[hsdd] High-Speed Digital Design Newsletter - - Current-Source Driver
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Wed, 15 Apr 2009 09:46:21 -0700
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2009 Signal Integrity Seminars
taught by Dr. Howard Johnson
Register NOW!
Rochester, NY U. Oxford, U.K.
<http://www.sigcon.com/seminars/seminarHSDD.htm> High- Speed Digital
Design: <http://www.sigcon.com/seminars/Oxford.htm> June 15-16,
2009
<http://www.sigcon.com/seminars/seminarAHSSP.htm> Advanced High-Speed
Signal Propagation:
<http://www.sigcon.com/seminars/RochesterAHSSP.htm> May 4-5, 2009
<http://www.sigcon.com/seminars/seminarHSNG.htm> High-Speed Noise and
Grounding: <http://www.sigcon.com/seminars/RochesterHSNG.htm> May
6-7, 2009 <http://www.sigcon.com/seminars/oxford.htm> June 17-18,
2009
Current-Source Driver
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 12 Issue 04
Spring is here, and I'll be hitting the road again soon, this time to the
East Coast and Europe. I'll be in Rochester, NY from May 4-7 teaching two
seminars. The first, "Advanced High-Speed Signal Propagation", treats the
subject of going so fast (or so far) that you must take into account
high-frequency signal losses, reflections, and other artifacts associated
with transmission media. My latest course, "High-Speed Noise and Grounding",
explores the world of mixed-signal crosstalk.
If you wish to preview either course, check out my DVDs now available
through the IEC. These DVD features contain the lab demonstrations normally
shown during class, plus some additional features. Each DVD runs about 1-1/2
hours, broken into roughly half-hour chunks. Each chunk is self-explanatory.
They make great discussion topics for departmental meetings and
get-togethers.
Each DVD is associated with one of my public courses:
"High-Speed Digital Design" (VOL I): http://www.iec.org/pubs/pub.asp?pid=110
<http://www.iec.org/pubs/pub.asp?pid=110&bsi=4> &bsi=4
"Advanced High-Speed Signal Propagation" (VOL II):
http://www.iec.org/pubs/pub.asp?pid=111
<http://www.iec.org/pubs/pub.asp?pid=111&bsi=4> &bsi=4
"High-Speed Noise and Grounding" (VOL III):
http://www.iec.org/pubs/pub.asp?pid=112
<http://www.iec.org/pubs/pub.asp?pid=112&bsi=4> &bsi=4
Complete set of all three: http://www.iec.org/pubs/pub.asp?pid=113
<http://www.iec.org/pubs/pub.asp?pid=113&bsi=4> &bsi=4
PREFACE TO TODAY'S ARTICLE
This is the last article in a three-part series.
Starting in 1981, my technical mentor, Professor Martin (Marty) Graham,
worked with me to create a new distributed bus architecture that quadrupled
the performance of a large ROLM (later IBM) digital telephone exchange.
Marty revealed the principles behind his new bus structure in a series of
meetings, mostly around mealtime.
If you haven't read my latest two articles, go read them first. They contain
background material essential to understanding today's topic:
"Space-Time Diagrams" www.sigcon.com/Pubs/news/12_02.htm
"Nibble Effect" www.sigcon.com/Pubs/news/12_03.htm
_____
Current-Source Driver
By Dr. Howard Johnson,
Professor Graham relaxed, legs crossed, sitting back in his chair, slowly
clinking the ice in a freshly-poured scotch. He regarded the half-eaten
mound of black-bottom pie in front of him.
"This is the only good diner I know with a full bar," he said. "Plenty of
diners serve pie. Plenty of bars serve scotch." He straightened, locking his
eyes with mine. "When you want both at the same time you have to go to a
very special place."
Marty wiped his lips with a paper napkin and then carefully spread it flat
on the table. He pulled a black fountain pen from his jacket pocket. With
precise, delicate strokes on the tissue-thin paper he swiftly sketched a
familiar picture and began his lecture.
* * * * *
The time-space diagram in Figure 1 depicts two totem-pole drivers in action.
Both begin in the tri-stated condition. Driver A engages first, propagating
a two-bit sequence both right and left. At a later time, driver B engages,
emitting a three-bit signal. The timing of the two transmitted bit streams
is too tight, causing the two intended wave patterns to overlap in the
red-colored region.
There are two good scenarios you can use to understand the resulting wave
pattern interaction. They both lead to the same conclusion.
The first scenario takes into account the pre-existing state of the
transmission line at the time driver B initiates its action. At that precise
moment, in the absence of any action on the part of B, the left-moving
signal from A already holds the line high. As a result, within the red
region the activation of B's totem-pole high-side circuit drives no
additional new current onto the line, so the system behaves as if driver B
were not activated.
Only after the wave from A completely passes position B does the second
driver begin to source any significant current. As a result, the received
signal at the right end of the line arrives absent the leading portion of
B's transmission. Driver B was engaged for three bit times, but because of
the overlap only two appear at the right end of the line. That is the nibble
effect.
The second scenario assumes the existence of a signal from B, and then
calculates its effect on the signal from A (Figure 2). In this scenario,
prior to activation driver B exists in a high-impedance tri-stated
condition. At that time it exerts no influence on the initial portion of the
passing wave from A.
Now let driver B launch waves proceeding in both left and right directions.
Assume both waves have full-sized, unit amplitudes. Note that the act of
transmitting changes the topology of the transmission structure. At the
moment driver B becomes active, its output impedance changes from the
tri-state condition of high impedance to a switched-on condition of very low
impedance. The existence of a low-impedance node on the transmission line at
position B can create reflections. The reflection coefficient for any
signals interacting with that node would be negative one.
Consider the interaction of wave A with node B. While B remains active, the
left-moving wave from A reflects off driver B's low impedance, turning into
a right-moving wave with negative amplitude (marked in red, -1). The
superposition of A's negative reflection plus the pre-existing positive
signal from driver B (marked in green, +1) creates a signal of zero
amplitude moving to the right. That explains the absence of signal moving
to the right.
In the left-moving direction, the low impedance of node B attenuates the
signal from A to such a degree that none of it escapes past the driver (red
dotted line). The only signal that remains headed left is the signal from B
at unit amplitude (marked in green, +1). When the overlap from A ceases, the
right-moving reflection from A goes away. That leaves only the signal from
B, moving at unit amplitude in both directions.
I like the reflection scenario because it neatly explains the nibble effect
as the result of reflections caused by the low-impedance state of driver B.
TOTEM-POLE DRIVERS
According to the reflection scenario, the nibble effect goes away if you
eliminate the reflections at B. Let's see if a totem-pole driver can make
that happen.
A nibble-free distributed bus driver must meet two requirements.
(1) The driver must put out a healthy-sized signal. At the same time,
(2) The driver must allow other signals to pass without reflecting them.
Meeting requirement (1) is trivial. Any totem-pole bus driver built from
huge I/O transistors easily forces a full-sized signal onto a terminated
transmission line. Unfortunately, when turned on, such a big driver presents
to the transmission line a very low value of output impedance, violating
(2).
A mid-range value of output impedance won't fix the reflection problem. For
example, engage a 50-ohm driver midway along an otherwise good 50-ohm line.
When a pre-existing signal flies by it encounters the driver (50 ohms) in
parallel with the remaining portion of the line (also 50 ohms). The
composite load at the driver location therefore equals 25 ohms, creating a
33% reflection.
To eliminate the reflection problem you need a driver with a very high value
of output impedance, much higher than the transmission line impedance. You
could build such a driver using small, wimpy transistors with lots of
natural output resistance. Alternately, you could start with a more powerful
driver and connect it to the line through a large series resistor (1000 ohms
or more). Either approach creates a high-impedance structure, eliminating
reflections. Unfortunately, both approaches limit the driver's output
current to an unacceptably low value, violating (1).
You can not meet both requirements using a totem-pole driver structure.
WHAT MARTY REALLY MEANT
In the restaurant, when Marty said, "Plenty of diners serve pie," what he
really meant was, "Plenty of drivers serve up a nice, big signal. Other
drivers give you a high value of output impedance. But, when you want both
at the same time, you have to go to a very special place. "
The special place we have to visit is called the current-source driver.
A digital current source operates in only two states, ON and OFF. When
turned OFF, it presents a high-impedance load (zero current) under all
reasonable conditions of operation. The OFF state corresponds to digital
ZERO. It also serves the function of the DISABLED, or TRI-STATE, condition
of an ordinary totem-pole driver.
When turned ON a current source emits a calibrated amount of current
regardless of the voltage existing at the load (within reason). The main
parameters associated with a current-source are:
Output current. Most current-source drivers are configured as either NMOS or
bipolar NPN pull-down-only devices. They provide two levels of current: off
and on. In the off state they draw zero current. In the on state they sink
a prescribed amount of current, usually in the range of 10-50 mA.
Dynamic output admittance specifies the change in output current in reaction
to change in load voltage. A value of zero represents a perfect current
source (i.e., the current does not change at all in reaction to load
voltage). In a 50-ohm bus architecture, avoidance of reflections greater
than 5% requires an admittance less than .0021 mho, which equates to an
effective dynamic output resistance greater than 475 ohms.
Headroom refers to the range of output voltages over which the circuit works
as specified. If, for example, the nominal termination voltage for a
transmission structure is 2.5 volts, and the driver can work over a range of
2.5 down to 1.5 volts, then the circuit headroom equals 1 volt. A generous
headroom specification ensures the current source can always sink its
prescribed amount of current even as other unrelated voltage signals pass by
on the bus structure.
Output capacitance. A totem-pole (push-pull) structure requires two output
transistors. A pull-down-only driver requires just one output transistor,
giving it a natural advantage in terms of its output capacitance. This is
why most current-source drivers are configured as pull-down-only.
Switching time is simply the rise and fall time of output current, as
measured with a resistive load.
Figure 3 illustrates two simple NMOS current source circuits in operation.
The FET elements are operated within their constant-current region.
Practical circuits incorporate additional means to stabilize the circuit
against variations in temperature and manufacturing parameters, but this
simple circuit will suffice to show the basic operation.
Control signals A and B each vary between zero (off) and some positive
voltage that causes each FET to sink a known amount of current. Provided
that the load resistance (25 ohms in this case) is sufficiently low, so that
the FET drain voltages do not drop too low, the current from each source
superimposes with that of the other.
A current source, when connected to a transmission line, causes no
reflections because the dynamic output impedance remains high in both the on
and off states. It's the perfect device for a nibble-free distributed bus.
A current source can be designed to sink arbitrary amounts of current,
although there exist some fundamental tradeoffs involving the output current
capability, the output capacitance, the headroom requirement, and,
especially, the power dissipation.
In operation on a distributed structure, the current source resolves the
wave pattern overlap problem by simply producing, within the red-colored
region of Figure 1, a double-sized output voltage. This output voltage
represents the amplitude of the first wave from A superimposed upon the
output from B. When connected in shunt to a common bus, current-source
drivers do not interfere like totem-pole drivers. The resulting time-space
diagram, using current sources, appears in Figure 4.
Current-source drivers, combined in a master-slave architecture with the
master receiver at the right side of the bus and a synchronous clock source
broadcasting from the left side, pull free of the nibble-effect timing
restrictions. With current sources you can attain 100% bus utilization.
The pattern of transactions still requires pre-planning to avoid direct
conflicts between drivers, but as long as each device is assigned, from the
perspective of the master receiver, a unique timeslot then no interference
occurs regardless the pattern of access. That is a major simplification of,
and upgrade to, the standard totem-pole structure with nibble-effect timing
limitations.
Figure 5 illustrates the sort of complex transaction patterns made possible
with current-source drivers. In the figure, as many as four waves overlap,
stacking up a maximum output voltage 4 times the nominal unit output level.
With enough headroom, each driving circuit still functions properly.
* * * * *
Marty described the current-source bus idea to me in 1981. Our use of
current-source drivers in the context of a digital telephone exchange was
granted U.S. patent 4,627,050 in 1986. The first implementation was 75 feet
long, carrying 16 data bits at a clock speed of 18.432 MHz. I did not
believe at the time that the computer industry would ever reach speeds so
great that the same techniques would apply on printed circuit boards running
100 times faster, with traces 100 times shorter. To me, such a progression
seemed fantastic, yet, to Marty, it seemed inevitable.
Marty understood how the computer industry repeats the same themes
generation after generation. At each stage, the physical packaging presents
significant obstacles to progress. Everyone complains about fundamental
tradeoffs among speed, power, heat, and cost.
Then someone invents a new device, or places an existing device in a smaller
physical package. Suddenly, the constraints are removed and the clock speed
lurches forward until, guess what, the new physical packaging once again
limits performance.
Study the physics of signal transmission, packaging, and manufacturing
technology, Marty said, and your talents will always be in great demand.
Best Regards,
Dr. Howard Johnson
_____
There is still time to sign up for my classes in Rochester, NY in May. Use
Promo Code NC9 for a tuition discount.
I'll also be making my annual trip to the University of Oxford, England,
June 15-19, to teach High-Speed Digital Design and, for the first time
there, High-Speed Noise and Grounding.
A full schedule of classes is available at www.sigcon.com
<http://www.sigcon.com/> .
If you have an idea that would make a good topic for a future newsletter,
please send it to <mailto:info03@xxxxxxxxxx> info03@xxxxxxxxxx .
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