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TCA9555: TCA9555 Recommended value of pull-down resistor

Noriaki Fukazawa
Expert 1620 points
Part Number: TCA9555
Other Parts Discussed in Thread: TCA9535
Hello Expert,

My customer is using TCA 9555. I received the following question,
Is it possible to cooperate in preparing answers?

<Question>
There are pins trying to use the IO pin of TCA9555 at the output.

CTA9555:
www.tij.co.jp/.../tca9555.pdf
This pin is connected to the input pin of DTC123JM of Rohm digital transformer.

At the time of start-up (from power-on to setting completion), the voltage of this pin changes at 0.7 V,
The DTC123JM is not turned on unintentionally. I want to turn off DTC 123JM at startup.
I am trying to add a pull-down to this circuit, but please tell me the recommended pull-down resistance value.

DTC123JM
www.rohm.co.jp/.../DTC123JM


We are sorry for the inconvenience caused by the contents concerning 
connection with other company's products.
Also, please tell the customer if there are points to check.


Best,Regars.

Nori

  • 0
    TI__Mastermind 19495 points

    Hello Hori,

    The TCA9555 is setup as an input during startup but it does has a 100k pull-up resistor which allows people to not add external pull-up resistors, which is needed on c-mos inputs so that there isn't excessive current due to floating inputs creating a shoot through event.  

    That being said, in your particular situation you need to figure what voltage you want on input of the the DTC123, which effectively looks like a 48.4 k resistor.  You need to calculate the resistance value to get the voltage you want at that node.  I don't know what Vcc you are using so I didn't do the math for you but it should be simple.

    The other option you have is to use the TCA9535.  It has the same pin out and functionality but doesn't have 100 k Ohm pull resistors on inputs.  Then you still will have to deal with termination of c-mos inputs, but you get to choose how and where to (Vcc or gnd)

    -Francis Houde

  • 0 in reply to fhoude
    Expert 1620 points
    Hello Francis-san.

    Hi Francis.

    Thank you for your reply and support.

    Since the TCA9555 has a 100 kΩ resistor on the input side,
    You are thinking that FET should not be turned on by shoot-through.

    Also, if you know the value of Vcc you use,
    Is it OK to recognize the recommended resistance value connected to an external pull-down resistor?

    Best Regars.
    Nori
  • 0 in reply to Noriaki Fukazawa
    TI__Guru 62910 points
    Hello Nori-san,

    The P ports of the TCA9555 have a worst-case input low-level leakage of 100 uA (see "IIL" specification in the Electrical Characteristics table). So, you should consider this current when choosing a pull-down resistance. For example, if you wanted to ensure that the voltage was no larger than 0.7 V at the DTC123JM input, you could compute the maximum pull-down resistance as (0.7 V)/(100 uA) = 7 kOhm.

    Max