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TCAN1044-Q1: spec check

Arie Li
Expert 3390 points
Part Number: TCAN1044-Q1
Other Parts Discussed in Thread: TCAN1051

Hi Experts,

Could you please help to explain the design consideration of below two specs of the CAN transceiver?

  1. Common mode voltage: for old product such as TCAN1042, TCAN1051, the common mode voltage is +/-30V, but for TCAN1044, TCAN1057, the common mode voltage changed to +/-12V, could you please help to explain the reason why you made this change? Is it because the previous +/-30V is a little over spec? Where should customer choose +/-30V and where should choose +/-12V?
  2. Bus fault voltage: For device without “H”, the bus fault voltage is usually +/-58V, what’s your design consideration for it? Is it an industrial standard? Is it for 12V and 24V battery systems? For device with “H”, the bus fault voltage is +/-70V, why you choose 70V? Is it for 48V battery system? Thank you.
  • 0
    TI__Mastermind 49995 points

    Hi Arie,

    1. The common mode voltage specification describes the operational common-mode range of the receiver on the device. The range of this spec allows for different common-mode voltages to be used by different nodes on the same CAN bus. This commonly occurs when there is a ground potential difference (GPD) between the nodes. Because this GPD is rarely more than a few volts and is independent of supply or battery voltage, +-12V is more than enough range to accommodate for this. 
    A second source of common-mode shift is from system faults or shorts. In a fault case where either of the CAN lines are shorted to battery voltage through a low-impedance path, the transceiver may remain operational if a valid differential can still be driven. Such examples are when CANH is shorted to +12V, or CANL is shorted to GND. In systems with higher battery voltages of 24V for example, a short on CANH to +24V would exceed the common-mode range of TCAN1044 and the receiver could no longer function properly until the fault is cleared. A device with a higher common-mode voltage ranting such as TCAN1042 may remain operational under such faults. 
    Note that the "bus fault protection" specification describes the absolute maximum voltage on the CAN lines. This specification describes conditions that if exceeded may cause damage to the device. A short to +24V on TCAN1044 for example would interrupt communications until the fault is cleared, but would not cause damage to the device as the bus fault protection voltage has not been exceeded. 

    2. I'm not sure if the actual specified values were driven by market design standards, competition, or simply what was possible with the current technology. As none of these values are typical supply voltages for automotive systems, it's likely that a target was set to achieve standard levels (such as 48V) with a buffer margin. The resulting device was able to be specified to the levels we see in the datasheets. The higher level of protection of the "H" variants is for applications that have special focus on reliability or are expecting particularly hostile conditions for their CAN bus. In the end, this provides more options for system designers so they can meet system requirements more easily.

    Regards,
    Eric Schott