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TCA6408A: Operation with no VCCi

Mark Guastaferro
Genius 11645 points
Part Number: TCA6408A

Customer needs to support a scenario where the Master I2C controller device goes into a standby or sleep state while the TCA6408A maintains the existing IO state on its GPIO pins. We believe this can be accomplished by making sure VccP is maintained above POR voltage and RESET voltage remains logic HI.

In addition, the customer also wants to minimize leakage paths in this condition.  There is a concern that the I2C output of the Master device may not remain Hi-Z when in sleep/standby state and would lead to a current leakage path from the SDA/SCL pull-ups. One idea is to place a switch in the Vcci path to remove the power supply from both the TCA6408A and the I2C pull-up resistors as shown below: 

Is there any concerns about this idea? For example, the SDA/SCL inputs could be floating in this case which is usually not good for CMOS inputs. But since there is no Vcci, maybe the SDA/SCL input buffers not powered in this case so floating inputs would not matter?  

An alternative idea is to add an I2C buffer/switch between the Master and TCA6408A to isolate the SDA/SCL interface when the Master goes to sleep/standby. This would add cost and power and is not preferred. 

  • 0
    TI__Genius 15345 points

    Hi Mark,

    The RESET pin is referenced to VCCI. If VCCI gets disconnected the device will reset and the p-ports will be Hi-Z again. I think a simple solution to this would be using a switch further along the VCCI path so that it is always supplying power to the TCA6408A but just the I2C signals are disconnected:

    I don't think there would be any problem with the SDA and SCL pins floating. Obviously if they are floating the device won't function but since you are expecting it not to function that is not an issue. Having the pins float won't damage the device. If you are concerned about the SCL and SDA pins floating you could add extremely weak pull downs (300K +) to the SCL and SDA pins. When the VCCI gets disconnected they would be pulled to GND and when it is connected the strong pull up would power the pins.

    I am going to double check this solution to make sure I am not missing anything.

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Genius 11645 points

    Hi Chris,

    Thanks for the quick reply.  One comment about your response. The customer's idea has the RESET pin still connected to VCCi.  The Vcci suppl yis being switched out after the RESET connection and removes Vcci from the I2C pull-ups, INT and the TCA6408A. RESET would still be pulled-up by Vcci. My understanding is the RESET input is powered by VCCP and Vcci loss does not trigger a device reset.

    The concern with floating SDA/SCL inputs is if this would increase quiescent current due to the input stage (Delta ICCP). Weak pull-downs may be a good idea unless input stage operates from Vcci and floating inputs doesn't matter. 

  • 0 in reply to Mark Guastaferro
    TI__Genius 15345 points

    Hi Mark,

    The RESET pin is still referenced to whatever the input on the VCCI pin is. It looks like, in the original circuit you sent, the switch would disconnect VCCI from the VCCI pin. This would give the RESET pin no reference voltage and would force a reset on the device:

    VCCP does not control the RESET pin unless you are talking about a power on reset. That would be a different case.

    Could you clarify what you are referring to when you say input stage?

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Genius 11645 points

    Hi Chris,

    Thanks for clarification.  Based on the block diagram and some descriptions in other posts, it seemed the reset function (both RESET pin and POR) are powered by the VCCP supply. I read in one post that removing Vcci from the device would not lead to a POR so I assumed it wouldn't lead to a RESET response if the RESET pin voltage is maintained.  But if the RESET state is dependent on Vcci, you effectively have a POR when Vcci is removed.  Is this correct? 

    Moving the switch to just isolate the pull-ups as you suggested will avoid this situation so I will recommend that. 

    By input stage, I am referring to the SDA/SCL inputs in the TCA6408A. If we have floating inputs on these pins when Vcci is present, I assume there can be increased Icci current.  My original question: is this still a concern if Vcci to the device is removed? But if we decide to keep the Vcci connection to the TCA6408A,  then the question becomes is there added Iq when these inputs float as I assume? 

  • 0 in reply to Mark Guastaferro
    TI__Genius 15345 points

    Hi Mark,

    You are correct in saying that the POR function is powered by the VCCP supply. In fact in our old TCA6508 device I believe the RESET pin was referenced to VCCP as well.

    A POR is just a function of the device so that whenever you cycle power you reset the values of the internal registers of the device. This is different then when you do a pin reset on the device by just driving the RESET pin low for longer than t_reset. At the end of the day they both provide the same function of setting the internal registers back to their defaults. They are just preformed differently. When you remove VCCI you are preforming a pin reset.

    I'm going to check with my team to see if there will be an increase in the ICCi current if SDA and SCL are floating and VCCI is still connected to the device. I will get back to you with an answer. 

    Best,

    Chris

  • 0 in reply to Chris Ayoub
    TI__Genius 15345 points

    Hi Mark,

    So I double checked everything with my team. The 300K pull down resistors may not be the best solution if you are worried about leakage current since when the VCCI is powered on there will be a leakage current through those resistors.

    You are correct in your original statement, if SDA and SCL are floating and VCCI is powered then shoot through current occurs.

    The best solution to this problem might be just adding another I2C buffer/switch between the 6408 and the controller. You wouldn't have to worry about the leakage current through the pull downs on the I2C lines.

    Let me know what you think.

    Best,

    Chris

  • +1 in reply to Chris Ayoub
    TI__Genius 11645 points

    Hi Chris,

    We will look at placing a analog switch between the Master I2C controller and the TCA6408A I2C inputs, leaving VCCi connected to the TCA6408A and the SDA/SCL pull-ups. 

    Thanks for your help