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THVD2410: [Vth]discuss in RS485

Allen Lu
Intellectual 2580 points
Part Number: THVD2410
Other Parts Discussed in Thread: THVD1400,

Tool/software:

Hi Team:

 i know to config out why the 485 device have different Vth setting, so i compare THVD2410 VS THVD1400 here.

THVD1400:

THVD2410:

First, for THVD1400, i summary like below VID w/ Vth±：

base on it ,we can know when the A=B=0V, The Vid=0V>-45mV, and make the R output 1, and the -45mV maximum value ensures that even under the worst process, VID=0V still has at least 45mV noise margin (0V-(-45mV)=45mV).

so what about the VID range of -150~-70mV? what s the R output behavior?

For THVD2410,  it will have fail-safe mode to make sure the Vid range (-40~40 ) will make R output 1.

and for the Vth+ and Vth-, does it also get the R=0 when Vid<-40mV?  and R=1 when Vid >40mV?  or  we should follow below ±125 threshold? but what about the Vid drop into the range of (-125~-40) and (40~125)?

tks for the comments here.

  • 0
    Guru 317480 points

    For the THVD1400, for VID between −200 mV and −45 mV, the output is undefined.

    For the THVD2410, for VID between −200 mV and 200 mV, the output is undefined. For VID between −40 mV and 40 mV, after tD(OFS), the output is high.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2580 points

    Hi Clemens ,

     tks for the comments here. it is clear of the behavior now.

     but due to the Vid may have chance cross over this range and our Vth± also have a range, so  we need to take care of it with our Vhys too. so what the suggestion when we do not have the pullup & pulldown R for the bus here?

    for example: there may be 4 status like ①②③④ may cross over or go through it on THVD2410,

    base on ±125mV typ Vth discussion:  i think there is no need for pull up/down R here. 250mV Vhys can cover the -125mV~ 125mV undefined range. am I correct?

     

    but base on the ±200mV max Vth discussion:  i think there still have a undefined range like : 

    -200mV< R=X <-50mV

    50mV< R=X < 200mV

    so this is the risk for no pull up/down R here. am I correct?

    if add a pull up/down R here, can you help to check the below setting correct or not for THVD2410?

  • 0 in reply to Allen Lu
    Guru 317480 points

    When there are no fail-safe biasing resistors, then the termination resistors will short the two bus lines, and VID = 0. In that case, for both THVD1400 and THVD2410, the output will be high.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2580 points

    hi Clemens ,

     it is clear of the Vid =0 behavior of  THVD2410 here.

     but we are still confuse the Vth 40~125mV here. i know that maybe some of devices may have 40Vth± or 200Vth± by batch error. but they are sill satisfy with the datasheet. 

     so combine the Vth as 40 / 125/ 200 and add Vhys as 250mV. there would be total different.

    Vth as 125mV typ, it is clear for all the R status

    Vth as 200mV typ, it will have undefined range as  -200mV< R=X <-50mV and  50mV< R=X < 200mV, correct?

    Vth as 40mV typ, How we handle the (Vth+) - VHYS = 40mV-250mV =-210mV > -200mV when Vid change from high to low? The R will turn to 0 at -210mV or -200mV?

  • 0 in reply to Allen Lu
    Guru 317480 points

    For any individual chip, the output is never undefined. ("Undefined" means that different chips might have different outputs.)

    In the first example, the output switches from low to high at 125 mV, and from high to low at −125 mV.
    In the second example with 200 mV, the output switches from low to high at 200 mV, and from high to low at −50 mV.
    In the second example with −200 mV, the output switches from low to high at 50 mV, and from high to low at −200 mV.
    The third examples cannot happen; VTH+ is never larger than 200 mV.

  • 0 in reply to Allen Lu
    TI__Guru* 75011 points

    Allan,

    It looks like Parker answered your question through an internal email.

    But ultimately, the hysteresis value is a typical number. Which means that it was derived by taking the typical the typical Vth+ and subtracting it from Vth- (so 125mV--125mV =250mV). The worst case scenario would be 40mV--40mV making up 80mV in theory however these max and min values come with guard bands so you shouldn't actually get a device with these limits. 

    It's easier to think of it as Vth+ and Vth- are designed around a specific number (the typical values) but these numbers can move due to process variation and temperature. You will not know this value unless you test each device. Some may have Vth+ at 150mV while others may be 100mV. But if you cross this value, then R will be high. We set the 200mV max as a way to guarantee if this value is crossed the R will definitely be high. 

    -Bobby