A Comparison of Key Parametrics of CMOS and Bipolar Integrated Circuits In Line Driver Applications

AND8060/D
A Comparison of Key
Parametrics of CMOS and
Bipolar Integrated Circuits
In Line Driver Applications
Prepared by: Fred Zlotnick
ON Semiconductor
APPLICATION NOTE
INTRODUCTION
CMOS has the reputation, among system designers, as a
low power technology; whereas, Bipolar ECL is considered
a power hog. However, many low power CMOS devices
consume many watts, and many ECL devices consume
much less than one watt. This paper addresses devices in
CMOS and ECL technology that can be used for similar
purposes and compares the differences in speed, power, and
other relevant specifications.
CMOS started its life as a low power alternative to high
density ICs for the watch and calculator industry. Operating
at 5 V and less than 1 MHz, CMOS was near zero power.
Indeed, the structure was created using a P–channel device
as a load resistor and an active N channel. With this
structure, power was consumed only during transition and
with clock rates at just a few hundred kilohertz, power was
nearly zero. In contrast, the ECL used a bipolar differential
circuit whereby speed was achieved by keeping its
transistors out of saturation. By the early 1970s, CMOS
speeds were about 10 MHz, while ECL could operate at
nearly 1 GHz.
Notwithstanding a few unique applications, CMOS is the
only viable technology for building high–density devices.
Microprocessors, memories, ASICs, and ASSPs are nearly
all CMOS. However, there is still room for optimal
performance using “mix and match” technologies. For
example, in I/O structures, density is not the critical factor
and ultra–small geometry is not necessarily the key to
improved performance.
According to Insight Onsite, 5.0 V logic represented as
much as 75% of all logic sold in 1999. Since all very small
geometry devices must run at less than 5.0 V, it appears that
the real world is far behind the leading–edge technology.
Many CMOS output drivers can drive from 24 – 64 mA. The
need for such high drive lies with the very low impedance of
bus lines and the quest for speed.
Achieving speed depends on several factors. A signal
propagates across a board at the speed of light. If the board
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April, 2001 – Rev. 0
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is fabricated from a standard material like glass–epoxy, the
speed is 5 – 6 ns/m. It takes approximately 2 ns to propagate
a signal across a board a distance of 0.33 m. A very fast
CMOS IC might switch in 2 ns; therefore, a total of 4 ns are
lost waiting for the signal to get to the device and switch it
on. If a designer was trying to use this trace as part of a
backplane operating at 100 MHz, nearly half a time slot will
be used to deliver the signal to its target IC and switch it on.
In actuality, running a backplane–bus at 100 MHz is quite
difficult. It is easier, but more costly, to widen the bus.
A CMOS IC input at the end of a 0.33 m transmission line
constitutes a nominal 5 – 10 pf capacitor termination.
Transmission line theory shows a signal will travel back and
forth down the line, creating overshoot and undershoot.
Since a CMOS input signal must be monotonic, a
termination resistance must be placed on the trace to prevent
ringing. With standard printed circuit techniques, Blood [1]
shows the characteristic impedance of a line is between
40 – 120 Ω. A single resistor of 100 Ω can be placed in
parallel to the input of the receiving IC; thus, terminating the
line in its assumed characteristic impedance. If the supply
voltage is 5.0 V and the saturation voltage of an output
device is assumed to be 0.5 V, the current in the driving
devices is 45 mA. This implies greater than 100 mW of
power dissipation per trace, assuming a 50% duty cycle. A
64–bit bus would draw a whopping 6 watts, just to terminate
the lines.
Forstner and Huchzemeier report that a backplane looks
like a transmission line with many capacitive loads across it.
The characteristic impedance of the line may drop by a
factor of 5 or more (e.g., from 100 Ω to 20 Ω). Matching a
20 Ω, 64–bit bus requires peak current of greater than
200 mA per line or a nearly 13 A peak. Operating from a
5.0 V supply with a 50% duty cycle, the average power is
30 W. It becomes obvious from this discussion that CMOS
is not necessarily a low–power technology, and in fact,
neither standard CMOS nor BiCMOS is very efficient for
driving high speed, rail–to–rail busses.
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comparator input that requires a small voltage swing. A
standard CMOS input, operating at 3.3 V, has an input
voltage requirement of +2.0 V for a high and 0.8 V for a low.
The input is replaced with a high–speed comparator,
requiring only a few hundred millivolts of swing. The output
driver is an open drain tied to a voltage less than Vcc. The
output swing is reduced along with output power. Typical
values for the output terminating voltage range between 1.2
and 1.5 V. The reference voltage of the comparator is
typically one–third the supply voltage (or 1.0 V). Figure 1
shows the circuit. The low supply voltage is 1.5 V and Vref
is 1.0 V +/–250 mV of threshold. The terminating resistor
can be as low as 20 Ω and still achieve reasonable power
dissipation.
Shrinking the device geometry has little effect on the
overall die size in many driver devices. Device size is
determined by pad pitch, ESD structures, and the very large
devices needed for high drive. Typical input loading is 10 pf
or more, irrespective of any internal geometry, owing to the
ESD structure of the device, and the board and connector
construction. Several bus solutions have recently become
available, all operating from lower voltage and with smaller
swing than standard TTL/CMOS, to reduce the power from
30 W.
There are two relatively new CMOS I/O structures
contending for bus driver applications. The first is Gunning
Transistor Logic (GTL). This output was created several
years ago to limit the power in an output stage. GTL uses a
+1.5 V
VCC, 3.3 Volts
Bus
GTL Input
20 Ω
Driver
Circuitry
Vref, 1.0 Volts
GTL Out
Figure 1. Circuit Diagram
specification, neither CMOS nor ECL, is a reference
document similar to RS–422, for voltage levels.
When implemented in CMOS, LVDS outputs look like
open–drain differential devices driven by a current source
(Figure 2). The input looks like GTL input with both inputs
being fed. The EIA–644 specification calls for 400 mV of
differential swing at the output and the input capable of
switching with 100 mV of swing. The terminating resistor
R is usually supplied externally by the system designer so a
value can be selected that matches the transmission line
used. In practice, the LVDS driver is limited to ~4 mA from
its current source, which limits the power dissipation in the
output stage to about 13 mW. As seen previously for a
backplane, 100 Ω is not a reasonable figure. A newer
specification, called Bus–LVDS (BLVDS), has been
developed to try to accommodate the very low impedance
encountered on a backplane. With differential I/Os, BLVDS
looks very similar to ECL.
In this configuration, the output current is approximately
50 mA, but the power supplied to the resistor is now only
37 mW, assuming 50% duty cycle. Thus, a 64–bit bus only
consumes 2.4 W. GTL provides a rather easy way to reduce
power consumption, while at the same time, matching
transmission line impedance. The downside risk is noise
immunity. With only +/–250 mV swing, the system has a
much lower noise margin than TTL or CMOS. In order to
overcome the noise immunity issues, a differential bus may
be used.
In a differential or “balanced” bus, any noise picked up on
one wire will usually be picked up on the second one, and the
common mode rejection at the input will cancel much of the
noise. ECL has been available as an option to the designer
and high noise/high–speed environments have used this
technology extensively. A newer technique developed in the
1990s uses CMOS devices in a differential mode. EIA–644
calls for a Low Voltage Differential Signal (LVDS). This
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VCC, 3.3 Volts
Twisted Pair Transmission Line
R = 100 Ω
Pre–Driver
Figure 2. CMOS LVDS
device doesn’t decrease with lower voltage, as long as the
device is specified to operate at that voltage. New ECL
devices have 2.5 volt specifications and are faster than older
higher voltage parts because of improved device geometry.
Also, unlike CMOS, power requirements are fairly
independent of toggle rate. Newer devices operating at even
lower voltage will be available soon. This translates directly
to lower power since the current is nearly independent of the
operating voltage.
ECL, by its very structure, is differential and the swing is
generally +/–800 mV, about twice that of LVDS. Larger
swing improves noise immunity, and unlike CMOS LVDS,
voltage swing is not achieved at the expense of device
power. ECL circuits are low impedance voltage sources
driven from emitter followers. It is relatively easy to limit
the swing of ECL outputs, either internally or externally, and
match the EIA–644 specification. ECL inputs are
differential as well and will switch with a 50 mV
differential, although the specification is generally much
higher. An ECL differential input pair is far better matched
than CMOS devices. This translates to more consistent
switching and results in lower pin–to–pin skew and lower
part–to–part skew, as well. ECL has made tremendous
strides in process technology. Submicron geometry
predominates, with much lower voltage and internal
capacitance. The fτ available now is 10 – 20 GHz or more.
In bus driver applications, ECL can be ten times faster than
CMOS (GTL or LVDS).
Figure 3 illustrates a differential ECL driver with emitter
follower outputs and comparator inputs. As with CMOS,
processing innovations allow power requirements to drop
dramatically for ECL. Unlike CMOS, the speed of the
ECL Output
ECL Input
Differential
Transmission Line
Figure 3. ECL Driver with Emitter Follower Outputs
and Comparator Inputs
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Comparing ECL to CMOS LVDS and
Standard CMOS Devices
consumption stated per pair. All values come from original
manufacturers’ data sheets. The supply voltage is the value
recommended by the manufacturer. It is assumed the VCX
device is driving an 80 Ω resistor.
Table 1 shows the performance of several devices. The
methodology uses a single receiver/transmitter pair. It is
assumed the devices are terminated with power
Table 1. Device Comparison
Characteristic
Units
Vendor “A”
ECLinPS Plus
VCX
Transmitter
–
LV017A
100EP16
VCX04
Receiver
–
LV018A
100EP16
VCX04
Technology
–
CMOS/LVDS
ECL
CMOS
Standards
–
EIA–644
ECL
CMOS
Operating Voltage
–
3.3
3.3
3.3
MHz
300
3000
200
Max Propagation Delay*
ps
4000
280
5.6
Max Differential Skew*
ps
1100
40
N/App
ps
2000
140
4400
mW
133
158
100
Max Operating Frequency
Max Part to Part Skew*
Power Cons at Max Frequency (typical)*
*Per transmitter/receiver pair
Conclusion
For backplane applications, BLVDS needs to match
impedance as low as 20 Ω. This will be the subject of a
forthcoming article.
It would seem that GTL is much better suited than ECL or
standard CMOS to backplane requirements at less than 300
MHz, since it requires only a single line per I/O. For optimal
backplane speed, ECL is still the only technology that will
permit speed greater than 300 MHz reliably, and
surprisingly, at power levels not very different from CMOS
running at much lower speed.
There were no GTL line drivers available to compare, so
VCX was used as the closest CMOS technology. GTL, if
made into a line driver, would be expected to have power
requirements somewhat greater than LVDS since the
technology is similar. GTL is essentially single–ended
LVDS with a fixed reference voltage. The power
requirements are nearly the same for ECL and LVDS, even
though ECL runs at ten times the speed.
For line driver applications, LVDS is useful below 300
MHz. Beyond 300 MHz, the designer has the choice of ECL
or multiple LVDS drivers. There are some LVDS clock
drivers available at the present and results are similar.
Pin–to–pin skew is much worse for LVDS when compared
to ECL and the power requirements are quite similar.
Although attempts have been made to create a differential
bus using BLVDS devices, BLVDS devices are useful to
about 300 MHz. They have skew and propagation delay
much higher than ECL without the noise immunity.
References:
1. MECL System Design Handbook, William Blood,
ON Semiconductor Publication HB205/D, Rev.1A,
May, 1988.
2. “Fast GTL Backplane with the GTL 1655,” P.
Forstner and J. Huchzermeier, T. I. Application
Note, 1999.
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Notes
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Notes
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Notes
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