DS90LV047A_16 Datasheet, PDF (16/25 Pages) Texas Instruments – 3-V LVDS Quad CMOS Differential Line Driver

DS90LV047A
SNLS044D – MAY 2000 – REVISED JULY 2016
www.ti.com
10 Power Supply Recommendations
Although the DS90LV047A draws very little power while at rest. At higher switching frequencies there is a
dynamic current component which increases the overall power consumption. The DS90LV047A power supply
connection must take this additional current consumption into consideration for maximum power requirements.
11 Layout
11.1 Layout Guidelines
• Use at least 4 PCB layers (top to bottom); LVDS signals, ground, power, TTL signals.
• Isolate TTL signals from LVDS signals, otherwise the TTL may couple onto the LVDS lines. It is best to put
TTL and LVDS signals on different layers which are isolated by a power/ground plane(s).
• Keep drivers and receivers as close to the (LVDS port side) connectors as possible.
11.1.1 Power Decoupling Recommendations
Bypass capacitors must be used on power pins. Use high frequency ceramic (surface mount is recommended)
0.1-µF and 0.001-µF capacitors in parallel at the power supply pin with the smallest value capacitor closest to the
device supply pin. Additional scattered capacitors over the printed-circuit board improves decoupling. Multiple
vias must be used to connect the decoupling capacitors to the power planes. A 10-µF (35-V) or greater solid
tantalum capacitor must be connected at the power entry point on the printed-circuit board between the supply
and ground.
11.1.2 Differential Traces
Use controlled impedance traces which match the differential impedance of your transmission medium (that is,
cable) and termination resistor. Run the differential pair trace lines as close together as possible as soon as they
leave the IC (stubs must be < 10 mm long). This helps eliminate reflections and ensure noise is coupled as
common-mode. In fact, we have seen that differential signals which are 1 mm apart radiate far less noise than
traces 3 mm apart since magnetic field cancellation is much better with the closer traces. In addition, noise
induced on the differential lines is much more likely to appear as common-mode which is rejected by the
receiver.
Match electrical lengths between traces to reduce skew. Skew between the signals of a pair means a phase
difference between signals, which destroys the magnetic field cancellation benefits of differential signals and
EMI, results.
NOTE
The velocity of propagation, v = c/Er where c (the speed of light) = 0.2997mm/ps or
0.0118 in/ps
Do not rely solely on the autoroute function for differential traces. Carefully review dimensions to match
differential impedance and provide isolation for the differential lines. Minimize the number or vias and other
discontinuities on the line.
Avoid 90° turns (these cause impedance discontinuities). Use arcs or 45° bevels.
Within a pair of traces, the distance between the two traces must be minimized to maintain common-mode
rejection of the receivers. On the printed-circuit board, this distance must remain constant to avoid discontinuities
in differential impedance. Minor violations at connection points are allowable.
11.1.3 Termination
Use a termination resistor which best matches the differential impedance or your transmission line. The resistor
must be between 90 Ω and 130 Ω. Remember that the current mode outputs need the termination resistor to
generate the differential voltage. LVDS does not work without resistor termination. Typically, connecting a single
resistor across the pair at the receiver end will suffice.
16
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