MSC8112 Datasheet, PDF (40/44 Pages) Freescale Semiconductor, Inc – Dual Core Digital Signal Processor

Hardware Design Considerations
3.4 External SDRAM Selection
The external bus speed implemented in a system determines the speed of the SDRAM used on that bus. However, because of
differences in timing characteristics among various SDRAM manufacturers, you may have use a faster speed rated SDRAM to
assure efficient data transfer across the bus. For example, for 133 MHz operation, you may have to use 133 or 166 MHz
SDRAM. Always perform a detailed timing analysis using the MSC8112 bus timing values and the manufacturer specifications
for the SDRAM to ensure correct operation within your system design. The output delay listed in SDRAM specifications is
usually given for a load of 30 pF. Scale the number to your specific board load using the typical scaling number provided by
the SDRAM manufacturer.
3.5 Thermal Considerations
An estimation of the chip-junction temperature, T_J, in °C can be obtained from the following:
T_J = T_A + (R_θJA × P_D)
Eqn. 1
where
T_A = ambient temperature near the package (°C)
R_θJA = junction-to-ambient thermal resistance (°C/W)
P_D = P_INT + P_I/O = power dissipation in the package (W)
P_INT = I_DD × V_DD = internal power dissipation (W)
P_I/O = power dissipated from device on output pins (W)
The power dissipation values for the MSC8112 are listed in Table 2-3. The ambient temperature for the device is the air
temperature in the immediate vicinity that would cool the device. The junction-to-ambient thermal resistances are JEDEC
standard values that provide a quick and easy estimation of thermal performance. There are two values in common usage: the
value determined on a single layer board and the value obtained on a board with two planes. The value that more closely
approximates a specific application depends on the power dissipated by other components on the printed circuit board (PCB).
The value obtained using a single layer board is appropriate for tightly packed PCB configurations. The value obtained using a
board with internal planes is more appropriate for boards with low power dissipation (less than 0.02 W/cm² with natural
convection) and well separated components. Based on an estimation of junction temperature using this technique, determine
whether a more detailed thermal analysis is required. Standard thermal management techniques can be used to maintain the
device thermal junction temperature below its maximum. If T_J appears to be too high, either lower the ambient temperature or
the power dissipation of the chip. You can verify the junction temperature by measuring the case temperature using a small
diameter thermocouple (40 gauge is recommended) or an infrared temperature sensor on a spot on the device case that is painted
black. The MSC8112 device case surface is too shiny (low emissivity) to yield an accurate infrared temperature measurement.
Use the following equation to determine T_J:
T_J = T_T + (θ_JA × P_D)
Eqn. 2
where
T_T = thermocouple (or infrared) temperature on top of the package (°C)
θ_JA = thermal characterization parameter (°C/W)
P_D = power dissipation in the package (W)
Note: See MSC8102, MSC8122, and MSC8126 Thermal Management Design Guidelines (AN2601/D).
MSC8112 Dual Core Digital Signal Processor Data Sheet, Rev. 0
40
Freescale Semiconductor