Question of TCA9546 3.3V to 1.8V Translation

Wayne Chen (Audio AE)

Hi Sirs,

We are supporting Quanta to design a 3.3V to 1.8V I2C level shifter with MUX as the following block diagram (project name: S1U):

According to the datasheet, it requires at least 5V for 3.3V Vpass:

However, the VIH(min) of the SDA and SCL is 0.7VCC = 3.5V, which is not compatible with 3.3V I2C on the left.

Would you pls advise how to calculate the inimum VCC requirement in this case?

Thank you and Best regards,

Wayne Chen
12/15/2015

over 9 years ago

0 Jonathan Valdez over 9 years ago

TI__Genius 16510 points

Hello Wayne,

The VCC for the device in your case should be 1.8V. It should be the lowest voltage of all the busses connected.

The graph you are referring to shows the minimum voltage that is supported on the I2C line for a given VcC. At 5V Vcc, the minimum i2C village is 3.5V

0 Wayne Chen (Audio AE) over 9 years ago in reply to Jonathan Valdez

TI__Genius 15950 points

Thank you for your advise, Jonathan.

Can we use 3.3V VCC in this case? It looks functional because 1.8V I2C is still within the 3/3V VCC specification. Am I right?

Wayne Chen
12/17/2015

0 Jonathan Valdez over 9 years ago in reply to Wayne Chen (Audio AE)

TI__Genius 16510 points

Hello Wayne,

You can to do this, however it is not recommended because you will be taking away all of your margin. You will have to ensure that the bus will go all the way to 1.8V, and not any less. In other words, if VCC = 3.3V and the SDA line is at 1.8V, the mosfet will have additional leakage through it because the VGS voltage is still decently large. This leakage will be minimal, and the device should function, but I would recommend you use a 1.8V VCC if you can, as this will give you much more margin on the VIH.

0 Wayne Chen (Audio AE) over 9 years ago in reply to Jonathan Valdez

TI__Genius 15950 points

Hello Jonathan,

Do we have an I2C MU device with two VCC pins for voltage translator as the NXP PCA9846 does?

Thank you and Best regards,

Wayne Chen
01/21/2016

0 Jonathan Valdez over 9 years ago in reply to Wayne Chen (Audio AE)

TI__Genius 16510 points

Hello Wayne,

I replied to your other thread.

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Question of TCA9546 3.3V to 1.8V Translation