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TCAN1042-Q1: Power Dissipation in Bus Short Circuit

Part Number: TCAN1042-Q1

Hello,

I'm wondering how to estimate the power dissipation of TCAN1042-Q1 when its CAN bus outputs are short-circuited to GND. Could you review my idea?

  • Supply current consumed by internal circuit (ICC(INT)): The data sheet provides the Max ICC (5-V supply current) value in dominant state with bus fault as 180 mA. I guess it contains IOS(SS_DOM) (short-circuit steady-state output current, dominant, Normal mode) of 100 mA which will be drawn from VCC supply pin. The supply current consumed by the device internal circuit in this fault state can be estimated as 80 mA by subtracting 100 mA from 180 mA.
  • Supply current consumed by short-circuit output current (ICC(OUT)): The short-circuit current flowing out from CANH pin will be drawn from VCC supply pin. The average short-circuit current IOS(AVG) can be estimated by the equation (1) on page 17 of the data sheet.
  • Power dissipation: The power dissipation of this device in this fault state can eventually be calculated as PD(SS) = VCC × (ICC(INT) + ICC(OUT)).

If the junction temperature raised by this heat dissipation exceeds the thermal shutdown threshold TTSD, will it turn off the device?

Additional Question

  • Is there any way for a system (MCU, etc.) to know whether a TCAN1042-Q1 is in a bus fault condition caused by bus short circuit?

Best regards,
Shinichi Yokota

  • Hello Yokota-san,

    Someone from our team will get back to you soon on this. Thanks for your patience.

    Max
  • Hello Yokota-san,

    Your approach to calculating and estimating the power dissipation of the TCAN1042-Q1 is very close to accurate, but some different specifications should be used in the calculations.

    The driver portion of the TCAN1042-Q1 will consume the majority of the current, and the rest of the device will consume a little. Therefore, just as you suggested, we will need to determine these two currents and add them together.

    First, to determine the current from the rest of the device, we can use the specification for ICC in Normal Mode (recessive) from table 7.5 in the datasheet on page 6 which gives a maximum of 2.5mA of current. When the driver is in the recessive state, it is essentially off and only the current from the rest of the device is consumed. Therefore we can assume this current is also the same when the driver is in a dominant state, or in a bus fault condition such as your short to GND.

    Second, we need to calculate the current from the driver. Generally speaking, the driver has a CANH half, and a CANL half with the VCM level being in the middle. In a normal mode, the current will flow from VCC through the CANH half to VCM, and then through the CANL half to GND. In your particular case, shorting the CANH/CANL lines to GND will set VCM to GND which shorts around the CANL portion of the driver and only the CANH half is consuming current. This essentially makes CANL = open.

    The device has a current limiting feature to limit the total current during short-circuit events. This is the Ios(ss_dom) parameter which gives a maximum current of 100mA. It is stated as a minimum of -100mA. The minus (-) sign indicates that the current is flowing out of the TCAN1042-Q1 device pin but all current comes from VCC. Therefore we will estimate that the current through the driver will be the maximum amount of 100mA.

    To get the total current during the short to GND situation, we can add the 100mA, and the 2mA, to get total device current of 102mA. If VCC = 5V, then the power dissipation is ~500mW.

    If the short is present long enough to allow the device junction temperature to heat past the Ttsd, then yes, the device will protect itself and go into thermal shutdown.

    The TCAN1042-Q1 is only an 8-pin device and does not contain any status pins that the MCU could use as an indication of a bus fault condition due to a short without some form of external monitoring circuitry that would be in addition to the TCAN1042-Q1 device. Using a 14-pin CAN transceiver with a "fault" pin could give you additional possibilities to monitor whether a fault condition has occurred, but not necessarily which specific fault condition has occurred.

    Thanks and Regards,
    Jonathan