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Maxim > Design Support > Technical Documents > Application Notes > Audio Circuits > APP 3924
Keywords: UCSP, Output Power, Thermal Limit, Thermal Shutdown, audio, amplifier
APPLICATION NOTE 3924
Thermal Considerations for a UCSP Package
By: Adrian Rolufs
Oct 15, 2007
Abstract: The amount of power that an audio amplifier will dissipate is primarily limited by its package
and external heat sink (whether a copper plane on the PCB or a metal heat sink). While more efficient
amplifiers like Class D do not dissipate as much as conventional Class AB amplifiers, all amplifiers
dissipate some power as heat. This application note discusses the power-dissipation capabilities of the
UCSP™ package, and how that package can limit dissipated output power compared to other package
options.
The UCSP Package
The UCSP is a packaging technology that eliminates the traditional
plastic package used to encapsulate integrated circuits (ICs).
Soldering the silicon directly to a PCB saves board space, but at
the sacrifice of some of the advantages of a traditional package,
especially heat dissipation.
Most packages used for audio amplifiers have some type of
exposed pad that allows the substrate of the IC to be connected
directly to either a heat sink or the PCB's ground plane. This design provides a low-thermal impedance
path for the heat transferred from the IC to its surroundings, and thus keeps the device from overheating.
With a UCSP package, however, the IC is directly soldered to the PCB using bumps on the bottom
surface of the device. While there are direct paths from the substrate to the PCB through the ground
bumps and these bumps have a low thermal impedance, their area is much less than a typical exposed
pad. Consequently, thermal dissipation is reduced. Nonground bumps also help dissipate heat, but at a
reduced capacity compared to ground bumps. Top-side thermal dissipation through a heat sink is
impractical due to the space constants of most systems using UCSP devices. The UCSP package is,
moreover, not as mechanically robust as other packages typically used with heatsinks. A UCSP can
actually become damaged while in contact with the heat sink. The thermal capabilities of a UCSP device
are therefore determined by combining the heat dissipated by the grounded bumps with the rest of the
bumps on the device.
Power Dissipation
Audio amplifiers are generally offered in multiple packages and characterized using the package option
with the most power dissipation. In most cases these packages prevent the power dissipated by the
package itself from limiting the possible output power.
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