LTM2881 查看數據表（PDF） - Linear Technology
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LTM2881 Datasheet PDF : 24 Pages
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LTM2881
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
2881 F11
Figure 11. Curve Trace Between A and B with Termination
Enabled and Disabled
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.
130
128
126
124
122
120
118
116
114
112
110
–10
–5
0
5
10
15
COMMON MODE VOLTAGE (V)
2881 G11
Figure 12. Termination Resistance vs Common Mode Voltage
150
10
PHASE
140
0
130
–10
MAGNITUDE
120
–20
110
–30
100
0.1
1
FREQUENCY (MHz)
–40
10
2881 F13
Figure 13. Termination Magnitude and Phase vs Frequency
250
230
210
LTM2881-3
190 R=54 CL=1000p
170
R=54 CL=100p
R=54 CL=0
150
130 LTM2881-5
R=54 CL=1000p
110 R=54 CL=100p
90 R=54 CL=0
70
50
0.1
1
DATA RATE (Mbps)
10
2881 F14
Figure 14. Supply Current vs Data Rate
For more information www.linear.com/LTM2881
2881fh
15
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