Application Information
INTERNAL TERMINATIONS
The DS15BR400 has integrated termination resistors on both
the input and outputs. The inputs have a 100Ω resistor across
the differential pair, placing the receiver termination as close
as possible to the input stage of the device. The LVDS outputs
also contain an integrated 100Ω ohm termination resistor, this
resistor is used to minimize the output return loss and does
not take the place of the 100 ohm termination at the inputs to
the receiving device. The integrated terminations improve
signal integrity and decrease the external component count
resulting in space savings. The DS15BR401 has 100Ω output
terminations only.
OUTPUT CHARACTERISTICS
The output characteristics of the DS15BRB400/DS15BR401
have been optimized for point-to-point backplane and cable
applications, and are not intended for multipoint or multidrop
signaling.
POWERDOWN MODE
The PWDN input activates a hardware powerdown mode.
When the powerdown mode is active (PWDN=L), all input and
output buffers and internal bias circuitry are powered off.
When exiting powerdown mode, there is a delay associated
with turning on bandgap references and input/output buffer
circuits as indicated in the LVDS Output Switching Charac-
teristics
Upon asserting the power down function (PWDN = Low), and
if the Pre-emphasis feature is enable, it is possible for the
driver output to source current for a short amount of time lifting
the output common mode to VDD. To prevent this occurrence,
a load discharge pull down path can be used on either output
(1 kΩ to ground recommended). Alternately, a commonly de-
ployed external failsafe network will also provide this path
(see INPUT FAILSAFE BIASING). The occurrence of this is
application dependant, and parameters that will effect if this
is of concern include: AC coupling, use of the powerdown
feature, presence of the discharge path, presence of the fail-
safe biasing, the usage of the pre-emphasis feature, and input
characteristics of the downstream LVDS Receiver.
PRE-EMPHASIS
Pre-emphasis dramatically reduces ISI jitter from long or
lossy transmission media. One pin is used to select the pre-
emphasis level for all outputs, off or on. The pre-emphasis
boost is approximately 6 dB at 750 MHz.
Pre-emphasis Control Selection Table
PEM Pre-Emphasis
0 Off
1 On
INPUT FAILSAFE BIASING
Failsafe biasing of the LVDS link should be considered if the
downstream Receiver is ON and enabled when the source is
in TRI-STATE, powered off, or removed. This will set a valid
known input state to the active receiver. This is accomplished
by using a pull up resistor to VDD on the ‘plus’ line, and a pull
down resistor to GND on the ‘minus’ line. Resistor values are
in the 750 Ohm to several kΩ range. The exact value depends
upon the desired common mode bias point, termination re-
sistor(s) and desired input differential voltage setting. Please
refer to application note AN-1194 “Failsafe Biasing of LVDS
interfaces” for more information and a general discussion.
DECOUPLING
Each power or ground lead of the DS15BR400 should be
connected to the PCB through a low inductance path. For best
results, one or more vias are used to connect a power or
ground pin to the nearby plane. Ideally, via placement is im-
mediately adjacent to the pin to avoid adding trace induc-
tance. Placing power plane closer to the top of the board
reduces effective via length and its associated inductance.
Bypass capacitors should be placed close to VDD pins. Small
physical size capacitors, such as 0402, X7R, surface mount
capacitors should be used to minimize body inductance of
capacitors. Each bypass capacitor is connected to the power
and ground plane through vias tangent to the pads of the ca-
pacitor. An X7R surface mount capacitor of size 0402 has
about 0.5 nH of body inductance. At frequencies above 30
MHz or so, X7R capacitors behave as low impedance induc-
tors. To extend the operating frequency range to a few hun-
dred MHz, an array of different capacitor values like 100 pF,
1 nF, 0.03 µF, and 0.1 µF are commonly used in parallel. The
most effective bypass capacitor can be built using sand-
wiched layers of power and ground at a separation of 2–3
mils. With a 2 mil FR4 dielectric, there is approximately 500
pF per square inch of PCB.
The center dap of the LLP package housing the DS15BR400
should be connected to a ground plane through an array of
vias. The via array reduces the effective inductance to ground
and enhances the thermal performance of the LLP package.
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DS15BR400/DS15BR401