SN74CBTD3384C: Maximum frequency for different family bus switch

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

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. 

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 C_io(ON), but lower r_on.

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.

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

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).

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.

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.

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

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.

Hi Kate,

Just wanted to make sure I am understanding it correctly, if the edge rate refers to 5V per 10us, this means my signal rise/fall time minimum has to be 10us to go from 0V to 5V. If that is the case, even with the edge rate, (1/ 10us), the fastest signal rate I can go will be 100kHz. 

Is this the specified edge rate for this part?

The datasheet does not show a specified edge rate for this part.

As Clemens mentioned earlier, a good way to determine if the level shifting effect is impacted in your application is to use an oscilloscope to measure the output of the switch.