Less than 1%

~20%

~80%

~20%

14

OPA521

SBOS872 – MAY 2018

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Copyright © 2018, Texas Instruments Incorporated

10 Layout

10.1 Layout Guidelines

10.1.1 Thermal Considerations

In a typical power line communications application, the device dissipates 2 W of power when transmitting to the

low-impedance AC line. This amount of power dissipation can increase the junction temperature, which can lead

to a thermal overload that results in signal transmission interruptions if the PCB thermal design is not

implemented properly. Proper management of heat flow from the device and good PCB design and construction

are required to ensure proper device temperature, maximize performance, and extend the operating life of the

device.

The device is assembled in a 5-mm × 5-mm, QFN-20 package. This QFN package has a large exposed thermal

pad on the underside that conducts heat away from the device and to the underlying PCB.

Some heat is conducted from the silicon die surface through the plastic packaging material and is transferred to

the ambient environment. However, this route is not the primary thermal path for heat flow because plastic is a

relatively poor conductor of heat. Heat flows across the silicon die surface to the bond pads through the wire

bonds to the package leads, to the top layer of the PCB. While these paths for heat flow are important, the

majority (nearly 80%) of the heat flows downward through the silicon die to the thermally-conductive die-attach

epoxy and to the exposed thermal pad on the underside of the package (as shown in Figure 5). Minimizing the

thermal resistance of this downward path to the ambient environment maximizes the life and performance of the

device.

Figure 5. Heat Flow in the QFN Package

The exposed thermal pad must be soldered to the PCB thermal pad. The thermal pad on the PCB must be the

same size as the exposed thermal pad on the underside of the QFN package. See QFN/SON PCB Attachment

for recommendations on attaching the thermal pad to the PCB. Figure 6 shows the direction of heat spreading to

the PCB from the device.