Texas Instruments | DS92LV010A Bus LVDS Transcvr Ushers New Era of High-Perf Backplane Design | Application notes | Texas Instruments DS92LV010A Bus LVDS Transcvr Ushers New Era of High-Perf Backplane Design Application notes

Texas Instruments DS92LV010A Bus LVDS Transcvr Ushers New Era of High-Perf Backplane Design Application notes
DS92LV010A,DS92LV1021A,DS92LV1023E
Application Note 1115 DS92LV010A Bus LVDS Transceiver Ushers in a New Era
of High-Performance Backplane Design
Literature Number: SNLA012
National Semiconductor
Application Note 1115
John Goldie
July 1998
Bus LVDS (BLVDS) is a new family of bus interface circuits
invented by National Semiconductor based on LVDS technology. This family of interface devices is optimized for multipoint cable and backplane applications. The DS92LV010A is
the first member of this family and is a single transceiver device which supports operation at 155 Mbps in heavily loaded
(20 Card) backplane applications. It differs from standard
LVDS in providing increased drive current to handle double
terminations that are required in the multi-point application.
BLVDS drivers are also enhanced in their contention protection, and feature balanced output impedance.
reduced to 10 mA. To double the noise margin over that of
the reduce-swing, single-ended technologies, BLVDS uses a
differential data transmission scheme similar to LVDS but
enhanced for multi-point applications. This enables
the 250 mV swings to operate at 100s of Mbps, while doubling noise margin and reducing noise generation. It also
supports live-insertion of devices into an active bus due to
the receiver common-mode rejection capability. The first of a
series of devices for BLVDS applications is the
DS92LV010A. The DS92LV010A “Single Bus LVDS Transceiver” device’s performance is the scope of this application.
BUS LVDS
BLVDS features a low voltage differential signal of z250 mV
and fast transition times. This allows the drivers to support
applications ranging from low speeds at a few MHz (or even
DC) to high speeds in the 500 MHz range and even beyond.
Additionally, the low voltage swing minimizes power dissipation and noise generation. The differential data transmission
scheme provides a ± 1V common mode range and live insertion (hot plug) of devices into an active bus.
In the past, the bus driving problem was solved by increasing
the drive current of the standard logic single-ended drivers
(244 function). With standard logic swings and increased
drive current, application speeds were increased to the
10 MHz – 20 MHz range, but not faster. Since increasing
drive current alone was not enough, the next enhancement
was made. As before, the drive current was once again
raised; however, the signal swing was also reduced. Thus
BTL (Backplane Transceiver Logic) was invented which supports 80 mA-sink capability and a 1V signal swing. This approach easily drives heavily loaded backplanes up to the 50
MHz – 66 MHz range. However, it is still single-ended (like
generic TTL), and only provides about 400 mV of noise margin. To break the 100 MHz barrier, a single-ended, reduced
swing approach is not feasible since noise margin is already
at the minimum acceptable level.
Bus LVDS removes the need for a large amount of drive current, by reducing the signal swing an order of magnitude
from TTL levels. With the small swing, drive current can be
AN100939-1
FIGURE 1. DS92LV010A Bus LVDS Single Transceiver
THE DEVICE
The DS92LV010A is a highly versatile 8-pin compact device.
It provides one line driver and one receiver with common bus
pins. The common bus I/O pins reduce bus loading to a minimum (5 pF typical). The logic side of the device supports four
pins for maximum flexibility. The four pins are driver input
(DI), driver enable (DE) – active high, receiver output (RO),
and receiver enable (RE*) – active low. The four inputs are
CMOS inputs with TTL compatible thresholds. They also
present high impedance in power-off conditions and will not
load down the driving gate. The block diagram is shown in
Figure 1. This allows the device to be configured as a driver,
a receiver, or a transceiver. Both the driver and receiver
functions can be enabled simultaneously to support a loop
back function. Or, the DE and RE* pins may be tied together
to form a direction control pin. The possible configurations
are shown in Figure 2.
DS92LV010A Bus LVDS Transceiver Ushers in a New Era of High-Performance
Backplane Design
DS92LV010A Bus LVDS
Transceiver Ushers in a
New Era of
High-Performance
Backplane Design
AN100939-2
FIGURE 2. Device Configurations
AN-1115
TRI-STATE ® is a registered trademark of National Semiconductor Corporation.
© 1998 National Semiconductor Corporation
AN100939
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ployed for use on data buses, or for clock distribution. Pointto-Point, Multi-drop, and Multi-point bus configurations are
shown in Figure 3.
The DS92LV010A may be used in standard LVDS Point-toPoint applications, Multi-drop data distribution applications,
or in a classical multi-point bus application. It may be em-
AN100939-3
FIGURE 3. Bus Configurations: Point-to-Point, Multi-Drop, and Multi-Point
put is also high (BLVDS High) while the inverting output is
low (BLVDS Low), thus yielding a +250 mV differential output
voltage. Mathematically the differential voltage is calculated
as A−B, or 1.375 – 1.125 = +250 mV. For a low driver input
( < 0.8V) the true output is low while the inverting output is
high, thus yielding a –250 mV differential output voltage.
Again, mathematically this is calculated as A−B, or 1.125 –
1.375 = −250 mV. The differential output signal swing (Vss)
is twice VOD, typically 500 mV with the 27Ω load across the
driver outputs. Figure 4 illustrated the IOD vs VOD curve for
the device when operating for a 3.3V or 5V rail. Data points
are shown for 100Ω, 90Ω, 80Ω, 70Ω, 60Ω, 50Ω, 45Ω, 40Ω,
35Ω, 30Ω, 27Ω, 20Ω and 10Ω differential loads.
The DS92LV010A may also be powered from a 5V or a 3.3V
rail directly. Respective parameters are listed separately in
the datasheet. BLVDS bus characteristics and AC parameter
performance is similar for either power supply operation.
This allows the device to be used in a wide variety of applications.
THE DRIVER
The driver translates standard TTL levels to differential
BLVDS levels. The driver output levels are nominally 1.375V
and 1.125V providing a 250 mV differential signal across a
27Ω load. The offset voltage (VOS) of the driver is nominally
1.25V. When the driver input is logic high ( > 2V), the true outIOD vs VOD (VCC = 3.3V)
IOD vs VOD (VCC = 5V)
AN100939-4
AN100939-5
FIGURE 4. IOD vs VOD Curves
above. Since the device has a fixed output impedance, the
differential output voltage will vary depending upon the applied termination load. With a 27Ω load, the nominal VOD is
250 mV (IOD = 9.4 mA). With a 100Ω load, the nominal VOD
is 725 mV (IOD = 7.25 mA) when the device is powered from
a 3.3V rail. The single termination load to typically employed
Output impedance is balanced on the BLVDS driver. The
sourcing impedance is matched to the sinking impedance.
This can be seen in the slope of the respective VOH/IOH and
VOL/IOL curves for the device. These curves are shown in
Figure 5. By subtracting the VOL curve from the VOH curve,
the differential output voltage curve can be drawn (see Figure 4). This curve provides VOD/IOD information discussed
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2
in point-to-point applications (single-direction) where a termination is only needed at the far end of the line (see Figure 3).
VOH, VOL, VOS vs IOD (VCC = 3.3V)
VOH, VOL, VOS vs IOD (VCC = 5V)
AN100939-7
AN100939-6
FIGURE 5. IOH vs VOH and IOL vs VOL Curves
Also plotted on the VOH/IOD (IOH) and VOL/IOD (IOL) is the
driver offset voltage (VOS). The flatness of this line is also a
measure of balanced output impedance. For LVDS and
BLVDS drivers, this driver offset voltage is typically 1.25V.
The offset voltage is also one component of the common
mode voltage at the receiver input on the bus. The active signal levels present at the receiver input is a function of four
components:
1. The active driver offset voltage — VOS
2.
Any ground potential
devices — VGPD
3.
Any
longitudinally
mode) — Vnoise
difference
coupled
between
noise
VCM = VOS ± VGPD ± VNoise ± 0.5 VOD
the
(common
AN100939-9
FIGURE 7. The Three Components of VCM
4. The active driver differential output voltage — VOD
Mathematically this is: VCM = VOS ± VGPD ± Vnoise
± 0.5VOD. For LVDS and BLVDS systems, a ± 1V common
mode range is typically allowed, which is around the driver
offset voltage of +1.25V. Therefore the input common-mode
voltage range is +125 mV to +2.375V. This assumes an offset of +1.25V, a differential voltage of 250 mV and the combination of ground potential difference (VGPD) and noise
(VNoise) is ± 1V maximum. Note, how VOD, VOS and VCM are
related. Figures 6, 7 illustrate the measurement of these
common data transmission parameters.
FAULT TOLERANT AND CONTENTION SAFE
The driver is also very fault tolerant. Being a current mode
device, output current is regulated. Shorting a high output to
ground results in a small fault current. Even with a logic-high
output shorted to ground, fault power dissipation is limited to
less than 100 mW (IOSMAX x VCC = > 20 mA x 5V). A low
driver output can also be shorted to a supply rail without excessive current or damage to the device. In this case output
short current is limited to typically 15 mA, minimizing fault
current power dissipation. If multiple drivers are enabled at
the same time, the drivers will scale back output current until
a compliance voltage is met. This prevents damage to the
drivers under contention conditions that may occur due to
software error, or hardware configuration. The DS92LV010A
features a rugged I/O.
NOISE MINIMIZER
The ‘010 driver is a current mode device, and output loop
current is controlled. This provides multiple benefits to the
end system. First, dynamic power dissipation increases
slowly as switching frequency is increased. This enables the
device to operate at high data rates, easily exceeding
AN100939-8
FIGURE 6. VOA, VOB, VOD, and VOS
Driver Measurements
3
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RECEIVER FAILSAFE
The receiver also supports a Failsafe function. For three different input conditions, a stable HIGH output level on the receiver will be obtained. The three conditions occur due to different system configurations and events, they are:
155 Mbps. The current mode driver also minimizes shootthrough current in the driver output stage during transitions;
thus a flat ICC vs frequency curve is obtained without a lot of
noise generated on the power and ground planes of the
board. Lastly, the current mode outputs provide soft transitions with less high frequency content, reducing generated
EMI. Crosstalk can also be a problem in high-density applications where signal lines are closely spaced in backplane
routing channels. BLVDS drivers help to reduce system
crosstalk since their voltage swings are so small and
crosstalk is proportional to amplitude.
1.
2.
3. Shorted Inputs
An “Open Inputs” condition occurs when a powered-up card
or node is removed from the bus. If this node does not include the termination resistor, then the inputs are now open.
Internal to the receiver, the plus input is pulled high, and the
minus input is pulled low with high impedance resistive networks in the 200 kΩ range.
A “Terminated Inputs” condition occurs under multiple scenarios. This can occur when the bus is idle (no active driver),
when all drivers are powered-off, or when a card is removed
from the bus and it includes the termination resistor, then the
inputs are “terminated”. Once again, internal bias network
will set the output high for these conditions. Under this condition, a small differential voltage is created, if there is differential noise, then additional external failsafe resistors may
be required. Recall that the receiver is a high-gain, highspeed device, and its function is to detect small (amplitude
and duration) differential pulses and amplify them to full
CMOS levels. If this is the case – see the section on “External Failsafe Biasing”.
“Shorted Inputs” condition occurs when the “plus” input is
shorted to the “minus” input and the resulting differential voltage is 0V. Shorted input failsafe is not supported across the
common mode range of the receiver. It is defined as the input pins “shorted” together and no external common mode
voltage applied. It is typically not a good approach to pull
both inputs to the same common mode point also. This can
cause distortion to the first driven bit of valid data when the
bus becomes active.
External Failsafe Bias resistors can be used to boost the failsafe protection in noisy applications. The external bias resistors should be an order of magnitude greater than the termination resistor to minimize loading effects to active drivers.
The common mode point should be set to the 1.2V – 1.5V
range, and for the DS92LV010A should be always below
1.7V (see datasheet).
HIGH IMPEDANCE BUS PINS
The bus pins (DO/RI) provide high-impedance to the bus
when the device is in TRI-STATE ® or powered-off. This is extremely important in multi-point applications. Communication
between other active nodes should not be impacted if one or
more of the devices on the bus is powered-off. The
DS92LV010A supports this feature along with glitch-free bus
pins on power-up and down. Until VCC OK is reached, the
driver output pins are held in TRI-STATE by an internal circuit. At VCC-OK (typically 2.5V), the outputs follow the enables pins. If the driver enable (DE) is low out this point; the
driver outputs will remain off, and will not disturb traffic on the
bus while it is being powered-up. Bus loading presented by
the transceiver is also minimized since small geometry devices are used in the output of the driver (10 mA). Additionally, driver output and receiver input pins are connected together internally to reduce bus loading (only two pads
required instead of four). The bus pins typically present a
small 5 pF load. The light loading helps to maintain a high
working impedance for the bus.
AC PERFORMANCE
The DS92LV010A driver provides tight AC parameter limits
to support 155 Mbps operation. Driver rise and fall times are
typically 300 ps and are specified at 2 ns maximum. This
supports a ratio of 1-to-3 of transition time to unit interval and
provides a good balance between waveshape and noise
generation. Slew rate of the driver is held to typically 1 V/ns.
Also of importance is the propagation delay through the
driver for system timing calculations. Propagation delay is
specified at 3 ns typical and 5 ns maximum. Skew (pulse
skew) is typically 200 ps, and specified to be less than 1 ns.
Enable / Disable times are less than 10 ns, allowing for quick
turn around on the bus. Refer to the datasheet for complete
AC parameter limits, and conditions.
COMMON MODE RANGE
The receiver detects differential voltages, and rejects common mode voltages. The input stage design of the receiver is
optimized for switching performance over the common mode
range. This is the minimum common mode range of the device (GND to +2.4V). Input voltages may be up to one diode
below ground, and also as high as one diode above the device’s VCC potential. Exceeding this range may forward bias
the ESD protection circuitry of the device and clamping of
the active signal. The three components of the common
mode voltage seen at a receiver input are shown in Figure 7.
Noise coupled equally onto both lines is shown in Figure 8.
This noise is seen in the single-ended waveforms and not in
the differential waveform (A−B). To preserve the common
mode rejection of the receiver, twisted-pair cable and closely
coupled differential traces should be used as interconnect.
This will help ensure that noise is coupled equally to both
lines, and will be seen as common-mode and rejected.
THE RECEIVER
The receiver is a high-gain, high-speed device. It detects differential voltages as small as 20 mV (100 mV threshold
spec) and amplifies them to full CMOS levels with a propagation delay of only 5 ns. The bus pins are high impedance
(greater than 200 kΩ) to minimize loading effects to the bus
(see High Impedance Bus Pins – section above). The Highimpedance loading to the bus pins is also presented when
the device is powered-off. This allows for communication between other active nodes on the bus, while the power is off to
some of the nodes. This is especially important in applications that employ multiple receiver or transceiver (multipoint) devices. The receiver output transition times are controlled to limit switching noise from disturbing the bus, and
from generating system noise. The receiver’s performance
and features are discussed next.
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Open Inputs
Terminated Inputs
4
AN100939-10
FIGURE 8. Coupled Noise is Common Mode and is Rejected
abled. To ensure power biasing of the devices, standard
power sequencing is recommended. Ground should be applied first, then power, and then I/O pins on insertion. For removal, the reverse order is recommended.
AC PERFORMANCE
The DS92LV010A receiver like the driver provides tight AC
parameter limits to support 155 Mbps operation. Propagation
delay through the receiver for system timing calculations is
specified at 5 ns typical and 12 ns maximum. Skew, or pulse
skew is typically 500 ps, and specified to be less than 2 ns.
The receiver’s rise and fall times are typically 1.5 ns and are
specified at 4.0 ns maximum. Transition times are smooth
and balanced through the use of graduated turn-on circuitry.
This provides a symmetrical waveshape and generates little
noise. Enable/Disable times are less than 13 ns, allowing for
quick turn around on the bus. Refer to the datasheet for complete AC parameter limits, and conditions.
BLVDS ACHIEVES MEGABITS @ MILLIWATTS!
BLVDS drivers are capable of operating at 100’s of Mbps in
heavily loaded backplane applications. The data throughput
is achieved by the combination of differential data transmission and a small signal swing. However, not only is Megabit
service provided by BLVDS, but also ultra low power dissipation is provided. This is gained by the use of a core submicron CMOS process, a current mode driver and also the
small signal swing. With these low power levels, standard IC
packaging may be employed, and also the integration of digital blocks is enabled. The DS92LV1021/1210 Bus LVDS Serializer / Deserializer is an example of integration that BLVDS
has enabled. These devices include the BLVDS PHY (Line
Driver, Receiver, or Transceiver functional block), and also
the serial-parallel / parallel-serial conversion, clock embedding, clock recovery functions and digital control circuitry all
in a 28L SSOP package. These devices operate at
400 Mbps, and together consume typically less than
250 mW of power (at maximum switching rate).
BACKPLANE BENEFITS
The DS92LV010A is a highly versatile transceiver designed
to deliver Megabits @ millwatts. The concept of BLVDS and
differential data transmission also improves many aspects of
system design. These topics are discussed next briefly, and
are the subject of other National Bus LVDS application
notes.
COMMON SUPPLY RAILS
Unlike
other
high-performance
technologies,
the
DS92LV010A Bus LVDS Single Transceiver is powered from
common logic power rails of 5V or 3.3V. ECL is also a highperformance technology, but requires unique supply rails
such as −5.2V ECL or ± 2.5V for split-rail ECL operation.
These unique rails may complicate system power generation
and distribution. They also complicate the task of interfacing
directly to standard logic devices and other non-ECL application specific devices (ASICs).
SUMMARY
The DS92LV010A is the first of a series of devices for highspeed multi-point applications. The device can operate in the
100 Mbps – 155 Mbps range while consuming minimal
power and directly interface with standard logic devices. The
Bus LVDS products provide designers with new alternatives
for solving high-speed multipoint bus interface problems.
PASSIVE TERMINATION SIMPLIFIES BACKPLANE,
ELIMINATES ACTIVE DEVICES, AND IMPROVES
RELIABILITY!
BLVDS eliminates the need for a special termination pull-up
rail (Vt). Again this simplifies system power generation and
termination voltage distribution. It eliminates the need for active termination devices commonly required by the opencollector/drain single-ended technologies. This reduction in
termination complexity improves system reliability, and
saves PCB space.
LIVE INSERTION SUPPORT
The differential transmission scheme supports live insertion
of cards into active busses. This is due to the fact that when
a card is plugged into the bus the resulting glitch is seen as
common mode and ignored by active receivers. The driver
also includes a glitch-free power up circuit that keeps the
driver outputs in high-impedance until powered-up and en5
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DS92LV010A Bus LVDS Transceiver Ushers in a New Era of High-Performance
Backplane Design
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