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MC10H641FN Folha de dados(PDF) 5 Page - ON Semiconductor |
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MC10H641FN Folha de dados(HTML) 5 Page - ON Semiconductor |
5 / 10 page MC10H641, MC100H641 http://onsemi.com 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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