LTM2881 datasheet(13/20 Pages) LINER

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LTM2881

2881f

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short circuits to

any voltage within the absolute maximum range of (VCC2

–15V) to (GND2 +15V) levels. The maximum VCC2 current

in this condition is 250mA. If the pin voltage exceeds

about ±10V, current limit folds back to about half of the

peak value to reduce overall power dissipation and avoid

damaging the part.

The device also features thermal shutdown protection

that disables the driver and receiver output in case of

excessive power dissipation (See Note 4 in the Electrical

Characteristics section).

SLO Mode

The LTM2881 features a logic-selectable reduced slew rate

mode (SLO mode) that softens the driver output edges to

reduce EMI emissions from equipment and data cables.

The reduced slew rate mode is entered by taking the SLO

pin low to GND2, where the data rate is limited to about

250kbps. Slew limiting also mitigates the adverse effects

of imperfect transmission line termination caused by stubs

or mismatched cables.

Figures 9a and 9b show the frequency spectrums of the

LTM2881 driver outputs in normal and SLO mode operating

at 250kbps. SLO mode signiﬁcantly reduces the high

frequency harmonics.

Receiver and Failsafe

With the receiver enabled, when the absolute value of the

differential voltage between the A and B pins is greater than

200mV, the state of RO will reﬂect the polarity of (A-B).

During data communication the receiver detects the state

of the input with symmetric thresholds around 0V. The

symmetric thresholds preserve duty cycle for attenuated

signals with slow transition rates on high capacitive busses,

or long cable lengths. The receiver incorporates a failsafe

feature that guarantees the receiver output to be a logic-

high during an idle bus, when the inputs are shorted, left

open or terminated, but not driven. The failsafe feature

eliminates the need for system level integration of network

pre-biasing by guaranteeing a logic-high on RO under

APPLICATIONS INFORMATION

Figure 9a. Frequency Spectrum SLO Mode 125kHz Input

Figure 9b. Normal Mode Frequency Spectrum 125kHz Input

FREQUENCY 1.25MHz/DIV

2881 F09a

12.5

6.25

FREQUENCY 1.25MHz/DIV

2881 F09b

12.5

6.25

the conditions of an idle bus. Further network biasing

constructed to condition transient noise during an idle

state is unnecessary due to the common mode transient

rejection of the LTM2881. The failsafe detector monitors

A and B in parallel with the receiver and detects the state

of the bus when A-B is above the input failsafe threshold

for longer than about 3μs with a hysteresis of 25mV. This

failsafe feature is guaranteed to work for inputs spanning

the entire common mode range of –7V to 12V.

The receiver output is internally driven high (to VL) or low

(to GND) with no external pull-up needed. When the receiver

is disabled the RO pin becomes Hi-Z with leakage of less

than ±1μA for voltages within the supply range.

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