2001 Fairchild Semiconductor Corporation
AN500190
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Fairchild Semiconductor
Application Note
September 1998
Revised February 2001
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AN-5002
GTLP: Single vs. Multiple Output Switching Technical
Discussion
Abstract
Single Output Switching (SOS) specifications are provided
by the supplier as a tool to allow a cursory look at the per-
formance of a device. Actual performance is highly depen-
dent on the application in which it is used. The inclusion of
Multiple Output Switching (MOS) specifications gives an
additional data point to use when determining the change
in relative performance of a device. Derating curves pro-
vide a number of additional data points to assist in deter-
mining the change in relative performance of a device.
This application note provides a description of SOS and
MOS specifications that can be used to integrate new
devices into a design. These tools must be used with cau-
tion when calculating parameters such as timing budgets in
actual applications. The testing conditions used in setting
standard specifications for a datasheet are most likely dif-
ferent than the actual loading and conditioning of the
devices in an application.
Definitions
Single Output Switching
Single Output Switching (SOS) describes the single bit
propagation delay performance of a device. The tested
propagation delay performance is statistically processed
into a standardized specification that is used in datasheets
to provide a way to compare similar products from compet-
ing suppliers.
Multiple Output Switching
Multiple Output Switching (MOS) describes the multiple bit
propagation delay performance of a device. The word "mul-
tiple" usually refers to 8, 16 or 32 bits, depending on the
total number of data bits that the device has. It can also
refer to any other combination of multiple switching data
bits, but is always two or more.
The MOS measurement conditions usually mimic the SOS
conditions except for multiple bit switching. There is not a
standardized methodology for specifying performance,
making it somewhat difficult to compare similar products
from competing suppliers.
Specifications
The datasheet specifications of propagation delay perfor-
mance usually have only SOS specifications. Some IC
suppliers provide Extended AC Electrical Characteristics
that include MOS propagation delay performance and der-
ating curves. This extended data gives a useful compari-
son to standard SOS specifications in a controlled test
environment.
The test measuring conditions or testing environment for
SOS and MOS rely on controlled parameters such as test
loads, trace lengths and impedances, and frequency of
operation. While the measurement conditions seem far
removed from an actual application, they are currently the
best standardized test conditions that are available to
describe device propagation delay performance.
Understanding Specifications
Datasheet specifications (SOS, MOS or any other parame-
ter) try to best describe the device performance in near-
realistic applications. Currently the SOS and MOS propa-
gation delay performance is measured using 30pF/25
or
50pF/500
lumped loads for the Gunning Transceiver
Logic Plus (GTLP) family of products. The configuration of
the TTL load can be modelled as shown in Figure 1. The
output pull-up value of 6V is used for 3.3V V
CC
operation.
When testing SOS propagation delay performance, each of
the single bit paths are measured separately with the test
load. Each single bit is remeasured over the range of oper-
ating V
CC
and temperature. The statistical minimum, maxi-
mum, and mean of the sample of single bit paths are then
used to calculate the databook specification.
When testing MOS propagation delay performance all bit
paths are simultaneously switched in phase with the test
sense probe being moved to measure each output. The
load capacitance can be varied for additional extended AC
electrical characteristics. Typically, derating curves of prop-
agation delay versus load, use lumped load capacitances
of 10 pF, 30 pF, 50 pF, 100 pF and at times 250 pF.
FIGURE 1. AC Test Circuit
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AN-5002
How is MOS Used
When available, MOS specifications are used to determine
relative deltas in propagation delay performance of the
device. Because MOS testing uses the AC test circuit of
Figure 1, the user must be careful in using the propagation
delay values for timing budget analysis. The actual propa-
gation delay performance will depend on the type and dis-
tribution of the load the device is driving.
The usefulness of MOS data applies more to applications
that may be synchronous in nature when more than one
output is switching simultaneously. Synchronous switching,
especially in-phase synchronous switching, is generally
considered the worst case application from the driving
device point of view and is consequently the setup used for
MOS testing.
Common Mistakes
There are some common mistakes when interpreting SOS
propagation delay specifications. The most common mis-
take is assuming that the specification guarantees maxi-
mum propagation delay if all outputs were simultaneously
switching. MOS derating curves explain the degradation
beyond the specified SOS propagation delay.
The other common mistake is to assume the SOS propa-
gation delay maximum specification guarantees perfor-
mance across all loading conditions. There are often
datasheet derating curves for the change in propagation
delay over capacitive load. The test load in all cases is
lumped versus distributed.
Data Specifications Format
Table 1 and Table 2 are examples of datasheet specifica-
tions. Table 1 gives the maximum and minimum specifica-
tions of SOS propagation delay over the industrial/
commercial temperature range, V
CC
range, and standard
loading. Table 2 gives MOS specifications of propagation
delay with the same testing conditions as SOS but with all
outputs switching.
TABLE 1. AC Electrical Characteristics
TABLE 2. Extended AC Electrical Characteristics
T
A
=
-
40
C to
+
85
C,
C
L
=
30 pF, R
L
=
25
Symbol
Parameter
V
CC
=
3.3V
0.15V
Units
V
CCQ
=
5.0V
0.25V
(A to B)
Min
Max
t
PLH
Propagation Delay
1.0
6.5
ns
t
PHL
Propagation Delay
1.0
8.2
ns
Symbol
Parameter
T
A
=
-
40
C to
+
85
C,
Units
C
L
=
30 pF, R
L
=
25
V
CC
=
3.3V
0.15V
V
CCQ
=
5.0V
0.25V
(A to B)
18 Outputs Switching
Min
Max
t
PLH
Propagation Delay
1.0
8.8
ns
t
PHL
Propagation Delay
1.0
9.7
ns
3
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Derating Curves
Derating curves provide device performance data beyond
standard datasheet specifications. These curves are often
provided when a new family of products are introduced and
can be used for all the functions in the family. The MOS
derating curves describe the change in propagation delay
of a device as the number of switching outputs for that
device increases. Examples of t
PLH
and t
PHL
derating
curves include a table of the statistical mean @ 3.3V/5.0V,
30 pF along with the corresponding plot of the data.
TABLE 3. Mean Propagation Delays (t
PLH
)
TABLE 4. Mean Propagation Delays (t
PHL
)
FIGURE 2. Derating Curve (t
PLH
)
FIGURE 3. Derating Curve (t
PHL
)
Conclusion
Single Output Switching (SOS) specifications are provided by the supplier as a tool for the user to allow a cursory look into
the performance of a device. Actual performance is highly dependent on the application in which it is used. The inclusion of
Multiple Output Switching (MOS) specifications give the user an additional data point to use when determining the change
in relative performance of a device. Derating curves provide a number of additional data points to assist the user in deter-
mining the change in relative performance of a device.
With a clearer understanding of SOS and MOS specifications it is possible to be better prepared to integrate new devices
into a design. These tools must be used with caution when calculating parameters, such as timing budgets, in actual appli-
cations. The testing conditions used in setting standard specifications for a datasheet are most likely different than the
actual loading and conditioning of the devices in an application.
The use of standard specifications, such as SOS, make the selection of which family to use an easier task. Before testing in
the actual application, using MOS and derating curves offers more detailed information available to make the selection pro-
cess easier.
Outputs
Mean Propagation Delay
Switching
(A to B)
LH
GTLP16612
1
4.1
5
4.71
9
5.37
13
5.93
18
6.44
Outputs
Mean Propagation Delay
HL
GTLP16612
1
5.02
5
5.47
9
5.89
13
6.23
18
6.54
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and
Fairchild reserves the right at any time without notice to change said circuitry and specifications.
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FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD
SEMICONDUCTOR CORPORATION. As used herein:
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which, (a) are intended for surgical implant into the
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to perform when properly used in accordance with
instructions for use provided in the labeling, can be rea-
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user.
2. A critical component in any component of a life support
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