Philips Semiconductors
Application note
AU5790 Single wire CAN transceiver
AN2005
level is dominant, i.e. at least one transmitter is active, then RxD is Low. The RxD is an open drain output, and needs an external pull-up
resistance. To insure the RxD has the same voltage swing as other digital signal, the RxD pin should be pulled-up to the digital power supply
Vcc. The AU5790 provides appropriate high frequency filtering to ensure minimum susceptibility against electromagnetic interference. Further
enhancement is possible by applying an external inductor L and a capacitor C at the CANH pin as shown in Figure 6.
UL
The AU5790 features special robustness at its BAT and CANH pins. Hence the device is well suited for applications in the automotive
environment. The BAT input is protected against 40V load dump, jump start conditions and all the conventional Automotive transients as defined
in SAE J1113/ISO7637. In addition the CANH output pin is protected against ESD transients of at least 8KV without any external device
protection. Protection against wiring fault conditions e.g. short circuit to ground or battery voltage is also included in the design.
A thermal protection shutdown function with hysteresis is incorporated aimed at protecting the device against system fault conditions leading to
excessive operating junction temperature. In case the chip junction temperature reaches the trip point (>155 _C), the temperature protection
circuit will turn-off the transmit function. The transmit function is available again after a small decrease of the junction temperature. The thermal
shutdown hysteresis is about 5 _C.
NSTB and EN are mode control input pins. They are typically provided by a controller device. The AU5790 has four operation modes: sleep
mode, wake-up mode, high-speed transmission mode, and normal transmission mode.
3.3 Operating Mode and Control
The microcontroller controls the transceiver’s operating mode via the EN and NSTB pins. It is the microcontroller’s responsibility to insure that
the mode changes take place between the message frames. The following is the mode control summary table.
Table 1. Mode Control Summary
NSTB
EN
0
1
0
T D
X
don’t care
Description
sleep mode
wake-up mode
high-speed mode
normal mode
CANH
0 V
0 V, 12 V
0 V, 4 V
0 V, 4 V
R D
X
0
0
1
1
float(high)
bus state
bus state
bus state
T D-data
X
T D-data
X
T D-data
X
1
Times that the transceiver needs to change its operation mode are shown in following table.
Table 2. Mode Switching Time
From Mode
Normal
Normal
Normal
High speed
Wake-up
Sleep
To Mode
High speed
Wake-up
Sleep
Normal
Normal
Mode Switching Time (µs)
<30
<30
<500
<30
<30
<50
Normal
3.3.1 Sleep Mode and Power Management
Battery power management is extremely important while there are more and more electronic components on the in-vehicle network. The
AU5790 supports partial networking, i.e. individual nodes can communicate in normal and/or high-speed mode without disturbing the sleep
nodes in the network.
If the NSTB and EN control inputs are both pulled low or floating, the AU5790 enters a low-power or “sleep” mode. This mode is dedicated to
minimizing ignition-off current drain, to enhance system efficiency. In sleep mode, the typical quiescent current is 70 uA, and the transmit
function is disabled, e.g. the CANH output is inactive even when TxD is pulled low.
Sleeping nodes will ignore normal and/or high-speed communication on the bus, i.e. for 4.0 V dominant bus level, the RxD is still floating or
High. Sleeping nodes may be activated using the dedicated wake-up mode. An internal network active detector monitors the bus for any
occurrence of higher voltage signal level, typically 12 V, on the bus line with normal battery levels. If such levels are detected, a message will be
passed on to the CAN controller via the RxD output. Since the receive delay in sleep mode is much longer than that in normal or high speed
mode, the first wake-up message may be lost within the system. The controller should switch the transceiver to normal mode after it receives
the wake-up signal even though the message itself may be corrupt, otherwise the node will be back to sleep mode after the wake-up message.
7
2001 Apr 16
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