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Maxim > Design
support > App notes
> Audio
Circuits > APP 3879

Aug 16, 2006

Keywords: class D, thermal, performance,
heatsink, exposed, pad, TQFN
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APPLICATION NOTE 3879

Thermal Considerations for a Class D Amplifier

Abstract: A
Class D amplifier provides much better efficiency and thermal
performance than a comparable Class AB amplifier. Despite
these improvements, a Class D amplifier's thermal performance
must be considered. This application note examines the thermal
performance of Class D amplifiers and employs general examples
to illustrate good design practices.

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Continuous Sine Wave vs. Music
When a Class D
amplifier is evaluated in the lab, a continuous sine wave is
often used as the signal source.
Although this is convenient for measurement purposes, it
represents a worst-case scenario for thermal loading on the
amplifier. It is not uncommon for a Class D amplifier to enter
thermal
shutdown if driven near maximum output power with a
continuous sine wave.

Audio content, both music and
voice, has a much lower RMS value relative to its peak output
power. The ratio of peak to RMS value, known as crest factor,
is typically about 12dB for voice and 18dB to 20dB for musical
instruments. Figure 1 shows an audio signal in the time
domain along with a sine wave. Both are measured for their RMS
values by the oscilloscope. Although the audio signal has a
slightly higher peak value than the sine wave, its RMS value
is almost half that of the sine wave. Also, the audio signal
displayed corresponds to a burst of music; average values will
be even less than that shown. Therefore, although an audio
signal may reach peaks similar to that of a continuous sine
wave, its thermal impact on the Class D amplifier is highly
reduced. If the thermal performance of a system is being
evaluated, it is important to use actual audio signals rather
than sine waves for testing. If sine waves must be used, the
thermal performance will be less than the system's actual
capacity during normal use.

Figure
1. The higher RMS level of a sine wave vs. an audio signal
predicts the additional thermal burden placed on a Class D
amplifier when tested with a sine wave.

PC Board Thermal Considerations
In an
industry-standard TQFN package, the exposed pad is the primary
path for heat to move away from the IC. For a package that has
a bottom-side exposed pad, the PC
board and its copper are the Class D amplifier's primary
heatsink. When working with a Class D amplifier mounted on a
typical PC board, as shown in Figure 2, the following
directions should be helpful: Solder the exposed pad to a
large copper polygon. Add as much copper as possible from this
polygon to any adjacent pin on the Class D amplifier as well
as to any adjacent components, provided these connections are
at the same potential. In this particular case, the copper
connects the upper right side and the lower right side of the
thermal pad (Figure 2). These copper paths must be as wide as
possible, as they contribute to the overall thermal capability
of the system.

Figure
2. The exposed pad is the primary thermal path away from a
Class D amplifier when it is housed in a TQFN or TQFP
package.

The copper polygon to which the exposed
pad is attached should have multiple vias to the opposite side
of the PC board, where they would connect to another copper
polygon. This polygon should be as large as possible within
the system's constraints for signal routing.

Additional
improvements are possible if all traces from the device are
made as wide as possible. Although the IC pins are not the
primary thermal path out of the package, they do dissipate a
small amount of the heat. Figure 3 shows a PC board in
which a wide trace connects the Class D amplifier outputs to
two inductors on the figure's right side. In this case, the
copper windings of the inductors provide an additional thermal
path away from the Class D amplifier. The total improvement is
no more than about 10%, but it could make the difference
between acceptable thermal performance and thermal
problems.

Figure
3. The wide traces to the right of the Class D amplifier help
conduct heat.

Auxiliary Heatsinking
When Class D amplifiers are
operating at higher ambient temperatures, it is possible to
improve the thermal performance of a PC board with the
addition of an external heatsink. The thermal resistance
of such a heatsink must be kept as low as possible to maximize
its performance. With a bottom-side exposed pad, the lowest
resistance thermal path is on the bottom of the PC board. The
top side of the IC is not a significant thermal path for the
device, and therefore is not a cost-effective location for a
heatsink. Figure 4 shows an example of a
PC-board-mounted heatsink (218 series, available from
Wakefield Engineering). This heatsink, soldered to the PC
board, is a good compromise between size, cost, ease of
assembly, and thermal performance.

Figure
4. An SMT heatsink, such as the one pictured here, may be
needed when a Class D amplifier is operated at higher ambient
temperatures. (Photo courtesy of Wakefield
Engineering.)

Thermal Calculations
The die temperature of a Class D
amplifier can be estimated with some basic calculations. The
temperature in this example is calculated for the following
conditions:

TAM = +40°C

POUT = 16W

Efficiency (η) = 87%

ΘJA = 21°C/W

First the Class D
amplifier's power dissipation must be calculated:

This
power dissipation is then used to calculate the die
temperature, TC, as follows:

From
these numbers, we can conclude that the device will operate
with reasonable performance. It is important to start these
calculations based on a realistic estimate of the ambient
temperature inside the system. Very seldom do these systems
have the luxury of +25°C as the actual ambient
temperature.

Load Impedance
The on-resistance of the MOSFET
output stage in Class D amplifiers affects both the efficiency
and the peak-current capability. Reducing the peak current
into the load reduces the I2R losses in the
MOSFETs, thus increasing efficiency. To keep the peak currents
low, choose the highest impedance speaker that can still
deliver the desired output power within the voltage swing
limits of a Class D amplifier and its supply voltage, as in
Figure 5. In this case, assume that the Class D
amplifier has an output-current capability of 2A and a
supply-voltage range of 5V to 24V. A 4Ω load will exceed this
2A current limit at supply voltages of 8V and above, with a
corresponding maximum continuous value of 8W. If 8W is an
acceptable output power level, consider using a 12Ω speaker
and a supply voltage of 15V. Then, the peak current is limited
to 1.25A, with a corresponding maximum continuous output power
of 9.4W. Furthermore, the 12Ω load will operate with 10% to
15% higher efficiency than the 4Ω load, resulting in lower
power dissipation. Actual efficiency improvements will vary
with different Class D amplifiers. Although most loudspeakers
are either 4Ω or 8Ω, other impedances are available that can
provide a more thermally efficient solution.

Figure
5. Selecting an optimal impedance and supply voltage maximizes
output power.

Another consideration is load
impedance across the audio frequency band. A loudspeaker is a
complex electromechanical system with a variety of resonances.
In other words, an 8Ω speaker usually exhibits an 8Ω impedance
within only a very narrow range. For much of the audio bandwidth,
the impedance is higher than the nominal specified, as shown
in Figure 6. Over much of the audio bandwidth, this
loudspeaker has an actual impedance that is much higher than
its nominal value of 8Ω. Unfortunately, this impedance is
usually reduced by the addition of a tweeter and a crossover
network. The total system impedance must be considered to
ensure adequate current and thermal capability.

Figure
6. The impedance of this 8Ω, 13cm wide-range loudspeaker
varies significantly with frequency.

Conclusion
Class D amplifiers provide significant
improvement in efficiency over Class
AB amplifiers. Although this efficiency reduces the
thermal considerations during system design, it does not
eliminate them. However, by using good design practice and
establishing realistic expectations, Class D amplifiers make
audio system design simpler.

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Download,
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APP 3879: Aug 16, 2006

AN3879, AN 3879, APP3879,
Appnote3879, Appnote 3879

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