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TCAN1044-Q1: External resistor limiting current on STB pin

Kenneth Lee
Intellectual 1708 points
Part Number: TCAN1044-Q1

Tool/software:

Hello Team,

My customer wants to know more about below comment about STB. 

Why current limiting by series resistor(R4) is needed? As I understand, STB has internal pull up to Vcc and it doesn't have internal leakage path.

Is it for protecting STB from unexpected excessive current? Or, it's for protecting GPIO from MCU which is controlling STB pin? 

If STB pin requires current limiting resistor, how much current can be flow into? Thank you.

  • +1
    TI__Guru 50306 points

    Hi Kenneth,

    Inputs with pull-ups (TXD/STB) have clamp diodes to VCC/VIO and ESD diodes to ground. The recommended series resistor (1 k to 10 kohms) limits any current if you drive the pin when VIO is down or if VIO spikes.

    I.e., for example, they turn on and clamp any voltage above VIO + 0.3 V. Hence, the diodes can conduct when STB is driven prior to VIO going up or have automotive transients / overvoltage  on VIO. The current limiting resistors prevents the current to be dumped unto the device. This can also protect the MCU, preventing the current to backflow into the MCU if the spike is above MCU’s I/O.

    If you care about pin-slew or want to transition the transceiver very fast, lean towards the lower end of 1k; if you want ultra-low leakage and EMI, lean higher towards 10k.

    In practice, 4.7 k is a common compromise. Just be sure that whichever you choose still lets STB cross the valid logic threshold within your timing budget, thanks  

    Best Regards,

    Michael.