LNBH21
12/20
THERMAL DESIGN NOTES
During normal operation, the LNBH21 device dissipates some power. At maximum rated output current
(750mA), the voltage drop on the linear regulator lead to a total dissipated power that is typically 1.65W.
The heat generated requires a suitable heatsink to keep the junction temperature below the over
temperature protection threshold. Assuming a 45°C temperature inside the Set-Top-Box case, the total
R
thj-amb
has to be less than 48°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.
GivenanR
thj-case
equal to 2°C/W, a maximum of 46°C/W are left to the PCB heatsink. This figure is
achieved if a minimum of 6.5cm
2
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 figure 4, it is shown a suggested layout for the PSO-20 package with a dual
layer PCB, where the IC exposed pad connected to GND and the square dissipating area are thermally
connected through 32 vias holes, filled by solder. This arrangement, when L=25mm, achieves an R
thc-amb
of about 32°C/W.
Different layouts are possible, too. Basic principles, however, suggest to keep the IC and its ground
exposed pad 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.
Figure 4 : PowerSO-20 SUGGESTED PCB HEATSINK LAYOUT
