LTC2851 LINER [Linear Technology], LTC2851 Datasheet - Page 10

• Manufacturer Part Number: LTC2851
• Description: 20Mbps RS485 Transceivers
• Manufacturer: LINER [Linear Technology]

applicaTions inFormaTion
LTC2859/LTC2861
Driver
The driver provides full RS485 and RS422 compatibility.
When enabled, if DI is high, Y-Z is positive for the full
duplex device (LTC2861) and A-B is positive for the half-
duplex device (LTC2859).
When the driver is disabled, both outputs are high-
impedance. For the full duplex LTC2861, the leakage on
the driver output pins is guaranteed to be less than 10µA
over the entire common mode range of –7V to +12V. On
the half-duplex LTC2859, the impedance is dominated by
the receiver input resistance, R
Driver Overvoltage and Overcurrent Protection
The driver outputs are protected from short circuits to
any voltage within the Absolute Maximum range of (V
–15V) to +15V. The maximum 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.
10
Driver Output at 125kHz into 100Ω Resistor
Driver Output at 125kHz into 100Ω Resistor
Y–Z
Y–Z
Y, Z
Y, Z
1V/DIV
1V/DIV
IN
2µs/DIV
2µs/DIV
.
Figure 8a. Driver Output in Normal Mode
Figure 8b. Driver Output in SLO Mode
CC
The LTC2859/LTC2861 also feature thermal shutdown
protection that disables the driver, terminator, and receiver
in case of excessive power dissipation.
SLO Mode: Slew Limiting for EMI Emissions Control
The LTC2859/LTC2861 feature a logic-selectable reduced-
slew mode (SLO mode) that softens the driver output
edges to control the high frequency EMI emissions from
equipment and data cables. The reduced slew rate mode
is entered by taking the SLO pin low, 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 8a and 8b show the LTC2861 driver outputs in
normal and SLO mode with their corresponding frequency
spectrums operating at 250kbps. SLO mode significantly
reduces the high frequency harmonics.
Y–Z
Y–Z
Frequency Spectrum of the Same Signal
Frequency Spectrum of the Same Signal
10dB/DIV
10dB/DIV
1.25MHz/DIV
1.25MHz/DIV
285961 F08b
285961 F08a
285961fc