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PCA9515A: Level shifted GND from level shifting IC's

Yashveer Ramparsad
Prodigy 50 points
Part Number: PCA9515A
Other Parts Discussed in Thread: ISO1540, PCA9515, PCA9306, TCA9509

Hi, 

On the particular design that is being worked on, there are a mixture of 3V3 and 5V signals and IC's. These IC's are controlled by I2C. Now unfortunately not all the devices of similar voltage levels can be populated on one board. The problem that is being experienced is the level shifted ground arising from shifting between 5V to 3V3 and then back again. 

Take the PCA9515 which adds a 0.4V offset off 0V (GND), then in this design there is also an opto-isolator ISO1540 which also adds another 0.4V, unfortunately on the same board there is yet another level shift which results in a further shift away from ground. Yes, this is not ideal, but the IC's that are being worked with are at the lowest voltage available. 

What are the resolutions to such an issue of shifting in GND when using multiple level shifting IC's?

As it stands now, I2C devices fail to communicate because of the level shifted GND signals etc.

Any help appreciated. 

  • 0
    TI__Mastermind 23116 points

    Hello Yashveer,

    How much of a capacitive load do you have?  You could go with an unbuffered solution that uses a pass FET like the PCA9306.  Do you have a schematic of the system?  We do have some buffers that lower static voltage offsets, like the TCA9509.  It is available in the DGK package and is pin for pin compatible.  You may have to removed pull up resistors on A side because it has an internal constant current source to pull up the bus.  There is also a requirement that Vccb > (Vaa +1V), which your application satisfies. 

    -Francis Houde

  • 0 in reply to fhoude
    Prodigy 50 points

    Hi Francis, 

    Thank you for the reply, will look at those IC's you suggested.