Data Sheet ADM3051
Rev. A | Page 13 of 16
CIRCUIT DESCRIPTION
CAN TRANSCEIVER OPERATION
A CAN bus has two states: dominant and recessive. A dominant
state is present on the bus when the differential voltage between
CANH and CANL is greater than 0.9 V. A recessive state is
present on the bus when the differential voltage between CANH
and CANL is less than 0.5 V. During a dominant bus state, the
CANH pin is high and the CANL pin is low. During a recessive
bus state, both the CANH and CANL pins are in the high
impedance state.
The driver drives CANH high and CANL low (dominant state)
if a logic low is present on TxD. If a logic high is present on
TxD, the driver output is placed in a high impedance state
(recessive state). The driver output states are shown in Table 7.
The receiver output is low if the bus is in the dominant state and
high if the bus is in the recessive state. If the differential voltage
between CANH and CANL is between 0.5 V and 0.9 V, the bus
state is indeterminate and the receiver output may be high or
low. The receiver output states for given inputs are listed in
Table 8.
OPERATIONAL MODES
Three modes of operation are available: high speed, slope
control, and standby. RS (Pin 8) allows modification of the
operational mode by connecting the RS input through a resistor
to ground, or directly to ground, or to a CAN controller, as
shown in Figure 30.
With RS connected to ground, the output transistors switch on
and off at the maximum rate possible in high speed mode, with
no modification to the rise and fall slopes. EMI in this mode
can be alleviated using shielded cables.
Alternatively, connecting RS to a resistor, R
SLOPE
, allows
slope control mode, with the value of the resistor modifying
the rise and fall slopes. The reduced EMI allows the use of
unshielded cables.
Applying a logic high to RS initiates a low current standby mode.
The transmitter is disabled, and the receiver is connected to a
low current. RxD goes low upon receiving dominant bits, allowing
an attached microcontroller that detects this to wake the
transceiver via Pin 8, which returns it to standard operation.
The receiver is slower in standby mode and loses the first
message at higher bit rates.
Table 5. Mode Selection Using RS Pin (Pin 8)
Mode Condition to Force
Resulting
Voltage/Current
Standby V
RS
> 0.75 V
CC
−I
RS
< 10 µA
Slope Control 10 µA < −I
RS
< 200 µA 0.4 V
CC
< V
RS
< 0.6 V
CC
High Speed V
RS
< 0.3 V
CC
−I
RS
< −500 µA
TRUTH TABLES
The truth tables in this section use the abbreviations found in
Table 6.
Table 6. Truth Table Abbreviations
Letter Description
H High level
L Low level
X Don’t care
I Indeterminate
Z High impedance (off)
NC Disconnected
Table 7. Transmitting
Supply Input Outputs
V
CC
TxD State CANH CANL
On L Dominant H L
On H Recessive Z Z
On Z Recessive Z Z
Off X Z Z Z
Table 8. Receiving
Supply Inputs Output
V
CC
V
ID
= CANH − CANL Bus State RxD
On ≥0.9 V Dominant L
On ≤0.5 V Recessive H
On 0.5 V < V
ID
< 0.9 V I I
On Inputs open Recessive H
Off X X I
THERMAL SHUTDOWN
The ADM3051 contains thermal shutdown circuitry that
protects the part from excessive power dissipation during fault
conditions. Shorting the driver outputs to a low impedance
source can result in high driver currents. The thermal sensing
circuitry detects the increase in die temperature under this
condition and disables the driver outputs. The design of this
circuitry ensures the disabling of driver outputs upon reaching
a die temperature of 150°C. As the device cools, reenabling of
the drivers occurs at a temperature of 140°C.
