MSC7116_08 Datasheet, PDF (41/60 Pages) Freescale Semiconductor, Inc – Low-Cost 16-bit DSP with DDR Controller and 10/100 Mbps Ethernet MAC

Hardware Design Considerations
3 Hardware Design Considerations
This section described various areas to consider when incorporating the MSC7116 device into a system design.
3.1 Thermal Design 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 MSC7116 are listed in Table 4. 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. Use the following equation to determine T_J:
T_J = T_T + (Ψ_JT × P_D)
Eqn. 2
where
T_T = thermocouple (or infrared) temperature on top of the package (°C)
Ψ_JT = thermal characterization parameter (°C/W)
P_D = power dissipation in the package (W)
MSC7116 Data Sheet, Rev. 13
Freescale Semiconductor
41