MPC823 datasheet(3/70 Pages) FREESCALE

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MPC823 ELECTRICAL SPECIFICATIONS

POWER CONSIDERATIONS

The average chip-junction temperature, T

°C can be obtained from

J = TA + (PD ¥ qJA)

(1)

where

Ambient Temperature,

¥C

Package Thermal Resistance, Junction to Ambient,

¥C/W

INT + PI/O

INT =IDD x VDD

WattsÑChip Internal Power

I/O

Power Dissipation on Input and Output PinsÑUser Determined

For most applications P

I/O < 0.3 ¥ PINT and can be neglected. If PI/O is neglected

an approximate

relationship between P

D and TJ is:

Õ (T

J + 273¥C)

(2)

Solving equations (1) and (2) for K gives

K =

D ¥ (TA + 273¥C) + qJA ¥ PD

(3)

where K is a constant pertaining to the particular part. K can be determined from equation (3)

by measuring P

(at equilibrium) for a known T

. Using this value of K, the values of P

D and TJ

can be obtained by solving equations (1) and (2) iteratively for any value of T

Layout Practices

Each VCC pin on the MPC823 should be provided with a low-impedance path to the boardÕs

supply. Each GND pin should be provided with a low-impedance path to ground. The power

supply pins drive distinct groups of logic on chip. The VCC power supply should be bypassed to

ground using at least four 0.1

mF bypass capacitors located as close as possible to the four

sides of the package. The capacitor leads and associated printed circuit traces connecting to

chip VCC and GND should be kept to less than half an inch per capacitor lead. A four-layer

board that employs two inner layers as VCC and GND planes should be used.

All output pins on the MPC823 have fast rise and fall times. Printed circuit (PC) trace

interconnection length should be minimized in order to minimize undershoot and reflections

caused by these fast output switching times. This recommendation particularly applies to the

address and data busses. 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 VCC 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, Inc.

For More Information On This Product,

Go to: www.freescale.com

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