Thermal Characteristics
4.4
Estimation Using Simulation
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
Experimental Determination
To determine the junction temperature of the device in the application after prototypes are available, the
thermal characterization parameter (Ψ
JT
) can be used to determine the junction temperature with a
measurement of the temperature at the top center of the package case using the following equation:
T
J
= T
T
+ (Ψ
JT
×
P
D
)
where:
Ψ
JT
= thermal characterization parameter
T
T
= thermocouple temperature on top of package
P
D
= power dissipation in package
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
Layout Practices
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 bypass 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
recommended. 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 SoC 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.
MPC8272 PowerQUICC II Family Hardware Specifications, Rev. 3
16
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
