I2C Translator Design

[ApurvaAgre]

Hello,

In one of our projects, we have used the PCA9517AD,118. Attached is an image of our design. Could you please review it and let us know if there are any electrical or functional characteristics that need to be changed?

 Regards
Apurva Agre

[ApurvaAgre]

Hello Jozef,

Thanks for the reply. VDD_EXT is 1.8V and our I2C speed will be 400 KHz. The approximate length of the SDAA, SCLA, SDAB and SCLB PCB traces will be less than 10-15 cm.

Regards
Apurva 

[JozefKozon · Accepted solution]

Hello Apurva,

thank you for the information. 

For the A side for 400kHz, which is I2C Fast mode and for 1.8V the Rp(min) is approximately 0.4kOhm. 

The Rp(max) can be calculated from the formula (1). 

The maximum allowed rise time (tr) for the 400kHz is 300ns. 

The estimated bus capacitance (Cb) is 70pF.

Rp(max)=300ns/(0.8473*70pF)=0.0000003s/(0.8473*0.00000000007F)

Rp(max)=0.0000003s/0.000000000059311

Rp(max)=5058Ohm=5.058kOhm

The 2.1kOhm pull-up resistor value is correct for the A side. It is between the Rp(min)=0.4kOhm and Rp(max)=5.058kOhm.

For the B side the Rp(min) is approximately 1kOhm.

The Rp(max) can be calculated again from the formula (1) and the result is the same. 

Rp(max)=5.058kOhm.

The 1.1kOhm is very close to boundary Rp(min) 1kOhm value. I would recommend you to increase to some middle value between Rp(min) and Rp(max). Please use e.g. 3kOhm for the pull-up resistors on the B side.

With Best Regards,

Jozef

[ApurvaAgre]

Hello Jozef,

Thank you for the recommendations and support. This is very helpful to us.

Regards,

Apurva

[JozefKozon]

Hello Apurva,

the pull-up resistors seems quiet low to me. Depends on communication speed and bus capacitance. Please let me know the VDD_EXT voltage, the required I2C communication speed and approximate length of the SDAA, SCLA, SDAB and SCLB PCB traces and/or cables. So I can estimate the Bus capacitance and calculate the required pull-up resistor values. Please refer to the section 7.1 in the UM10204 attached.

Is there any issue with your application? 

With Best Regards,

Jozef