MPC875 datasheet(11/84 Pages) FREESCALE

11 / 84 page

MPC875/MPC870 Hardware Specifications, Rev. 3.0

Freescale Semiconductor

PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE

Thermal Calculation and Measurement

For the following discussions, PD = (VDDL × IDDL) + PI/O, where PI/O is the power dissipation of the I/O

drivers.

NOTE

The VDDSYN power dissipation is negligible.

7.1 Estimation with Junction-to-Ambient Thermal Resistance

An estimation of the chip junction temperature, TJ, in °C can be obtained from the following equation:

TJ = TA + (RθJA × PD)

where:

TA = ambient temperature ºC

RθJA = package junction-to-ambient thermal resistance (ºC/W)

PD = 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 TJ–TA) 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:

RθJA = RθJC + RθCA

where:

RθJA = junction-to-ambient thermal resistance (ºC/W)

RθJC = junction-to-case thermal resistance (ºC/W)

RθCA = case-to-ambient thermal resistance (ºC/W)

RθJC is device-related and cannot be influenced by the user. The user adjusts the thermal environment to

affect the case-to-ambient thermal resistance, RθCA. 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.

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

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