Datasheet

Additional improvements are possible if all the traces from
the device are made as wide as possible. Although the IC
pins are not the primary thermal path of the package, they
do provide a small amount. The total improvement would
not exceed about 10%, but it could make the difference
between acceptable performance and thermal problems.
Auxiliary Heatsinking
If operating in higher ambient temperatures, it is possible
to improve the thermal performance of a PC board with
the addition of an external heatsink. The thermal resis-
tance to this 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 topside of the IC is not a significant
thermal path for the device, and therefore is not a costef-
fective location for a heatsink.
Thermal Calculations
The die temperature of a Class D amplifier can be esti-
mated with some basic calculations. For example, the die
temperature is calculated for the below conditions:
● T
A
= +40°C
● P
OUT
= 2x8W = 16W
● R
L
= 16Ω
● Efficiency (η) = 87%
● θ
JA
= 27°C/W
First, the Class D amplifier’s power dissipation must be
calculated.
OUT
DISS OUT
P 16W
P P 16W 2.4W
0.87
= − = −=
η
Then the power dissipation is used to calculate the die
temperature, T
C
, as follows:
T
C
= T
A
+ PDISS x θ
JA
= 40°C + 2.4W x 27°C/W
= 104.8°C
Decreasing the ambient temperature or reducing θ
JA
will improve the die temperature of the MAX9704. θ
JA
can be reduced by increasing the copper size/weight of
the ground plane connected to the exposed paddle of
the MAX9704 TQFN package. Additionally, θ
JA
can be
reduced by attaching a heatsink, adding a fan, or mount-
ing a vertical PC board.
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 I
2
R losses in the MOSFETs, thereby increas-
ing efficiency. To keep the peak currents lower, choose
the highest impedance speaker which can still deliver the
desired output power within the voltage swing limits of the
Class D amplifier and its supply voltage.
Although most loudspeakers are either 4Ω or 8Ω, there
are other impedances available which can provide a more
thermally efficient solution.
Another consideration is the 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 is usually only 8Ω imped-
ance within a very narrow range, and often extends well
below 8Ω, reducing the thermal efficiency below what is
expected. This lower-than-expected impedance can be
further reduced when a crossover network is used in a
multi-driver audio system.
Optimize MAX9703/MAX9704 Efciency with
Load Impedance and Supply Voltage
To optimize the efficiency of the MAX9703/MAX9704,
load the output stage with 12Ω to 16Ω speakers. The
MAX9703/MAX9704 exhibits highest efficiency perfor-
mance when driving higher load impedance (see the
Typical Operating Characteristics). If a 12Ω to 16Ω load is
not available, select a lower supply voltage when driving
6Ω to 10Ω loads.
MAX9703/MAX9704 10W Stereo/15W Mono, Filterless,
Spread-Spectrum, Class D Ampliers
www.maximintegrated.com
Maxim Integrated │ 13