Thermal Calculation and Measurement
4
Input
capacitance is periodically sampled.
TSIZ0/REG, TSIZ1, D(0:31), IRQ(2:4), IRQ6, RD/WR, BURST, IP_B(3:7), PA(0:11), PA13, PA15, PB(14:31),
PC(4:15), PD(3:15), PE(14:31), MII1_CRS, MII_MDIO, MII1_TXEN, MII1_COL.
6
BDIP/GPL_B(5), BR, BG, FRZ/IRQ6, CS(0:7), WE(0:3), BS_A(0:3), GPL_A0/GPL_B0, OE/GPL_A1/GPL_B1,
GPL_A(2:3)/GPL_B(2:3)/CS(2:3), UPWAITA/GPL_A4, UPWAITB/GPL_B4, GPL_A5, ALE_A, CE1_A, CE2_A,
OP(0:3) BADDR(28:30)
5
A(0:31),
7
Thermal Calculation and Measurement
NOTE
The V
DDSYN
power dissipation is negligible.
For the following discussions, P
D
= (V
DDL
×
I
DDL
) + PI/O, where PI/O is the power dissipation of the I/O drivers.
7.1 Estimation with Junction-to-Ambient Thermal Resistance
An estimation of the chip junction temperature, T
J
, in °C can be obtained from the following equation:
T
J
= T
A
+ (R
θJA
×
P
D
)
where:
T
A
= ambient temperature ºC
R
θJA
= package junction-to-ambient thermal resistance (ºC/W)
P
D
= power dissipation in package
The junction-to-ambient thermal resistance is an industry standard value that provides a quick and easy estimation
of thermal performance. However, the answer is only an estimate; test cases have demonstrated that errors of a factor
of two (in the quantity T
J
–T
A
) are possible.
7.2 Estimation with Junction-to-Case Thermal Resistance
Historically, thermal resistance has frequently been expressed as the sum of a junction-to-case thermal resistance
and a case-to-ambient thermal resistance:
R
θJA
= R
θJC
+ R
θCA
where:
R
θJA
= junction-to-ambient thermal resistance (ºC/W)
R
θJC
= junction-to-case thermal resistance (ºC/W)
R
θCA
= case-to-ambient thermal resistance (ºC/W)
R
θJC
is device-related and cannot be influenced by the user. The user adjusts the thermal environment to affect the
case-to-ambient thermal resistance, R
θCA
. For instance, the user can change the airflow around the device, add a
heat sink, change the mounting arrangement on the printed circuit board, or change the thermal dissipation on the
printed circuit board surrounding the device. This thermal model is most useful for ceramic packages with heat sinks
where some 90% of the heat flows through the case and the heat sink to the ambient environment. For most
packages, a better model is required.
MPC885/MPC880 Hardware Specifications, Rev. 3
12
Freescale Semiconductor
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
