LTC4313 cascade with passive translator

Hi DuongNg,

   This probably will work, but our ability to support competitor parts in this forum is limited.

    Looking at your system block diagram, I don't see the need/reason for including the level shifter since all three supply voltages appear to be 3.3V.

   This type of level shifter part adds a smallish series resistance from the Rds(on) of the pass transistor which will increase the shifter's output low voltage.  This can be problematic if some I2C devices have a higher output low voltage and others have a low maximum input low threshold.   Since it doesn't appear that you need to level shift between different bus voltages, in the best case this level shifter is not causing issues/problems (but isn't actually adding any benefit) and in the worst case the additional series resistance could cause problems for the bus operation.  Generally, I'd suggest removing parts which don't add value to simplify the design, reduce cost and avoid potential sources of problems.

   Note that the LTC4313 includes a level shifting function along with its bus buffering and rise time accelerator functions.  Thus, if there is a need for level shifting which isn't clear from the presented block diagram, its possible that the LTC4313 could provide that function without increasing the BOM / PCB area / cost.

   I also note that you have a stronger 1k pull-up resistor between the MCU and the LTC4313-1.  This resistor will certainly work, but with the 40mA rise time accelerator current source of the LTC4313-1, you probably could use a much larger resistor value here.  I don't know the effective bus capacitance of this segment (e.g. MCU Cbus, parasitic bus C, C of other potential devices, etc.), but as long as the resistor provides a large enough dv/dt to trigger the accelerator you should be able to increase the value of this R which will save power.   See resistor sizing equations on page 9 of the datasheet.

  Finally, as a system-level design check, you may want to confirm the output low voltage levels at the different bus segments when different devices are pulling low to make sure that all devices properly see a sufficiently low input voltage regardless of the device pulling the bus low.  Note that the LTC4313 bus buffer does add a small offset voltage which depends on the input low voltage as well as the pull-up current on the output side.  Page 9 of the datasheet explains how to calculate the offset voltage.

Eric