LTM2889 datasheet(21/30 Pages) LINER

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LTM2889

2889fa

For more information www.linear.com/LTM2889

APPLICATIONS INFORMATION

Figure 13. Slew Rate vs Slew Control Resistor RSL

RSL (k)

2889 F13

100

VCC2 = 5V

VCC2 = 3.3V

When a voltage between 1.1V and VCC2 is applied, the RS

pin acts as a high impedance receiver. A voltage above

VIH_RS puts the chip in shutdown, while a voltage below

VIL_RS but above 1.1V activates the chip and sets the

transmitter to the minimum slew rate.

TheslewcontrolcircuitontheRSpinisactivatedatapplied

voltages below 1.1V. The RS pin can be approximately

modeled as a 1.1V voltage source with a series resistance

of 2k and a current compliance limit of –100µA, and a

250k pull-up resistor to VCC2 (Figure 14). Lowering the

voltage on RS increases the slew control current ISC be-

ing drawn from the slew control circuit until the voltage

reaches ~0.9V, where the current drawn from the circuit

is ~ –100µA. Below an applied voltage of ~ 0.9V, the slew

control circuit sources no additional current, and the

current drawn from it remains at ~ –100µA down to 0V.

The total current IRS drawn from the RS pin for input volt-

age 0.9V ≤ VRS ≤ 1.1V is the sum of the internal pull-up

resistor current IRS and the slew control current ISC.

IRS(0.9V ≤ VRS ≤ 1.1V)=IPU+ISC

VCC2 – VRS

250k

1.1V – VRS

The transmitter slew rate is controlled by the slew control

currentISCwithincreasingcurrentmagnitudecorrespond-

ing to higher slew rates. The slew rate can be controlled

using a single slew control resistor RSL in series with the

RS pin. When the RS pin is pulled low towards ground

by an external driver, RSL limits the amount of current

drawn from the RS pin and sets the transmitter slew rate.

Alternatively,theslewratemaybecontrolledbyanexternal

voltage or current source.

Figure 14. Equivalent Circuit of RS Pin

2889 F14

ISC

(–100µA LIMIT)

IPU

VCC2

IRS

IDEAL

DIODE

GND2

1.1V

–

250k

High Symmetry Driver with Variable Slew Rate

The electromagnetic emissions spectrum of a differential

linetransmitterislargelydeterminedbythevariationinthe

commonmodevoltageduringswitching,asthedifferential

component of the emissions from the two lines cancel,

while the common mode emissions of the two lines add.

The LTM2889 transmitter has been designed to maintain

highly symmetric transitions on the CANH and CANL lines

to minimize the perturbation of the common mode voltage

during switching (Figure 15), resulting in low EME. The

common mode switching symmetry is guaranteed by the

VSYM specification.

Figure 15. Low Perturbation of Common

Mode Voltage During Switching

200ns/DIV

CANL

500mV/DIV

COMMON MODE

500mV/DIV

CANH

500mV/DIV

2889 F15

1Mbps

LTM2889-3

PVCC = VCC = 3.3V

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