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MC10H641 Scheda tecnica(PDF) 4 Page - ON Semiconductor |
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MC10H641 Scheda tecnica(HTML) 4 Page - ON Semiconductor |
4 / 8 page MC10H641 MC100H641 MOTOROLA MECL Data DL122 — Rev 6 2–4 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 = 3V 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 1020304050 607080 FREQUENCY (MHz) 0 1 2 3 4 5 Figure 2 illustrates the thermal resistance (in °C/W) for the 28–lead PLCC 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. ∅JA versus Air Flow Finally taking this value for junction temperature and applying it to Figure 3 allows the user to determine the propagation delay for the device in question. A more common use would be to establish an ambient temperature range for the H641’s in the system and utilize the above methodology to determine the potential increased skew of the distribution network. Note that for this information if the TPD versus Temperature curve were linear the calculations would not be required. If the curve were linear over all temperatures a simple temperature coefficient could be provided. Figure 3. TPD versus Junction Temperature –30 JUNCTION TEMPERATURE ( 5.2 –10 10 30 50 70 90 110 130 °C) 5.4 5.6 5.8 6.0 6.2 6.4 TPHL TPLH |
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