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MC10H641 Datasheet(PDF) 5 Page - ON Semiconductor

Part No. MC10H641
Description  Single Supply PECL to TTL 1:9 Clock Distribution Chip
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

MC10H641 Datasheet(HTML) 5 Page - ON Semiconductor

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MC10H641, MC100H641
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5
Determining Skew for a Specific Application
The H641 has been designed to meet the needs of very low
skew clock distribution applications. In order to optimize
the device for this application special considerations are
necessary in the determining of the part−to−part skew
specification limits. Older standard logic devices are
specified with relatively slack limits so that the device can
be guaranteed over a wide range of potential environmental
conditions. This range of conditions represented all of the
potential applications in which the device could be used. The
result was a specification limit that in the vast majority of
cases was extremely conservative and thus did not allow for
an optimum system design. For non−critical skew designs
this practice is acceptable, however as the clock speeds of
systems increase overly conservative specification limits
can kill a design.
The following will discuss how users can use the
information provided in this data sheet to tailor a
part−to−part skew specification limit to their application.
The skew determination process may appear somewhat
tedious and time consuming, however if the utmost in
performance is required this procedure is necessary. For
applications which do not require this level of skew
performance a generic part−to−part skew limit of 2.5 ns can
be used. This limit is good for the entire ambient temperature
range, the guaranteed VCC (VT, VE) range and the
guaranteed operating frequency range.
Temperature Dependence
A unique characteristic of the H641 data sheet is that the
AC parameters are specified for a junction temperature
rather than the usual ambient temperature. Because very few
designs will actually utilize the entire commercial
temperature range of a device a tighter propagation delay
window can be established given the smaller temperature
range. Because the junction temperature and not the ambient
temperature is what affects the performance of the device the
parameter limits are specified for junction temperature. In
addition the relationship between the ambient and junction
temperature will vary depending on the frequency, load and
board environment of the application. Since these factors are
all under the control of the user it is impossible to provide
specification limits for every possible application.
Therefore a baseline specification was established for
specific junction temperatures and the information that
follows will allow these to be tailored to specific
applications.
Since the junction temperature of a device is difficult to
measure directly, the first requirement is to be able to
“translate” from ambient to junction temperatures. The
standard method of doing this is to use the power dissipation
of the device and the thermal resistance of the package. For
a TTL output device the power dissipation will be a function
of the load capacitance and the frequency of the output. The
total power dissipation of a device can be described by the
following equation:
PD (watts) = ICC (no load) * VCC +
VS * VCC * f * CL * # Outputs
where:
VS= Output Voltage Swing = 3.0 V
f = Output Frequency
CL = Load Capacitance
ICC = IEE + ICCH
Figure 1 plots the ICC versus Frequency of the H641 with
no load capacitance on the output. Using this graph and the
information specific to the application a user can determine
the power dissipation of the H641.
Figure 1. ICC versus f (No Load)
0
1020304050607080
FREQUENCY (MHz)
0
1
2
3
4
5
Figure 2 illustrates the thermal resistance (in °C/W) for
the PLCC−28 under various air flow conditions. By reading
the thermal resistance from the graph and multiplying by the
power dissipation calculated above the junction temperature
increase above ambient of the device can be calculated.
0
200
400
600
800
1000
AIRFLOW (LFPM)
30
40
50
60
70
Figure 2. jJA versus Air Flow
Finally taking this value for junction temperature and
applying it to Figure 3 allows the user to determine the


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