NCV7441
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4
If the main power supply V
CC
(nominal 5 V) is above its under−voltage (V
CC_UV
) level, each CAN channel can enter either
normal mode (when the corresponding STB1/2 digital input is pulled Low) or standby mode (when the corresponding STB1/2
signal is left High):
• In the normal mode:
♦ The bus transceiver is ready to transmit and receive CAN bus signals with the full CAN communication speed (up to
1 Mbps) and thus interconnect the CAN bus with the corresponding CAN controller through digital pins TxD1/2 and
RxD1/2
♦ The bus pins are internally biased to typically V
CC
/2 through the input circuitry
♦ TxD1/2 input pin is monitored by a timeout in order to prevent a permanent dominant being forced to the bus thus
preventing other nodes from communicating. If TxD1/2 is Low for longer than t
cnt(timeout)
, the transmitter switches
back to recessive. Only when TxD1/2 returns to High, the timeout counter is reset and the transmitter is ready to
transmit dominant symbols again. The TxD1/2 timeout protection is implemented individually for both CAN
transceivers.
♦ A common thermal monitoring circuit compares the circuit junction temperatures with threshold T
J(sd)
. If the thermal
shutdown level is exceeded, dominant transmission is disabled. The circuit remains biased and ready to transmit but
the logical path from TxD1/2 pin(s) is blocked. The transmission is again enabled when the junction temperature
decreases below the shutdown level and the TxD1/2 pin returns to the High level, thus avoiding thermal oscillations.
• In the standby mode:
♦ The respective transmitter is disabled and the current consumption of the channel is fundamentally reduced. Only the
low−power receiver on the channel remains active in order to detect potential CAN bus wakeups. The logical signal
on TxD1/2 input is ignored.
♦ The bus pins are biased to GND through the input circuitry
♦ Digital output RxD1/2 signals the output of the low−power receiver and can be used as a wakeup signal in the
application. A filtering time td
BUS
is applied between the bus activity and the RxD1/2 signal in order to ensure that
only sufficiently long dominant signals on the bus will be propagated to the digital output. In addition, dominant bus
signals are ignored in case they were present during normal−to−standby mode transition; in this way unwanted
wakeups are avoided in case of permanent dominant failure on the bus. Example waveforms illustrating bus activity
detection in standby mode are shown in Figure 3.
In order to ensure a safe device state, the digital inputs STB1/2 and TxD1/2 are connected through internal pull−up resistors
to V
CC
thus ensuring that both channels remain in standby mode and/or no dominant can be transmitted in case any of the digital
inputs gets disconnected.
PD20100209.08
STB1
STB2
RxD1
RxD2
CANH/L1
CANH/L2
<t
dbus
w t
dbus
<t
dbus
w t
dbus
<t
dbus
w t
dbus
Figure 3. NCV7441 Dual CAN: Bus Activity Detection in Standby Mode
