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MC34059

Hex EIA–485 Transceiver with Three–State Outputs

Motorola => Freescale 

MC34058 MC34059

Appendix A. Optimizing the Thermal Performance of the MC34058/9

Figure 12. Electrical Model of Package Heat Transfer

Ambient Temperature

RCA

RJCU

Device Junction

IPD

RJL

5.0 A

RJCD

RLB

RCDB

Board Temperature

An equivalent electrical circuit for the thermal model for the

MC34058/9 package is shown in Figure 12. It is a simplified

model that shows the dominant means of heat transfer from

the thermally enhanced 48–ld package used for the

MC34058/9. The model is a first order approximation and is

intended to emphasize the need to consider thermal issues

when designing the IC into any system. It is however

customary to use similar models and Equation 1 to estimate

device junction temperatures.

Equation 1 is the common means of using the thermal

resistance of a package to estimate junction temperature in a

ǒ Ǔ ) particular system.

TJ + PD · qjx TA

[1]

The term θjx in Equation 1 is usually quoted as a øja value

in °C/Watt. However, since the 48–ld package for the

MC34058/9 has been thermally enhanced to take advantage

of other heat sinking potentials, it must be modified. θjx must

actually be considered a composite of all the heat transfer

paths from the chip. That is, the three dominant and parallel

paths shown in Figure 12. Of those three paths, potentially

the most effective is the corner package leads. This is

because these corner leads have been attached to the flag

on which the silicon die is situated. These pins can be

connected to circuit board ground to provide a more efficient

conduction path for internal package heat. This path is

modeled as the Rjl (junction–to–leads) and Rlb

(leads–to–board) combination in Figure 12. This path

provides the most effective way of removing heat from the

device provided that there is a viable temperature potential

(i.e. heat sinking source) to conduct towards. However, if it is

not properly considered in the system design, the other

paths, (Rjcd + Rcdb) and (Rjcu + Rca) attain greater

importance and must be more carefully considered.

So Equation 1, modified to reflect a more complete heat

transfer model becomes;

ȧȡȢ ȧȡȢ ȧȣȤ ȧȣȤ + TJ TA·

1

1 )1

)AAA Rjcd Rjlb

1

) Rjca

1 )1

Rjcd Rjlb

[2]

AAA TB ·

Rjca

) PDISS · qja

ȧȡ ȧȣ1

) Rjca

1 )1

Ȣ Ȥ Rjcd Rjlb

where;

TJ= Junction Temperature

TA = Ambient Temperature

TB = Board Temperature

PDISS = Device Power

and θja = Total Thermal Resistance and is composed the

parallel combination of all the heat transfer paths from

the package.

While Equation 2 is still only a first order approximation of

the heat transfer paths of the MC34058/9, at least now it

includes consideration for the most effective heat transfer

path for the MC34058/9; the board to which the device is

soldered. The modified equation also better serves to

explain how external variables, namely the board and

ambient temperatures, affect the thermal performance of

the MC34058/9.

Methods of removing heat via the flag connected pins can

be classified into two means; conduction and convection.

Radiation is omitted as the contribution is small compared to

the other means. Conduction is by far the best method to

draw heat away from the MC34058/9 package. This is best

accomplished by using a multilayer board with generous

ground plane. In this case, the flag connected pins can be

connected directly to the ground plane to maximize the heat

transfer from the package. Figure 13 shows the results of

thermal measurements of a board with an external ground

plane (the actual ground area was approximately 6 1/4 in2).

The thermal leads are connected to the board ground plane

per the recommended strategy.

10

MOTOROLA ANALOG IC DEVICE DATA

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