BUF08800 Datasheet, PDF (15/23 Pages) Burr-Brown (TI) – Programmable Reference Generator and 400mA VCOM Driver

BUF08800
www.ti.com
GENERAL POWERPAD DESIGN
CONSIDERATIONS
The BUF08800 is available in a thermally-enhanced
PowerPAD package. This package is constructed using a
downset leadframe upon which the die is mounted; see
Figure 19(a) and Figure 19(b). This arrangement results in
the lead frame being exposed as a thermal pad on the
underside of the package; see Figure 19(c). This thermal
pad has direct thermal contact with the die; thus, excellent
thermal performance is achieved by providing a good
thermal path away from the thermal pad.
The PowerPAD package allows for both assembly and
thermal management in one manufacturing operation.
During the surface-mount solder operation (when the
leads are being soldered), the thermal pad must be
soldered to a copper area underneath the package.
Through the use of thermal paths within this copper area,
heat can be conducted away from the package into either
a ground plane or other heat-dissipating device. Soldering
the PowerPAD to the printed circuit board (PCB) is always
required, even with applications that have low power
dissipation. This process provides the necessary thermal
and mechanical connection between the lead frame die
pad and the PCB.
The PowerPAD must be connected to the most negative
supply voltage on the device, GND_A and GND_D.
1. Prepare the PCB with a top-side etch pattern. There
should be etching for the leads as well as for the
thermal pad.
2. Place recommended holes in the area of the thermal
pad. Ideal thermal land size and thermal via patterns
(2×4) for the HTSSOP-20 DAP package can be seen
in the technical brief, PowerPAD Thermally-
Enhanced Package (SLMA002), available for down-
load at www.ti.com. These holes should be 13 mils in
diameter. Keep them small, so that solder wicking
through the holes is not a problem during reflow.
SBOS380A − FEBRUARY 2007 − REVISED MAY 2007
3. Additional vias may be placed anywhere along the
thermal plane outside of the thermal pad area. The
vias help dissipate the heat generated by the
BUF08800 IC. The additional vias may be larger than
the 13-mil diameter vias directly under the thermal
pad. They can be larger because they are not in the
thermal pad area to be soldered; thus, wicking is not
a problem.
4. Connect all holes to the internal plane that is at the
same voltage potential as the GND pins.
5. When connecting these holes to the internal plane, do
not use the typical web or spoke via connection
methodology. Web connections have a high thermal
resistance connection that is useful for slowing the
heat transfer during soldering operations, making the
soldering of vias that have plane connections easier.
In this application, however, low thermal resistance is
desired for the most efficient heat transfer. Therefore,
the holes under the BUF08800 PowerPAD package
should make their connection to the internal plane
with a complete connection around the entire
circumference of the plated-through hole.
6. The top-side solder mask should leave the terminals
of the package and the thermal pad area with its ten
holes exposed. The bottom-side solder mask should
cover the holes of the thermal pad area. This masking
prevents solder from being pulled away from the
thermal pad area during the reflow process.
7. Apply solder paste to the exposed thermal pad area
and all of the IC terminals.
8. With these preparatory steps in place, the BUF08800
IC is simply placed in position and run through the
solder reflow operation as any standard
surface-mount component. This preparation results in
a properly installed part.
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