TCA9539-Q1: Cross current through SDA / SCL ports via pullups

Raphael Schroth

Part Number: TCA9539-Q1
Other Parts Discussed in Thread: TCA9538

Hello,

is there a riskt of cross current when power supply of the TCA9538 and the pull up resistors of the I2C interface aren´t on the same power domain?

Intention is to use several I2C slaves on the same bus like the TCA9538. Due to functional reasons the slaves and the MCU are on a continous power domain and has to be functional any time, but the port expander is supplied with a power domain, which can be switched of. So is there arisk to cross current into the switched power domain, via the pull up resistors?

Best regards

Raphael

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 265780 points

No. The absolute maximum ratings and recommended operating conditions allow the voltage at the SCL/SDA pins to go up to 5.5 V, regardless of the value of V_CC.

0 BOBBY over 5 years ago in reply to Clemens Ladisch

TI__Guru 55850 points

Hey Raphael,

Can you draw a block diagram for me so I fully understand?

ATM this is how I am thinking of it.

In this example if VccA is GND and the pull up resistors were both equsl then SDA/SCL would be a voltage divider and voltage would be (Vcc1)/2.

Is this block diagram correct?

-Bobby

0 Raphael Schroth over 5 years ago in reply to BOBBY

Prodigy 40 points

Hello Bobby,

please find the scheme of the situation.

In fact two different power supplies will be used for the I2C participants, but only one pull up towards the continous power supply will be placed. By this a cross current issue can occure for the switched power supply, when this is switched off. In that case other systems on this power domain might tend to malfunction because of residual voltage levels..

Question is, are the SDA/SCL ports influenced from weak or parasitic connections to VCC pin? Sometimes, when internal protection diodes are used to deliminate excessive signal levels such a cross current situation will occure.

0 Raphael Schroth over 5 years ago in reply to Raphael Schroth

Prodigy 40 points

0 BOBBY over 5 years ago in reply to Raphael Schroth

TI__Guru 55850 points

Hey Raphael,

Thanks for clarifying. In this case, Clemens is correct. Our device is to be high impedance when powered off so the switched power rail should remain GND and also tolerant of a voltage on SDA/SCL while Vcc is powered off.

Thanks,

-Bobby

0 Raphael Schroth over 5 years ago in reply to BOBBY

Prodigy 40 points

Hello Bobby, Clemens,

Thanks a lot for clarify.

Best regards

Raphael

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TCA9539-Q1: Cross current through SDA / SCL ports via pullups