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LTM2889

2889fa

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In addition to full compliance with the ISO 11898-2 stan-

dard, LTM2889 meets the more stringent requirements

of ISO 11898-5 for bus driver symmetry. This requires

that the common mode voltage stay within the limits not

only during the static dominant and recessive states, but

during the bit transition states as well. Ultra-high speed

peak detect circuits are used during manufacturing test

to ensure that VSYM limits are not exceeded at any point

during the switching cycle.

The high frequency content may be reduced by choosing

a lower data rate and a slower slew rate for the signal

transitions. The LTM2889 CAN transceiver provides an

approximate 20 to 1 reduction in slew rate, with a cor-

responding decrease in the high frequency content. The

lowest slew rate is suitable for data communication at

200kbps or below, while the highest slew rate supports

4Mbps. The slew rate limit circuit maintains consistent

control of transmitter slew rates across voltage and

temperature to ensure predictable performance under all

operatingconditions.Figure16demonstratesthereduction

in high frequency content of the common mode voltage

achieved by the lowest slew rate compared to the highest

slew rate when operating at 200kbps.

APPLICATIONS INFORMATION

Figure 16. Power Spectrum of Common Mode Voltage Showing

High Frequency Reduction of Lowest Slew Rate (RSL = 200k)

Compared to Highest Slew Rate (RSL = 0)

500kHz/DIV

0dB

20dB/DIV

2889 F16

RSL = 0

RSL = 200k

SPLIT Pin Output for Split Termination Support

Split termination is an optional termination technique

to reduce common mode voltage perturbations that

can produce EME. A split terminator divides the single

line-end termination resistor (nominally 120Ω) into two

series resistors of half the value of the single termination

resistor (Figure 2). The center point of the two resistors

is connected to a low impedance voltage source that sets

the recessive common mode voltage.

Split termination suppresses common mode voltage

perturbations by providing a low impedance load to com-

mon mode noise sources such as transmitter noise or

coupling to external noise sources. In the case of single

resistor termination, the only load on a common mode

noise source is the parallel impedance of the input resis-

tors of the CAN transceivers on the bus. This results in a

common mode impedance of several kilohms for a small

network.Thesplittermination,ontheotherhand,provides

a common mode load equal to the parallel resistance of

the two split termination resistors, or ¼ the resistance of

the single termination resistor (30Ω). This low common

mode impedance results in a reduction of the common

mode noise voltage compared to the much higher com-

mon mode impedance of the single resistor termination.

The SPLIT pin on the LTM2889 provides a buffered volt-

age to bias the mid-point of the split termination resistors.

The voltage on the SPLIT pin matches the common mode

voltage established by the transmitter in the dominant

state and the receiver input resistor bias during the reces-

sive state: 2.5V when VCC2 = 5V and 1.95V when VCC2 =

3.3V. SPLIT is decoupled to GND2 with an internal 4.7nF

capacitor to lower the AC impedance to better suppress

fast transients. SPLIT is a high voltage fault tolerant output

that tolerates the same ±60V overvoltage faults and ±25kV

ESD discharges as CANH and CANL.

OnedisadvantageoftheSPLITterminationishigherpower

supply current if the two terminating transceivers differ in

their common mode voltage due to differences in VCC2 or

GND2 potential or to chip to chip variations in the internal

reference voltages. This will result in the transceiver with

the higher common mode voltage sourcing current into

the bus lines through its SPLIT pin, while the transceiver

with the lower common mode voltage will sink current

through its SPLIT pin.

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