DS92LV8028 Datasheet, PDF (11/16 Pages) National Semiconductor (TI) – 8 Channel 10:1 Serializer

Application Information
USING THE DS92LV8028
The DS92LV8028 is an easy to use serializer that combines
eight 10:1 serializers into a single chip with a maximum
payload of 5.28Gbps. Each of the eight serializers accepts
10 or less data bits. The serializers then multiplex the data
into a serial data stream with embedded clock bits and route
to the LVDS output at up to 660Mbps per channels. The
LVDS output is a 5 ma current loop driver that can be used
for point-to-point and lightly loaded multidrop applications.
Each of the eight channels has their own serializer function
but share a single Transmit Clock (TCLK) with a single PLL
for the entire chip. The data on all eight channels is latched
into the device with the rising edge of TCLK and the data
stream is compatible with the DS92LV1210, DS92LV1212A,
DS92LV1224, DS92LV1260 deserializers from National
Semiconductor.
If using less than 10 bits of data, it is recommended to tie off
adjacent bits to the embedded clock bits to prevent causing
a RMT in the data payload. For example, if only using 8 bits,
tie D0 High and D9 Low.
Power Considerations
All CMOS design of the Serializer and Deserializer makes
them inherently low power devices. Additionally, the constant
current source nature of the LVDS outputs minimize the
slope of the speed vs. I_CC curve of CMOS designs.
PCB LAYOUT AND POWER SYSTEM
CONSIDERATIONS
Circuit board layout and stack-up for the BLVDS devices
should be designed to provide low-noise power feed to the
device. Good layout practice will also separate high-
frequency or high-level inputs and outputs to minimize un-
wanted stray noise pickup, feedback and interference.
Power system performance may be greatly improved by
using thin dielectrics (2 to 4 mils) for power / ground sand-
wiches. This arrangement provides plane capacitance for
the PCB power system with low-inductance parasitic, espe-
cially proven effective at high frequencies above approxi-
mately 50MHz, and makes the value and placement of ex-
ternal bypass capacitors less critical. External bypass
capacitors should include both RF ceramic and tantalum
electrolytic types. RF capacitors may use values in the range
of 0.01 uF to 0.1 uF. Tantalum capacitors may be in the 2.2
uF to 10 uF range. Voltage rating of the tantalum capacitors
should be at least 5X the power supply voltage being used.
It is a recommended practice to use two vias at each power
pin as well as at all RF bypass capacitor terminals. Dual vias
reduce the interconnect inductance by up to half, thereby
reducing interconnect inductance and extending the effec-
tive frequency range of the bypass components. Locate RF
capacitors as close as possible to the supply pins, and use
wide low impedance traces (not 50 Ohm traces). Surface
mount capacitors are recommended due to their smaller
parasitics. When using multiple capacitors per supply pin,
locate the smaller value closer to the pin. A large bulk
capacitor is recommend at the point of power entry. This is
typically in the 50uF to 100uF range and will smooth low
frequency switching noise. It is recommended to connect
power and ground pins straight to the power and ground
plane, with the bypass capacitors connected to the plane
with via on both ends of the capacitor. Connecting a power or
ground pin to an external bypass capacitor will increase the
inductance of the path.
A small body size X7R chip capacitor, such as 0603, is
recommended for external bypass. Its small body size re-
duces the parasitic inductance of the capacitor. User must
pay attention to the resonance frequency of these external
bypass capacitors, usually in the range of 20-30MHz range.
To provide effective bypassing, very often, multiple capaci-
tors are used to achieve low impedance between the supply
rails over the frequency of interest. At high frequency, it is
also a common practice to use two via from power and
ground pins to the planes, reducing the impedance at high
frequency.
Some devices provide separate power and ground pins for
different portions of the circuit. This is done to isolate switch-
ing noise effects between different sections of the circuit.
Separate planes on the PCB are typically not required. Pin
Description tables typically provide guidance on which circuit
blocks are connected to which power pin pairs. In some
cases, an external filter many be used to provide clean
power to sensitive circuits such as PLLs.
Use at least a four layer board with a power and ground
plane. Locate CMOS (TTL) swings away from the LVDS
lines to prevent coupling from the CMOS lines to the LVDS
lines. Closely-coupled differential lines of 100 Ohms are
typically recommended for LVDS interconnect. The closely-
coupled lines help to ensure that coupled noise will appear
as common-mode and thus is rejected by the receivers. Also
the tight coupled lines will radiate less.
TRANSMISSION MEDIA
The DS92LV8028 Serializers can be used in point-to-point
configuration of a backplane across PCB traces or through
cable interconnect. In point-to-point configurations the trans-
mission media needs only to be terminated at the receiver
end. The DS92LV8028 may also be used with double termi-
nations for a total load or 50 Ohms for use in certain limited
multidrop applications. Termination impedances lower than
50 Ohms is not recommended.
TERMINATION
Termination of the LVDS interconnect is required. For point-
to-point applications termination should be located at the
load end. Nominal value is 100 Ohms to match the line’s
differential impedance. Place the resistor as close to the
receiver inputs as possible to minimize the resulting stub
between the termination resistor and receiver.
Additional general guidance can be found in the LVDS Own-
er’s Manual - available in PDF format from the national web
site at: www.national.com/lvds
DS92LV8028 BLVDS SERIALIZER BYPASS
RECOMMENDATIONS
General device specific guidance is given below. Exact guid-
ance can not be given as it is dictated by other board level
/system level criteria. This includes the density of the board,
power rails, power supply, and other integrated circuit power
supply needs.
For a typical application circuit, please see Figure 12.
DVDD = DIGITAL SECTION POWER SUPPLY
These pins supply the digital portion of the device. A 0.1uF
capacitor is sufficient for these pins.
11
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