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TCA9406: VOH_min issue

Part Number: TCA9406

Hi Experts,

My customer would like to clarify an VOH_min issue of TCA9406 BiDir translator, 
which is to be used as an I2C translator between 3.3V bus and 5V bus.
(thus VCCA = 3.3V, VCCB = 5V).

The I2C ICs on both sides have (usual) I2C definitions
of their VIHmin= 0.7xVCC which gives
VIH_min = 2.31V for 3.3V bus,
VIH_min = 3.5V for 5V bus.

However, according to the TCA9406 datasheet, the table in section 6.5
Electrical characteristics specifies minimal output high
voltage of the TCA9406 to be 0.67xVCC, resulting in
VOHA_min = 2.211V for 3.3V bus (< VIH_min = 2.31V), 
VOHB_min = 3.35V for 5V bus (< VIH_min = 3.5V). 

Which means that TCA9406 output high voltage can be lower
than the minimal input high voltage for the ICs on
the bus and thus a reliable operation is not guaranteed.
Given that a lot of I2C ICs use VIHmin= 0.7xVCC it seems
to our customer that TCA9406 is pretty much unusable (and 'it should
work' or 'it works most of the time' can't be used as 
an argument).

Customer questions: 
Does customer understanding this incorrectly or overlooking something?
Given that the source of VOH should basically be the pull-up resistors,
can this be improved by adding external pull-ups to reduce the
overall pull-up resistance?

Please advise. Thank you so much in advance.

Kind regards,

  • The specified VOH is measured with an output current of 20 µA. Your actual leakage current will be much lower.

    (And with such a voltage drop, the internal pull-up resistor would have more than 50 kΩ. This is probably unrealistic.)

  • Hi Gerald,

    The VOH that you see in the datasheet is the minimum guaranteed voltage that the rise time accelerator will drive the bus to. Once the accelerator is finished driving the bus high the pull up resistors will continue to pull up the bus to their pull up voltage. This device has internal pull ups so it will automatically pull the bus high unless you have it disabled.