AMIS-42700 Dual High-Speed CAN Transceiver
7.7 Time Out Timer
Preliminary Data Sheet
The Tx0 dominant time out timer circuit prevents the output drivers from driving a permanent dominant state (blocking all network
communication) if pin Tx0 or the bus lines of the other bus are forced permanently dominant by a hardware and/or software failure.
The timer is triggered by a negative edge of the TIMERIN signal. If the duration of the low-level on TIMERIN exceeds the internal
timer value TIMERDEL, the timer output TIMEROUT becomes high, disabling the transmitter (bus returns into the recessive state).
The timer is reset by a positive edge of the TIMERIN signal.
7.8 Over Temperature Detection
A thermal protection circuit is integrated to prevent the transceiver from damage if the junction temperature exceeds thermal
shutdown level. Because the transmitter dissipates most of the total power, the transmitter will be switched off only to reduce
power dissipation and IC temperature. All other IC functions continue to operate.
7.9 Fault Behavior
A fault like a short circuit is limited to that bus line where it occurs, hence data interchange from the protocol IC to the other bus
system is not affected.
When the voltage at the bus lines is going out of the normal operating range (-12V to +12V), the receiver is not allowed to
erroneously detect a dominant state.
7.10 Short Circuits
As specified in the maximum ratings, short circuits of the bus wires CANHx and CANLx to the positive supply voltage Vbat or to
ground must not destroy the transceiver. To provide sufficient safety for automotive applications the voltage range for permanent
short circuits is extended to 50V dc. A short circuit between CANHx and CANLx must not destroy the IC as well.
The dedicated comparator (L2VBAT) on CANL pin detects the short to battery and after debounce time-out switches off the
affected driver only.
The receiver of the affected driver has to operate normally.
7.11 Faulty Supply
In case of a faulty supply (missing connection of the electronic unit or the transceiver to ground, missing connection of the
electronic unit to Vbat or missing connection of the transceiver to Vcc) the power supply module of the electronic unit will operate
such that the transceiver is not supplied, i.e. the voltage Vcc is below the POR level. In this condition the bus connections of the
transceiver must be in the POR state.
If the ground line of the electronic unit is interrupted, Vbat may be applied to the Vcc pin (measured relative to the original ground
potential, to which the other units on the bus are connected).
7.12 Reverse Electronic Unit (ECU) Supply
If the connections for ground and supply voltage of an electronic unit (ECU) (max. 50V) which provides Vcc for the transceiver are
exchanged, this transceiver has a ground potential which may be up to 50V higher than that of the other transceivers. In this case
no transceiver must be destroyed even if several of them are connected via the bus system.
Any exchange among the six connections CANH1, CANH2, CANL1, CANL2, ground, and supply voltage of the electronic unit at
the connector of the unit must never lead to the destruction of any transceiver of the bus system.
AMI Semiconductor
– Rev. 1.4, May 05 - Preliminary
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