DSP56156ROM datasheet(65/76 Pages) MOTOROLA

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Design Considerations

DSP56156 Data Sheet

MOTOROLA

Design Considerations

JC is device-related and cannot be influ-

enced by the user. However,

CA is user-de-

pendent and can be minimized by such

thermal management techniques as heat

sinks, ambient air cooling, and thermal con-

vection. Thus, good thermal management on

the part of the user can significantly reduce

CA so that ΘJA approximately equals ΘJC.

Substitution of

JC for ΘJA in equation (1) will

result in a lower semiconductor junction

temperature. Values for thermal resistance

presented in this document, unless estimat-

ed, were derived using the procedure de-

scribed in Motorola Reliability Report 7843,

“Thermal Resistance Measurement Method

for MC68XX Microcomponent Devices”, and

are provided for design purposes only. Ther-

mal measurements are complex and depen-

dent on procedure and setup. User-derived

values for thermal resistance may differ.

Power, Ground, and

Noise

Each DSP56156 VCC pin should be provided

with a low-impedance path to +5 volts. Each

DSP56156 GND pin should likewise be pro-

vided with a low-impedance path to ground.

The power supply pins drive distinct groups

of logic on chip as shown in Table 26.

The VCC power supply should be by-

passed to GND ground using at least six

0.01 – 0.1 µF bypass capacitors located ei-

ther underneath the chip’s socket or as close

as possible to the four sides of the package.

The capacitor leads and the associated

printed circuit traces connecting to chip VCC

and GND should be kept to less than 0.5” per

Heat Dissipation

The average chip junction temperature, TJ, in

°C, can be obtained from:

TJ = TA + (PD × ΘJA)(1)

Where:

= ambient temperature,

°C

= package thermal resistance,

junction-to-ambient,

°C/W

=PINT + PI/O

PINT =ICC × VCC watts — chip internal

power

PI/O = power dissipation on input and

output pins — user determined

For most applications PI/O < PINT and PI/O can

be neglected. An appropriate relationship be-

tween PD and TJ (if PI/O is neglected) is:

PD = K/(TJ + 273)

(2)

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

K = PD × (TA + 273) + PD × ΘJA

(3)

Where K is a constant pertaining to the partic-

ular package. K can be determined from

equation (2) by measuring PD (at equilibri-

um) for a known TA. Using this value of K, the

values of PD and TJ can be obtained by solv-

ing equations (1) and (2) iteratively for any

value of TA. The total thermal resistance of a

package (

JA) can be separated into two com-

ponents,

JC and ΘCA, representing the barrier

to heat flow from the semiconductor junction

to the package (case) surface (

JC) and from

the case to the outside ambient (

CA). These

terms are related by the equation:

JA = ΘJC + ΘCA

(4)

Heat Dissipation

Power, Ground, and Noise

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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