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TCA9509: TCA9509 Port A pull-up restriction: Is a 4.7 k ohm pull-up from a connected sensor acceptable?

Hori Shota
Expert 1730 points

Part Number: TCA9509

Hi team,
 
Purpose
I would like to confirm whether it is acceptable for Port A of the TCA9509 to be pulled up by a connected device.
Background / Setup
  • Device: TCA9509
  • The datasheet states that a pull-up resistor should not be connected to Port A.
  • In my system, the device connected to Port A (a sensor) has a pull-up resistor on the line.
Issue
Because of the sensor configuration, Port A is effectively pulled up through the connected device.
Questions
  • Is this configuration acceptable, or could it cause any functional issues with the TCA9509?
  • If this configuration is not recommended, what failure mode or risk should be expected?
Best Regards,
Shota
  • 0
    TI__Guru 64978 points

    Hi Hori,

    Hori Shota said:
    Questions
    • Is this configuration acceptable, or could it cause any functional issues with the TCA9509?
    • If this configuration is not recommended, what failure mode or risk should be expected?

    The VILC spec of the TCA9509 requires VILC < 150 mV for VCCA > 1.5V and VCCB >3.15V. 

    This means any external pull-up resistance + 1 mA current source of the TCA9509 must be driven by the A-side I2C device and have a VOL < 150mV. External sources of current are not recommended because it increases VOL. 

    What is the resistive value of your pull-up on Port A? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Expert 1730 points
    Hi Tyler-san,
    Thank you for reply.
    Background / Setup
    • An external sensor is connected to Port A.
    • The sensor side already includes a pull-up resistor.
    • The resistance value of the pull-up on Port A is 4.7 kΩ.
    Question
    • Based on the recommendation that using an internal pull-up is not advised when an external pull-up exists, can we conclude that using both in this case is not recommended?
    Best Regards,
    Shota

  • 0 in reply to Hori Shota
    TI__Guru 64978 points

    Hi Hori Shota,

    If I assume 1.8V is used on A-side for TCA9509, I come up with the following sinking current. 

    This is a open-drain driver in your system pulling LOW. It must sink the current from the 4.7k PU resistor + 1 mA current source from the TCA9509. 

    I calculated under these circumstances that the total current to sink is ~ 1.351 mA. 

    The open-drain driver from the host or target on the A-side of the TCA9509 must have a minimum pull-down strength of 111 ohms. 

    Regards,

    Tyler