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DS90LV031B Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. DS90LV031B
Description  3V LVDS Quad CMOS Differential Line Driver
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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DS90LV031B Datasheet(HTML) 5 Page - National Semiconductor (TI)

   
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Parameter Measurement Information (Continued)
Typical Application
Applications Information
General application guidelines and hints for LVDS drivers
and receivers may be found in the following application
notes: LVDS Owner’s Manual (lit #550062-001), AN808,
AN977, AN971, AN916, AN805, AN903.
LVDS drivers and receivers are intended to be primarily used
in an uncomplicated point-to-point configuration as is shown
in Figure 6. This configuration provides a clean signaling
environment for the quick edge rates of the drivers. The
receiver is connected to the driver through a balanced media
which may be a standard twisted pair cable, a parallel pair
cable, or simply PCB traces. Typically, the characteristic
differential impedance of the media is in the range of 100
Ω.
A termination resistor of 100
Ω should be selected to match
the media, and is located as close to the receiver input pins
as possible. The termination resistor converts the current
sourced by the driver into a voltage that is detected by the
receiver. Other configurations are possible such as a multi-
receiver configuration, but the effects of a mid-stream con-
nector(s), cable stub(s), and other impedance discontinuities
as well as ground shifting, noise margin limits, and total
termination loading must be taken into account.
The DS90LV031B differential line driver is a balanced cur-
rent source design. A current mode driver, generally speak-
ing has a high output impedance and supplies a constant
current for a range of loads (a voltage mode driver on the
other hand supplies a constant voltage for a range of loads).
Current is switched through the load in one direction to
produce a logic state and in the other direction to produce
the other logic state. The output current is typically 3.5 mA, a
minimum of 2.5 mA, and a maximum of 4.5 mA. The current
mode requires (as discussed above) that a resistive termi-
nation be employed to terminate the signal and to complete
the loop as shown in Figure 6. AC or unterminated configu-
rations are not allowed. The 3.5 mA loop current will develop
a differential voltage of 350 mV across the 100
Ω termination
resistor which the receiver detects with a 250 mV minimum
differential noise margin neglecting resistive line losses
(driven signal minus receiver threshold (350 mV – 100 mV =
250 mV)). The signal is centered around +1.2V (Driver Off-
set, V
OS) with respect to ground as shown in Figure 7. Note
that the steady-state voltage (V
SS) peak-to-peak swing is
twice the differential voltage (V
OD) and is typically 700 mV.
The current mode driver provides substantial benefits over
voltage mode drivers, such as an RS-422 driver. Its quies-
cent current remains relatively flat versus switching fre-
10131107
FIGURE 5. Driver TRI-STATE Delay Waveform
10131108
FIGURE 6. Point-to-Point Application
www.national.com
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