TXB0104: power coupling to VCCB

Howard Zou

Genius 12365 points

Part Number: TXB0104
Other Parts Discussed in Thread: TXS0104E

Hi,

my customer is using TXB0104 with VCCA=1.8V and VCCB=3.3V.

Sometimes they will have condition that 3.3V is applied to B1 but VCCB is not powered up.

When OE is enabled, they will see 3.3V coupling to VCCB.

When OE is disabled, they will not see 3.3V coupling to VCCB.

We would not like 3.3V to be coupled to VCCB.

We would like to know why "when OE enabled, 3.3V is coupled to VCCB, and when OE disabled, 3.3V is not coupled to VCCB".

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 316380 points

The TXB disables all outputs when OE is low, or when a supply is at 0 V (see section 10 of the datasheet).

Apparently, V_CCB is not at 0 V but floating. In that case, a small charge is enough to power up the V_CCB side and active the clamping diodes (see [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?).

To prevent the coupling, set OE low, or force V_CCB to 0 V.

0 Howard Zou over 4 years ago in reply to Clemens Ladisch

TI__Genius 12365 points

Clemens,

My understanding is that the internal clamp diode to VCCB is always there, how can it be disabled by pulling OE low?

Is it possible that you could share some internal schematic to help me understand it?

Thanks.

0 Clemens Ladisch over 4 years ago in reply to Howard Zou

Guru 316380 points

You'd have to ask a TI employee for that …

0 Dylan Hubbard over 4 years ago in reply to Clemens Ladisch

TI__Mastermind 45635 points

Hey Howard,

I can't share any design schematics, but the OE circuitry will isolate the I/O pin from the diode. You can ascertain this from the spec below:

0 Howard Zou over 4 years ago in reply to Dylan Hubbard

TI__Genius 12365 points

Dylan,

I'm not sure about it, the table you show doesn't tell the voltage applied to any output when VCCB=0, but only VCCB=1.65 TO 5.5V.

Another question is:

with TXS0104E, we can still see 3.3V voltage coupled to VCCB when OE disabled, why?

0 Dylan Hubbard over 4 years ago in reply to Howard Zou

TI__Mastermind 45635 points

Hey Howard,

It says in the description, "or power-off state", which indicates this. Are you able to show this in a scope shot?

0 Howard Zou over 4 years ago in reply to Dylan Hubbard

TI__Genius 12365 points

Do you mean capturing VCCB, OE and B1 of TXS0104E using a scope?

0 Dylan Hubbard over 4 years ago in reply to Howard Zou

TI__Mastermind 45635 points

Hey Howard,

Yes that would helpful for me to see.

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TXB0104: power coupling to VCCB