Thermal Calculation and Measurement
4
Input capacitance is periodically sampled.
5
A(0:31), TSIZ0/REG, TSIZ1, D(0:31), IRQ(2:4), IRQ6, RD/WR, BURST, IP_B(0:1), PA(0:4), PA(6:7), PA(10:11), PA15,
PB19, PB(23:31), PC(6:7), PC(10:13), PC15, PD8, 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
7
Thermal Calculation and Measurement
NOTE
The V
DDSYN
power dissipation is negligible.
For the following discussions, P
D
= (V
DDL
×
I
DDL
) + P
I/O
, where P
I/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.
MPC875/MPC870 Hardware Specifications, Rev. 3.0
Freescale Semiconductor
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
11
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
