LTC4331 datasheet(17/22 Pages) AD

17 / 22 page

LT3960

Rev. B

For more information www.analog.com

Passive Leakage on I2CAN Bus Pins

When the power supply is removed or the chip is in shut-

down, the I2CAN pins are in a high impedance state. The

I2CAN receiver inputs are isolated from the CANxH and

CANxL pins by FET switches which open in the absence

of power, preventing the resistor dividers on the receiver

inputs from loading the bus. The high impedance state

of I2CAN pins is maintained over a range determined by

the ESD protection of the pins, typically -0.3V to 7V. For

bus voltages outside this range, the current flowing into

the receiver is governed by the conduction voltages of

the ESD device and the 35.7k nominal I2CAN receiver

input resistance.

I2CAN Bus Termination

I2CAN buses must be terminated at the ends of each

twisted pair with a 120Ω resistor. Split termination is an

optional termination technique to reduce common mode

voltage perturbations that can produce EME. A split ter-

minator divides the single line-end termination resistor

(nominally 120Ω) into two series resistors of half the

value of the single termination resistor (Figure 10).

The center point of the two resistors is connected to a

4.7nF capacitor. Split termination suppresses common

mode voltage perturbations by providing a low impedance

load to common 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 com-

mon mode noise source is the parallel impedance of the

input resistors of the I2CAN transceivers on the bus. This

results in a common mode impedance of several kΩ for a

small network. The split termination, on the other hand,

provides a common mode load equal to the parallel resis-

tance of the two split termination resistors (30Ω). This

low common mode impedance results in a reduction of

the common mode noise voltage compared to the much

higher common mode impedance of the single resistor

termination. Figure 11 and Figure 12 compare the com-

mon mode noise of an application with and without split

cap termination.

Figure 10.

4.7nF

LT3960

CANxH

CANxL

CANxH BUS

CANxL BUS

LT3960

CANxH

CANxL

CANxH BUS

CANxL BUS

120

3960 F10

SPLIT TERMINATION

RESISTOR TERMINATION

Split Termination for Improved

Common Mode Behavior

Figure 11.

VCC = 3.3V

200ns/DIV

SDA

5V/DIV

CANSDAH

1V/DIV

CANSDAL

1V/DIV

CANSDA CM

50mV/DIV

3960 F11

I2CAN Common Mode Noise (4.7nF Split Cap)

Figure 12.

VCC = 3.3V

200ns/DIV

3960 F12

SDA

5V/DIV

CANSDAH

1V/DIV

CANSDAL

1V/DIV

CANSDA CM

500mV/DIV

I2CAN Common Mode Noise (No Split Cap)

APPLICATIONS INFORMATION

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