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BPA-200 Application Note

The document discusses the thermal design considerations for using integrated monolithic power amplifiers beyond their specified capabilities in booster circuits. It explains that the power dissipation rating listed in datasheets can be misleading, and that the sensitivity of protection circuits means the actual power dissipation limit is lower to avoid prematurely activating these circuits. Thermal management is important to extend the output power capability without triggering protection mechanisms. Specific examples from the LM3886T datasheet are provided to illustrate how to determine the practical power dissipation limit from the guaranteed output power specifications.

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Elder Monte
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302 views16 pages

BPA-200 Application Note

The document discusses the thermal design considerations for using integrated monolithic power amplifiers beyond their specified capabilities in booster circuits. It explains that the power dissipation rating listed in datasheets can be misleading, and that the sensitivity of protection circuits means the actual power dissipation limit is lower to avoid prematurely activating these circuits. Thermal management is important to extend the output power capability without triggering protection mechanisms. Specific examples from the LM3886T datasheet are provided to illustrate how to determine the practical power dissipation limit from the guaranteed output power specifications.

Uploaded by

Elder Monte
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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will expand on the thermal design aspects of Overture ICs.

While the bridge/parallel configuration


is only one of many that can be made to obtain higher output power levels, the concept of "design
by power dissipation" is equally applicable to other types of booster circuits.

The BPA-200 schematics, and test results exemplify what can be achieved with proper component
selection, thermal design, and layout techniques. The BPA-200 is only an example and is not
intended for sale. This documentation is intended to show obtainable results and give general
guidance of conceptual design.

III. Thermal Background:


The voltage and current ratings of a power semiconductor are typically the first specs considered in
designing high power amplifiers. The same is true for an integrated monolithic power amplifier.
However, power dissipation ratings are equally important to the long-term reliability of the power
amplifier design. When using a monolithic IC in its intended application and within its specified
capabilities, the thermal design is relatively straightforward. When an IC is used beyond its
capabilities, as in booster circuits, power dissipation issues become more critical and not as
straight- forward. Therefore, the designer must understand the IC's power dissipation capabilities
before using the IC in a booster configuration.

-Typical Characteristic Data


The power dissipation capabilities of a power IC are either specified in the datasheet or can be
derived from its guaranteed output power specification. While the power dissipation rating for the
LM3886T is 125W, this number can be misleading. Its power dissipation specification is derived
from the IC's junction-to-case thermal resistance, ØJC=l °C/W, the maximum junction
temperature, TJ=150°C, and the ambient air, TA=25°C. As stated in the datasheet, the device must
be derated based on these parameters while operating at elevated temperatures. The heatsinking
requirements for the application are based on these parameters so that the IC will not go into
Thermal Shutdown (TSD). The real problem for Overture ICs, however, comes from the
sensitivity of the output stage's unique SPiKe Protection which dynamically monitors the output
transistor's temperature. While the thermal shutdown circuitry is enabled at TJ=150°C, SPiKe
circuitry is enabled at TJ=250°C for instantaneous power spikes in the output stage transistor. As
the overall temperature of the IC increases, SPiKe circuitry becomes even more sensitive causing it
to turn on before the 125W limit is reached. TSD circuitry will continue to function globally for
the IC in conjunction with SPiKe circuitry. However, protection circuitry should not be activated
under normal operating conditions. The question then becomes, what is the power dissipation
limit for the IC such that SPiKe circuitry is not enabled? Knowing the power dissipation limit and
keeping the case temperature of the IC as cool as possible will expand the output power capability
without activating SPiKe Protection.

The other way to determine IC power dissipation capabilities is to analyze the output power
specification in the datasheet. In the case of the LM3886T, there are two output power
specification guarantees: 60W(min) into a 4Ω load using ±28V supplies and 50W(typ) into an 8Ω
load from ±35V supplies. Using these two conditions and the theoretical maximum power
dissipation equation

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