TCA39306: Pull up resistor values

Koyo Asaishi

Part Number: TCA39306

Hello team,

I have some questions about I3C. TCA39306 will be used to change the voltage between 1.8V and 3.3V and speed is 12.5MHz.

TCA30306 have 12.5pF max of ON capacitance for SDAn. Does this mean both SDA1 and SDA2 have 12.5pF max and total parasitic capacitance at SDA is 25pF?
50pF is the max bus cap value that MIPI defined so just another 25pF is allowable bus capacitance?
Is there formula to decide suitable pull up resistor value?

Best regards
koyo

2 months ago

0 Tyler Townsend 2 months ago

TI__Guru 54798 points

Hi Koyo,

I will respond back tomorrow morning.

Regards,

Tyler

0 Tyler Townsend 2 months ago in reply to Tyler Townsend

TI__Guru 54798 points

Hi Koyo,

The Con spec includes both SDA1 and SDA2, the entire channel. 12.5pF is the max cap for both SDA1 and SDA2 together.

This app note applies to PU resistance for I2C:

https://www.ti.com/lit/an/slva689/slva689.pdf?ts=1720204726016&ref_url=https%253A%252F%252Fwww.google.com%252F

This will help solve for the pull-up resistor for I2C mode in the open-drain phase of I3C.

Regards,

Tyler

0 Koyo Asaishi 2 months ago in reply to Tyler Townsend

TI__Intellectual 2150 points

Hi Tyler,

Thanks for the answer.

What should be the rise time of 12.5MHz I3C? I can't find tr value in the MIPI I3C documents. If you have like below graph, it is very helpful.

Best regards
koyo

0 Tyler Townsend 2 months ago in reply to Koyo Asaishi

TI__Guru 54798 points

Hi Koyo,

The rise-time is more important for the 1 MHz open-drain phase of I3C.

The PU resistance calculations for open-drain mode I found in table 82. See the last row.

In table 87, I found the I3C push-pull timing requirements for SDR which includes rise and fall time.

Regards,

Tyler

0 Koyo Asaishi 2 months ago in reply to Tyler Townsend

TI__Intellectual 2150 points

Hi Tyler,

I'd found the tables but I don't clearly understand how can I calculate because tr/(0.8473*Cb) looks for I2C (legacy mode) and the calculation result is 189ohm if tr=8ns and Cb=50pF.

Could you calculte suitable pull-up resistor for me? Fscl=12.5MHz. Only TCA39306 is used on the bus to transfer between 1.8V and 3.3V.

Datasheet says pull-up resistors are needed on both side if bus voltage transition is required but no guidance to select the resistance for I3C.

Best regards
koyo

0 Tyler Townsend 2 months ago in reply to Koyo Asaishi

TI__Guru 54798 points

Hi Koyo,

The equation for tr/(0.8473*Cb) is used for calculating I2C open-drain phase.

When I3C push-pull phase kicks in, the rise/fall time spec's are required by the driver.

If your Host controller integrates the pull-up resistor for I3C, only the output side will need pull-up resistors.

I have included this FAQ that provides an excel calculator to help determine the correct PU resistor sizes when using LSF devices - similar to TCA39306.

[FAQ] How do I size my external RC components surrounding Auto-Bi Directional Translators to fit my System Requirements?

Please let me know if this helps.

Regards,

Tyler

0 Koyo Asaishi 2 months ago in reply to Tyler Townsend

TI__Intellectual 2150 points

Hi Tyler

Thanks. I will use the sheet for my reference. I don't know where the 2*3 value of B20 equation comes from but seems 130ohm may be the suitable value if Cb=50pF. Please kindly check if this is true. 130ohm is very small value than I expected.

Based on my simulation, 300~500ohm could be suitable value.

0 Tyler Townsend 2 months ago in reply to Koyo Asaishi

TI__Guru 54798 points

Hi Koyo,

I am adding Jack to this thread to help discuss the calculator.

Regards,

Tyler

+1 Jack Guan 2 months ago in reply to Tyler Townsend

TI__Mastermind 47111 points

Hi Asaishi-san,

Koyo Asaishi said:
I will use the sheet for my reference. I don't know where the 2*3 value of B20 equation comes from but seems 130ohm may be the suitable value if Cb=50pF. Please kindly check if this is true. 130ohm is very small value than I expected.

A pulse with a rise time that is equal to 1/3 of its pulse width is generally considered acceptable for a "recognizably square" output waveform. As we are calculating for Mbps (1MHz= 2Mbps), the value is also multiplied by 2, therefore it is 1/ (2x3xRC). I would suggest reducing Cload to below 50pF if possible if weaker pull-ups are desired.

In you waveform captured with the 300ohm, the output signal is not reaching 1V8 which would be the main concern.

Regards,

Jack

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TCA39306: Pull up resistor values