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SN74CBTD3384C: Maximum frequency for different family bus switch

Prodigy 60 points

Replies: 11

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Part Number: SN74CBTD3384C

Hi,

I am referring to the "Digital Bus Switch Selection Guide", on page 5, the comparison table for different bus switch family, what are the factors or different in family that caused the different in maximum frequency? Most of app note mentioned that the maximum frequency is depending on the external load and capacitance.

Assuming I have the same load condition (R & C) applied to each family of bus switch, what is the architecture of bus switch that limits the maximum frequency? Eg. CBTD, CBT, CB3T, CB3Q. As the frequency varies a lot across the mentioned bus switch family.

Thanks.

  • Hello,

    Thank you for your question.

    The internal parasitic capacitances and resistances have an impact on bandwidth in addition to external load. These internal characteristics are determined by configuration, architecture, and features of the device, which vary depending on the family.

    Best regards,

    Kate

  • Hello,

    Just checking in that your question was answered. I am marking the case resolved, but please don't hesitate to reach out if you have any more questions!

    Regards,

    Kate

  • In reply to Kate Dickson:

    Hi Kate,

    Understand that it is the architecture of the family affects the parasitic capacitance and resistance, indirectly affect the bandwidth.

    My follow up question is

    1. the difference between CBTD and CBTD-C is the undershoot protection feature. But why the family with undershoot protection can support higher frequency, since I assume there are additional circuits for the feature, and should add more parasitic capacitance?

    2. If I compare CBT and CBTD, now the difference is the level shifting feature. However, the max frequency difference between this two families is even higher, which are 10MHz and 200MHz respectively, what could be the reason of it?

    3. Previously, you mentioned on parasitic will affect the bandwidth. Is it the same as the frequency specification? Or the max frequency specification has already factor in the bandwidth of bus switch? For example, if the max frequency supported is 10MHz, the actual bandwidth of bus switch might be 30MHz. Is that the case?

    Thanks. 

  • In reply to Lee Zhi Xian:

    I don't know about that application note, but the web product list shows the same bandwidth (20 MHz) for CBTD and CBTD-C. The CBTD-C indeed has a much larger Cio(ON), but lower ron.

    I guess the diode in the CBTD family reduces the gate voltage of the pass transistor.

    "Bandwidth" or "frequency" can be either an analog or a digital measurement. These bus switches are intended for digital signals, so I'd guess the specified values are for a square wave, and the analog bandwidth would be higher.

  • In reply to Lee Zhi Xian:

    Hello,

    Based on the different IP and architecture of these families, certain characteristics of the designs will impact the bandwidth of the devices.

    The bandwidth specification refers to the -3dB point of the frequency response.

    Best regards,

    Kate

  • In reply to Clemens Ladisch:

    Thanks, Clemens for mentioning the web product list and the bandwidth. It is a bit confusing sometimes, since different resource is telling a different number (eg. app note vs web product list).

  • In reply to Kate Dickson:

    Hi Kate,

    Referring to SN74CBTD3384C datasheet, under page 3, note 7, it mentioned high frequency, fast edge rate and multiple output switching will not have level shifting effect.

    1. The datasheet does not specify what frequency is considered as high frequency. In general, should I refer to application note or web product list for this number? And why it is not specified directly in the datasheet?

    2. Similarly, note 7 does not mentioned what is considered as the fast edge rate. I also couldn't find the fastest edge rate supported in the datasheet? So what is this number or where can I refer?

    Thanks.

  • In reply to Lee Zhi Xian:

    Note 7 appears to imply that a rising edge overshoots above 3.3 V for a short time.

    I guess the only way to find out how large this effect is in your case, and how it affects your circuit, is to measure it with an oscilloscope.

  • In reply to Lee Zhi Xian:

    Hello,

    For this device, the application note is a good reference for this information.

    Edge rate, also known as slew rate, is the change of voltage per unit of time. In this case, fast edge rate means a rate of 5V per 10us or greater.

    Best regards,

    Kate

  • In reply to Clemens Ladisch:

    Hi Clemens,

    Thanks for the info. How do you get the information where note 7 refers to the overshoot effect, so that in the future I know where I can get it too? :D

    Thanks for the advice, will verify it with the oscilloscope on the overshoot effect.