TCA9803: Which VCC side, side A or side B should be connected to I2C master device?

Kazuya Nakai54 said:

Part Number: TCA9803

 Hello guys,

 One of my customers is consideitng using TCA9803 for their next products.

  They want to get any answer to the following questions. Could you please tell me your answers to the questions? 

Q1, Which VCC side (side A or side B) should be connected to I2C master device?

       If the master device I/O ports working voltage is 2.5V, slave device working voltage  is 3.3V, should the master device be connected to VCCA side because VCCA operation voltage range (0.8V~3.6V) 

is lower than VCCB one?

Our device does not know/care what side the master/slave is on. Both sides support 2.5V and 3.3V so this does not matter.

Q2. What factors or conditions determine which power side to connect to the master device?

A side support down to 0.8V and B side supports down to 1.65V but because you are above both values, this does not matter.

Q3. What are main different points between TCA9803 and TCA9517A? Are those working voltage range and internal pull-up current source on side-B?

9517 uses a static voltage offset which is different from 9803's static current offset. The VoL on B side of 9803 is lower than the VoL of 9517. 9517 has different voltage rules on VccA and VccB as well. B side does not support 2.5V on the 9517 device. B side of 9803 has a system requirement where it does not want to see any external current sources like a pull up resistor.

 Your reply would be much appreciated.

 Best regards,

 Kazuya.  

-Bobby

Hi Kazuya,

There is no requirement for the I2C master to be on a particular side of TCA9803. The buffer is bi-directional and will be able to repeat communications from either side of the device. Similarly in a voltage translation application, it does not matter which side is at the higher voltage as long as the respective recommended supply voltages are used. Note that the B-side of TCA9803 has a higher low-level output voltage (Vol). You may consider using this side for devices with more tolerant low-level input voltage levels so the lower output level of the A-side may be used for more sensitive inputs.

The supply pins for TCA9803 should be connected to the supply rails providing the voltage for their respective I2C bus. Example layouts for this can be seen in figures 16 to 20 on the data sheet. Note that the B-side has internal current sources that eliminate the need for external pull-up resistors.

Some of the characteristics of TCA9517A are the use of a Static Voltage Offset (SVO) to prevent contention instead of the internal current source used by TCA9803, a supply voltage restriction that requires VccA <= VccB, higher maximum operating frequency of 1000kHz, and higher low-level output voltages (Vols) on both A- and B-sides.

I hope this helps answer the questions that your customer has. Let me know if there is any follow-up or if you may like more information about this or other I2C buffers.

Regards,

Eric

 Hello Bobby, Eric,

 Thank you very much for your reply. I could undestand TCA9803 and TCA9517A features and functions well because of your replies.

 Could I ask you an additional question as the following?

 Q. Why is VOL of TCA9803 side B higher than side A one?

     What is problem in case that VOL of TCA9803 side B same as side A one?  

 Thank you and best regards,

 Kazuya.

"Q. Why is VOL of TCA9803 side B higher than side A one?

What is problem in case that VOL of TCA9803 side B same as side A one?

"

These two questions are basically the same thing. B side of the TCA980x was designed such that I2C compliant devices are expected to be able to drive a low from B side to A side during contention situations (such as an ACK hand off). If the VoL on B side were lower, it would require the master/slave on B side to have a stronger pull down driver in order for the low detection on B side to work properly. Essentially the higher VoLB is, the weaker the master/slave's drive strength needs to be to pass a low from B to A during hand offs.

Static voltage offset buffers (like TCA9517a) work in a similar fashion, the VoL is larger on the offset side in order to prevent the device from locking up and to allow for an external driver to be able to pass a low from that side during contention/hand-offs.

-Bobby