MPC8248 Freescale Semiconductor, Inc, MPC8248 Datasheet - Page 13

• Manufacturer Part Number: MPC8248
• Description: Mpc8248 Powerquicc Ii Integrated Communications Processor
• Manufacturer: Freescale Semiconductor, Inc

4.4
When the board temperature is not known, a thermal simulation of the application is needed. The simple
two-resistor model can be used with the thermal simulation of the application, or a more accurate and
complex model of the package can be used in the thermal simulation.
4.5
To determine the junction temperature of the device in the application after prototypes are available, the
thermal characterization parameter (Ψ
measurement of the temperature at the top center of the package case using the following equation:
where:
The thermal characterization parameter is measured per JEDEC JESD51-2 specification using a 40 gauge
type T thermocouple epoxied to the top center of the package case. The thermocouple should be positioned
so that the thermocouple junction rests on the package. A small amount of epoxy is placed over the
thermocouple junction and over 1 mm of wire extending from the junction. The thermocouple wire is
placed flat against the case to avoid measurement errors caused by cooling effects of the thermocouple
wire.
4.6
Each VDD and VDDH pin should be provided with a low-impedance path to the board’s power supplies.
Each ground pin should likewise be provided with a low-impedance path to ground. The power supply pins
drive distinct groups of logic on chip. The VDD and VDDH power supplies should be bypassed to ground
using by-pass capacitors located as close as possible to the four sides of the package. For filtering high
frequency noise, a capacitor of 0.1uF on each VDD and VDDH pin is recommended. Further, for medium
frequency noise, a total of 2 capacitors of 47uF for VDD and 2 capacitors of 47uF for VDDH are also
recommnded. The capacitor leads and associated printed circuit traces connecting to chip VDD, VDDH
and ground should be kept to less than half an inch per capacitor lead. Boards should employ separate inner
layers for power and GND planes.
All output pins on the MPC8272 have fast rise and fall times. Printed circuit (PC) trace interconnection
length should be minimized to minimize overdamped conditions and reflections caused by these fast
output switching times. This recommendation particularly applies to the address and data buses. Maximum
PC trace lengths of six inches are recommended. Capacitance calculations should consider all device loads
as well as parasitic capacitances due to the PC traces. Attention to proper PCB layout and bypassing
becomes especially critical in systems with higher capacitive loads because these loads create higher
transient currents in the VDD and GND circuits. Pull up all unused inputs or signals that will be inputs
during reset. Special care should be taken to minimize the noise levels on the PLL supply pins.
Freescale Semiconductor
Ψ
T
P
D
T
JT
Estimation Using Simulation
Experimental Determination
Layout Practices
= thermocouple temperature on top of package
= power dissipation in package
= thermal characterization parameter
T
J
= T
T
MPC8272 PowerQUICC II™ Family Hardware Specifications, Rev. 1.5
+ (Ψ
JT
× P
D
)
JT
) can be used to determine the junction temperature with a
Thermal Characteristics
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