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LTM2881

2881fe

APPLICATIONS INFORMATION

Switchable Termination

Proper cable termination is very important for signal fi-

delity. If the cable is not terminated with its characteristic

impedance, reflections will distort the signal waveforms.

The integrated switchable termination resistor provides

logic control of the line termination for optimal perfor-

mance when configuring transceiver networks.

When the TE pin is high, the termination resistor is enabled

and the differential resistance from A to B is 120Ω. Figure

11 shows the I/V characteristics between pins A and B

with the termination resistor enabled and disabled. The

resistance is maintained over the entire RS485 common

mode range of –7V to 12V as shown in Figure 12. The

integrated termination resistor has a high frequency re-

sponse which does not limit performance at the maximum

specified data rate. Figure 13 shows the magnitude and

Figure 13. Termination Magnitude and Phase vs Frequency

Figure 12. Termination Resistance vs Common Mode Voltage

Figure 11. Curve Trace Between A and B with Termination

Enabled and Disabled

2881 F11

COMMON MODE VOLTAGE (V)

130

128

126

124

122

120

118

116

114

112

110

2881 G11

–5

–10

FREQUENCY (MHz)

150

140

130

120

110

100

–10

–20

–30

–40

2881 F13

0.1

PHASE

MAGNITUDE

Figure 14. Supply Current vs Data Rate

DATA RATE (Mbps)

2881 F14

0.1

250

230

210

190

170

150

130

110

LTM2881-3

R=54 CL=1000p

R=54 CL=100p

R=54 CL=0

LTM2881-5

R=54 CL=1000p

R=54 CL=100p

R=54 CL=0

phase of the termination impedance versus frequency.

The termination resistor cannot be enabled by TE if the

device is unpowered, ON is low or the LTM2881 is in

thermal shutdown.

Supply Current

The static supply current is dominated by power delivered to

the termination resistance. Power supply current increases

with data rate due to capacitive loading. Figure 14 shows

supply current versus data rate for three different loads

for the circuit configuration of Figure 4. Supply current

increases with additional external applications drawing

current from VCC2.

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