LTC2875
13
Rev A
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APPLICATIONS INFORMATION
±25kV ESD Protection
The LTC2875 features exceptionally robust ESD protec-
tion. The transceiver interface pins (CANH, CANL, SPLIT)
feature protection with respect to GND to ±25kV HBM
without latchup or damage, during all modes of opera-
tion or while unpowered. All other pins are protected to
±8kV HBM to make the LTC2875 reliable under severe
environmental conditions.
4Mbps Operation
The LTC2865 features a high speed receiver and trans-
mitter capable of operating up to 4 Mbps. In order to
operate at this data rate, the transmitter must be set at its
maximum slew rate by pulling the RS pin low to ground
with no more than 4kΩ of resistance, including the output
impedance of the buffer driving the RS input (see RS Pin
and Variable Slew Rate Control below).
Driver
The driver provides full CAN compatibility. When TXD is
low with the chip enabled (RS low), the dominant state is
asserted on the CAN bus lines (subject to the TXD timeout
tTOTXD); the CANH driver pulls high and the CANL driver
pulls low. When TXD is high and RS is low, the driver is
in the recessive state; both the CANH and CANL driv-
ers are in the Hi-Z state and the bus termination resistor
equalizes the voltage on CANH and CANL. In the recessive
state, the impedance on CANH and CANL is determined
by the receiver input resistance, R
IN
. When RS is high the
LTC2875 is in shutdown; the CANH and CANL drivers are
in the Hi-Z state, and the receiver input resistance RIN is
disconnected from the bus by a FET switch.
Transmit Dominant Timeout Function
The LTC2875 includes a 2ms (typical) timer to limit the
time that the transmitter can hold the bus in the dominant
state. If TXD is held low, a dominant state is asserted on
CANH and CANL until the TXD timer times out at tTOTXD,
after which the transmitter reverts to the recessive state.
The timer is reset when TXD is brought high. The trans-
mitter asserts a dominant state upon the next TXD low.
The lowest data rate that can be communicated without
interference from the transmit dominant timeout timer is
22kbps, corresponding to 11 consecutive dominant bits
divided by a bit time equal to the minimum tTOTXD value
of 0.5ms. 11 dominant bits is the maximum allowed by
the CAN protocol, consisting of 5 dominant bits followed
by an error frame of 6 dominant bits.
Driver Overvoltage,Overcurrent, and Overtemperature
Protection
The driver outputs are protected from short circuits to any
voltage within the absolute maximum range of –60V to
60V. The maximum current in a fault condition is ±100mA.
The driver includes a progressive foldback current limiting
circuit that continuously reduces the driver current limit
with increasing output fault voltage. The fault current is
typically ±10mA for fault voltages of ±60V.
The LTC2875 also features thermal shutdown protection
that disables the driver in case of excessive power dis-
sipation (see Notes 3 and 4). When the die temperature
exceeds 170°C (typical), the transmitter is forced into the
recessive state. The receiver remains operational.
Power-Up/Down Glitch-Free Outputs
The LTC2875 employs a supply undervoltage detection
circuit to control the activation of the circuitry on-chip.
During power-up, the CANH, CANL, RXD and SPLIT out-
puts remain in the high impedance state until the supply
reaches a voltage sufficient to reliably operate the chip.
At this point, the chip activates if RS is low. The receiver
output goes active after a short delay tENRX and reflects
the state at the CAN bus pins, and the SPLIT output goes
active at approximately the same time. The transmitter
powers up in the transmit dominant timeout state regard-
less of the state of the TXD pin, and remains in the reces-
sive state until the first high to low transition on TXD after
the TXD enable time tENTX. This assures that the transmit-
ter does not disturb the bus by glitching to the dominant
state during power-up.
During power down, the reverse occurs; the supply under-
voltage detection circuit senses low supply voltage and
immediately puts the chip into shutdown. CANH, CANL,
RXD, and SPLIT outputs go to the high impedance state.
The voltage on RXD is pulled high by the 500k pull-up
resistor.