LNBP21 datasheet(17/24 Pages) STMICROELECTRONICS

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LNBP21

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THERMAL DESIGN NOTES

During normal operation, this device dissipates

some power. At maximum rated output current

(500mA), the voltage drop on the linear regulator

lead to a total dissipated power that is of about

1.7W. The heat generated requires a suitable

heatsink to keep the junction temperature below

the

over

temperature

protection

threshold.

Assuming

40°C

temperature

inside

the

Set-Top-Box case, the total Rthj-amb has to be

less than 50°C/W.

While this can be easily achieved using a

through-hole power package that can be attached

to a small heatsink or to the metallic frame of the

receiver, a surface mount power package must

rely on PCB solutions whose thermal efficiency is

often limited. The simplest solution is to use a

large, continuous copper area of the GND layer to

dissipate the heat coming from the IC body.

The SO-20 package of this IC has 4 GND pins that

are

not

just

intended

for

electrical

GND

connection, but also to provide a low thermal

resistance path between the silicon chip and the

PCB heatsink. Given an Rthj-c equal to 15°C/W, a

maximum of 35°C/W are left to the PCB heatsink.

This figure is achieved if a minimum of 25cm2

copper area is placed just below the IC body. This

area can be the inner GND layer of a multi-layer

PCB, or, in a dual layer PCB, an unbroken GND

area even on the opposite side where the IC is

placed. In both cases, the thermal path between

the IC GND pins and the dissipating copper area

must exhibit a low thermal resistance.

In figure 40, it is shown a suggested layout for the

SO-20 package with a dual layer PCB, where the

IC Ground pins and the square dissipating area

are thermally connected through 32 vias holes,

filled by solder. This arrangement, when L=50mm,

achieves an Rthc-a of about 25°C/W.

Different

layouts

are

possible,

too.

Basic

principles, however, suggest to keep the IC and its

ground pins approximately in the middle of the

dissipating area; to provide as many vias as

possible; to design a dissipating area having a

shape as square as possible and not interrupted

by other copper traces.

Due to presence of an exposed pad connected to

GND below the IC body, the PowerSO-20

package has a Rthj-c much lower than the SO-20,

only 2°C/W. As a result, much lower copper area

must be provided to dissipate the same power and

minimum of 12cm2 copper area is enough, see

figure 41.

Figure 40: SO-20 Suggested PCB Heatsink Layout

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