TCA9802: Whether TCA9546A can be located on B side

Mitsuhiro Hanabe

Part Number: TCA9802
Other Parts Discussed in Thread: TCA9546A

Tool/software:

In my trial board, TCA9546A is connected on B side of TCA9802.
Sometimes a slave device located in the opposite side of TCA9546A doesn't respond.... then I found my circuitry fully matches the figure below if TCA9546A is one of "FET Based" device expressed here.

Is it the case?
If that's the case, what kind of incorrect behavior can happen on the bus line?

over 1 year ago

0 Tyler Townsend over 1 year ago

TI__Guru 59108 points

Hi Mitsuhiro,

TCA9802 uses a current detection method on B-side to determine the direction that a LOW is being passed either from B-to-A or A-to-B.

In order for this to work, the current mirror inside the device is sensitive to external sources of current including pull-up resistors, leaky esd cells, etc. See I-EXT-I and I-EXT-O specs in the datasheet.

Because a FET based architecture requires pull-up resistors on both sides for the device to work, the pull-up resistors on the slave side in the figure you attached are seen by the TCA9802 which could interfere with the current detection mechanism of the buffer.

If you can't change your setup, you could try to weaken the pull-up strength on the slave side making the pull-up resistors larger to reduce the I-EXT current.

Regards,

Tyler

0 Mitsuhiro Hanabe over 1 year ago

Prodigy 30 points

Hi Tyler

Thanks a lot for your prompt answer. It helped me to understand the mechanism very well.
Let me make sure just in case, is TCA9546A a component having a FET based architecture which we have to take care of?

Best regards,
Hanabe

0 Tyler Townsend over 1 year ago in reply to Mitsuhiro Hanabe

TI__Guru 59108 points

Hi Mitsuhiro,

Yes. Please see the functional block diagram of the TCA9546A:

This is FET based architecture and requires pull-up resistors on both sides of the device in order for it to work.

Regards,

Tyler

+1 Mitsuhiro Hanabe over 1 year ago in reply to Tyler Townsend

Prodigy 30 points

Hi Tyler

Thanks, it's clear!

Best regards,
Hanabe

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TCA9802: Whether TCA9546A can be located on B side