thank you very much !

  • Hi Jerry,

    The prop delay of the wire is also going to add to the total prop delay, 1/2 * R_wire * C_wire. The PW/TSSOP package plus switch, when loaded by a 50 ohm resistor has a prop delay of 95 pS at 25C. The prop delay from the switch itself  ~ 1/2 * R_on * C_On is not greatly affected by temperature as max approximated prop delay is going to be 1/2 * 4.5Ohms * 12pF ~27 ps prod delay, with the packaging causing the rest of the prop-delay. 

    If operating around 25C with a 33 Ohm load using the TSSOP package, I don't think there would be much of an increase on prop delay, if any, from the typical value in the datasheet. The caveat here is that longer wires are going to slow down propagation times.

    Please let me know if you have any other questions.

    Best,

    Parker Dodson

  • Hi Parker ,

    Sorry for listing the query for you , but the confusion will be more easy to clarify for me , thanks

    1. Could you inform me how long is the distance by a 50 ohm resistor with a prop delay of 95 pS at 25C ?

    Just in cast , i could refer the distance by layout evaluation . And depend on your experience , how long do you recommend by operating around 25C with a 33 Ohm load using the TSSOP package ?

    2. Just curious , how would you get the value by temperature as max approximated prop delay ?

    3. We chose the SN74CBTLV by the vendor recommend at the first time , but we checked the SN74CBTLV datasheet , it has Max value of tDP , why..?  

  • Hi Jerry.

    1. The measurement points for the test are measured at the input of the IC and on the output. The load is calibrated to be 50 ohms during testing. Length has be included in the 50 ohm measurement. That being said tests like this are typically done by placing the resistor as close to the pin as possible to not add much delay to the circuit. 

    At 25C I'd recommend placing the load resistor as close to the pin as possible to shorten length. At those operating conditions and with a smaller load however I don't think you will have a major problem. The traces impedance is what is going to affect the prop time, best practice is to have a short trace connected directly to load, wider traces will lower resistance but increase capacitance and vice versa. 

    2. We could only get max Tpd for one load. To do that the operating temperature has to be at max, this is the worst case for resistance of the IC. Temperature is what is going to affect Tpd beside load, as temp increases the silicon increase its resistance. This effect isn't seen as much in the package because metal doesn't change its resistance as much under higher temperatures. Based on the operating conditions of 25C with a 33 Ohm load, as long as you use as short as possible traces between load and IC, I don't think you will go above 95ps.

    3. We have a few SN74CBTLV parts, could you tell me which one you are referring to so that I can see what you are talking about?

    Best,

    Parker Dodson

  • Hi Parker ,

    Very appreciated for your detailed explain , I got the situation .

    Otherwise , reference the item 3 that you mentioned is SN74CBTLV3257PWR .

    The SN74CBTLV3257PWR datasheet has the maximum value of the Tpd , and just curious that why it could appear the value on maximum ?

    Because of any situation are affected for the prop time , e.g. , temperature  , distance , resistance and capacitance ...

    Thanks !

  • Hi Jerry,

    So this is from an older part, and we have since updated our newer parts without spec-ing a max because of all the variables. 

    For this part the prop delay is calculated based on the conditions in note 1. They have the following system set-up:

    Where Ron is the typical switch resistance. So the "max tpd" in this case is the theoretical maximum calculation.  

    Best,

    Parker Dodson

  • Hi Parker ,

    Thank you for your all help in this series discussing 

    We got it and appreciated !