TCA6416A: I/O expander with bus isolation

alex a said:

Part Number: TCA6416A

Hi Support Team,

In our application (battery powered), we have multiple boards, the host μC resides on board A.
on Board B, I have I/O lines for status monitoring and control and I was looking at TCA6416A.

Now, if I understand correctly, VCCI can be powered ON while VCCP is OFF, this keeps the TCA6416A in RESET state 

[Bobby] I've seen on a rare occasion where this caused our device to latch SDA low until VCCP powered up. Testing seemed that this occurrence was dependent on the slew rate of VCCI (where anything slower than .5V/ms caused this to occur). For this reason, I would not suggest doing this.

#1 is it possible to power VCCP while VCCI is NOT powered?

There is a scenario where board B is powered UP meanwhile board A and μC is powered OFF, and the concern here is power back feeding to the μC.
For protection, my first thought is to use a switch, i.e. SN74CB3T3245 in front of the TCA6416A 

[Bobby] Yes it is possible. The GPIOs should remain HI-Z and applying power (while not violating the absolute ratings) should not back bias the part. Adding the switch isn't necessary.

#2 what is correct configuration for unused pins for lowest current consumption?

Configure as INPUT and 
  a) tie HI directly [Bobby] Not recommended in the case where you accidently set the GPIO to output low.
  b) tie LO directly [Bobby] Not recommended in the case where you accidently set the GPIO to output high.
  c) tie HI with pull-up [Bobby] This is best.
  d) tie LO with pull-down [Bobby] Biasing high is more efficient for lower leakage current.

leave floating and configure as OUTPUT HI? [Bobby] This is best.
leave floating and configure as OUTPUT LO? [Bobby] Biasing high is more efficient for lower leakage current.

 

#3 can you please explain main difference between TCA6416A and TCA9535/TCA9555 (with internal pull-ups) ?

The TCA64xx series allows for level translation on the P-Ports so that your VoH can be different from your VCC on the I2C logic. (like interfacing with devices running on 1.8V logic through I2C comms but you want to be able to drive your GPO to a higher biasing voltage like 5V for switching on a mechanical relay where 1.8V wouldn't cut it)

The TCA9535 is basically the TCA6416A but without level translation in applications where your GPO VOH can be the same as your I2C voltage. The other notable difference is more hardware configurable address pins than the 64xx.

TCA9555 is the same as TCA9535 but includes pull up resistors. This can help in some scenarios where space savings may be a concern so you want integrated pull up resistors. In other cases where you want to bias a node low, this could hurt you because you would then you would need to populate a pull down resistor and there would be a leakage path from the weak pull up to the external pull down. (important for battery powered end equipments).

Best,
AJ

There is a scenario where board B is powered UP meanwhile board A and μC is powered OFF, and the concern here is power back feeding to the μC.
For protection, my first thought is to use a switch, i.e. SN74CB3T3245 in front of the TCA6416A 
μC 

Comments above in blue.

-Bobby