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TCAN1043A-Q1: Series resistor for pin EN/STB/FAULT

Josef Warta
Expert 4360 points
Part Number: TCAN1043A-Q1


Hello Team,

do we have a recommendation on series resistors for these pins (EN/STB/FAULT) when connecting to a uController? If yes, which value do we recommend.

Many Thanks

Josef

  • 0
    TI__Guru 50688 points

    Hi Josef,

    Internal pull-down resistors typically 60kohm are connected internally on the EN and STB pins. The FAULT pin is an output pin, and does not require a pull-up/down resistor. 

    When choosing a resistor to bias the EN and STB pins, customer would need to choose a pull-up/down resistor strong enough to overcome the 60kohm internal pull-down resistance. 

    4.7k, 10k, resistors would be general values to use for these resistors. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Expert 4360 points

    Hello Tyler, I think I described it in a wrong way. I mean resistors which are in-line of the connection between uC and interface chip to protect the uC such as recommended for example with our high-side switch. See enclosed picture.

    Our high-side product group is strongly recommending these resistors due to the fact that strong pulses outside the box could damage the uC through some internal paths of the chip. 

    BR Josef

  • 0 in reply to Josef Warta
    TI__Guru 50688 points

    Hi Josef,

    I found a really good read on digikey that describes how to size input series resistance in order to protect the inputs to a microcontroller:

    https://www.digikey.com/en/articles/protecting-inputs-in-digital-electronics#:~:text=For%20most%20digital%20inputs%2C%20a,of%201%20k%CE%A9%20is%20used.

    I have seen customers use 22.6 ohm resistors for I2C applications, I have seen 100 ohm resistors. I think it just depends on the kind of filter that you want to implement on the pin, and what transients you might see. Choose a resistor accordingly in order to filter out high-frequency noise, and possibly add some filter capacitance to ensure smooth rise/fall times in order to mitigate overshoot/undershoots. 

    These values for series resistance and filter capacitance are also determined by the speed at which the customer toggles these pins. It would be helpful information to know how fast each pin will be toggled first before making a decision on what size of resistor and cap to use. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Expert 4360 points

    Hello Tyler,
    thanks for feedback but do TRX team recommend any series resistor for their CAN/LIN products?

    I know that HighSide Switch team is doing recommendaton on some products but this is more due to signals which can find a path through the chip back to the uC and protect against high-voltage pulses and reverse polarity.

    Not sure if we have similar requirements for interface parts?

    BR Josef

  • 0 in reply to Josef Warta
    TI__Guru 50688 points

    Hi Josef,

    I think this question has multiple variables. The added series resistance needs to be selected based off of RC low-pass filter characteristics (rise-time) which is based off of data rate at which the digital pin will toggle. We also need know the drive strength of the MCU pin, and its ability to drive logic TXD. If the series resistance is too large, there will be a large voltage drop across the series resistance that might affect the logics levels at the input of the transceiver, or on the RXD pin logic thresholds back to the MCU. 

    TRX does not offer a one-size fits all resistor sizing for protecting the MCU, this is more dependent on the customer to decide what bests works in their system. I have seen customers use 20 ohm or 100 ohm for RS232, I have seen 1k. It really dependents on the tradeoffs that the customer desires. If they add more series protection, they might have better defense against transient events that could potentially cause damage, but in the same scenario, they may have to reduce their overall data rate for the system to work. 

    Regards,

    Tyler