SN74CBTD3384C: query on propagation delay & small eye diagram capture

Leo Chow

Part Number: SN74CBTD3384C
Other Parts Discussed in Thread: LSF0108

Hi,

I'm having SN74CBTD3384C as a 5V to 3.3V translator in one of the schematic design. However, based on my testing, it seems that when I tried to send a signal at through the bus switch at 80MHz, the eye diagram of the signal is relatively small. I am suspecting the parasitic capacitance from the bus switch caused the eye to be small. I would need some help to interpret the specs of this part correctly.

From the specs, it is mentioned that the I/O capacitance is typical at 12.5pF. So when the switch is on, does that mean both side of bus switch (A and B) each has a 12.5pF, or only one side? I'm capturing eye diagram at 80MHz
From the specs, it only provides the propagation delay through the bus switch. How can I estimate the maximum operating frequency of the bus switch? And does this propagation delay changes when passing through a 3.3V signal (without translation) versus a 5V signal (with translation)?
the on resistance of the bus switch is given when the input is 2.4V. How can I use this info to estimate the on resistance when the input is at 3.3V or 5V? I would need this information to calculate the total signal path resistance under different condition. Is there any application note on this or graph on the bus switch on resistance?

over 6 years ago

0 Saminah C over 6 years ago

TI__Genius 9132 points

Hi Leo,

Please see details below for your query on SN74CBTD3384C:

1. When the switch is on, it means both sides of bus switch (A and B) each has a 6.25pF (Total 12.5pF I/O capacitance between A and B)

2. What maximum operating frequency are you looking for your use case? I can also help look at a level translator that will best fit your requirements.

3. Please see this application note "5-V to 3.3-V Translation With the SN74CBTD3384" that covers the Mechanics of 5-V to 3.3-V Translation - It shows the impact of an external diode on voltage translation.

Thanks

Saminah

0 Saminah C over 6 years ago in reply to Saminah C

TI__Genius 9132 points

Looping in the SLL team:

SLL team,

Please suggest a translator and return to INT-MSS after completion

Thanks,

Saminah

0 Karan Kotadia over 6 years ago in reply to Saminah C

TI__Genius 16735 points

Hi,

It looks like you are looking for a bi directional translator? Something similar to that would be the LSF0108.

Here is a series on how our LSF device operates. https://training.ti.com/introduction-voltage-level-translation-lsf-family?cu=1134826

Thanks!

-Karan

0 Saminah C over 6 years ago in reply to Karan Kotadia

TI__Genius 9132 points

Hi Leo,

Please take a look at LSF0108 and the link Karan shared and let us know if you have further questions.

Thanks

Saminah

0 Leo Chow over 6 years ago in reply to Saminah C

TI__Expert 3120 points

Hi,

Unfortunately engineers have decided to stay with SN74CBTD3384C as they are nearing pilot builds. Ideally maximum frequency we're dealing with would be 80MHz. Assuming I want to maintain good outputs which i manage to obtain at 40MHz, what would the estimated max frequency be to meet this criteria?

With respect to the app notes Figure 4 & 5, it only mentioned Vcc=5V & output obtain under ambient temp. I'm wondering if there are additional specs/information required to simulated the result as per graph plot?

http://www.ti.com/lit/an/scda003b/scda003b.pdf

0 Saminah C over 6 years ago in reply to Leo Chow

TI__Genius 9132 points

Hi Leo,

It is recommended to select a switch with bandwidth that is 3x the fundamental frequency.

For an 80 MHz signal, the switch should have bandwidth of at least 240 MHz. If it is below 240 MHz, you will notice signal attenuation.

You can use the device selection tool (link) to filter the switches that offer high bandwidth (240MHz or higher).

Figure 4 & 5 in the app note refer to the measurements taken at Ambient temperature and VCC at 5V. No additional specs are required to simulate the graph results.

Thanks

Saminah

Switches & multiplexers

Switches & multiplexers forum

SN74CBTD3384C: query on propagation delay & small eye diagram capture