LTC2859/LTC2861

10

285961fb

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

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short circuits to

–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.

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.

Figure 8a. Driver Output in Normal Mode

Driver Output at 125kHz into 100Ω Resistor

Frequency Spectrum of the Same Signal

Figure 8b. Driver Output in SLO Mode

Driver Output at 125kHz into 100Ω Resistor

Frequency Spectrum of the Same Signal

APPLICATIONS INFORMATION

285961 F08a

Y, Z

Y–Z

2μs/DIV

1V/DIV

Y–Z

1.25MHz/DIV

10dB/DIV

285961 F08b

Y, Z

Y–Z

2μs/DIV

1V/DIV

Y–Z

1.25MHz/DIV

10dB/DIV