max9709etntd Maxim Integrated Products, Inc., max9709etntd Datasheet - Page 16

• Manufacturer Part Number: max9709etntd
• Description: Max9709 25w/50w, Filterless, Spread-spectrum, Stereo/mono, Class D Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplifier
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 cost-
effective location for a heatsink. If an LC filter is used in
the design, placing the inductor in close proximity to the
IC can help draw heat away from the MAX9709.
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
• P
• Efficiency (η) = 87%
• θ
First, the Class D amplifier’s power dissipation must be
calculated:
Then the power dissipation is used to calculate the die
temperature, T
The on-resistance of the MOSFET output stage in Class
D amplifiers affects both the efficiency and the peak-cur-
rent capability. Reducing the peak current into the load
reduces the I
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.
16
T
C
A
JA
OUT
=
______________________________________________________________________________________
= +40°C
T
P
= 21°C/W
A
DISS
= 16W
+
P
DISS
=
2
R losses in the MOSFETs, which increases
C
P
, as follows:
×
OUT
η
θ
JA
−
=
P
40
OUT
° +
C
=
16
24
0 87
.
Auxiliary Heatsinking
Thermal Calculations
W
W
×
−
16
21
Load Impedance
°
W
C W
/
=
2 4
.
=
W
90 4
.
°
C
Another consideration is the load impedance across
the audio frequency band. A loudspeaker is a complex
electromechanical system with a variety of resonance.
In other words, an 8Ω speaker usually has 8Ω imped-
ance within a very narrow range. This 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 multidriver audio system.
The MAX9709 can be configured into multiple amplifier
systems. One concept is a 2.1 audio system (Figure 5)
where a stereo audio source is split into three channels.
The left- and right-channel inputs are highpass filtered
to remove the bass content, and then amplified by the
MAX9709 in stereo mode. Also, the left- and right-chan-
nel inputs are summed together and lowpass filtered to
remove the high-frequency content, then amplified by a
second MAX9709 in mono mode.
The conceptual drawing of Figure 5 can be applied to
either single-ended or differential systems. Figure 6
illustrates the circuitry required to implement a fully
differential filtering system. By maintaining a fully differ-
ential path, the signal-to-noise ratio remains uncompro-
mised and noise pickup is kept very low. However,
keeping a fully differential signal path results in almost
twice the component count, and therefore performance
must be weighed against cost and size.
The highpass and lowpass filters should have different
cutoff frequencies to ensure an equal power response
at the crossover frequency. The filters should be at
-6dB amplitude at the crossover frequency, which is
known as a Linkwitz-Riley alignment. In the example
circuit of Figure 6, the -3dB cutoff frequency for the
highpass filters is 250Hz, and the -3dB cutoff frequency
for the lowpass filter is 160Hz. Both the highpass filters
and the lowpass filters are at a -6dB amplitude at
approximately 200Hz. If the filters were to have the
same -3dB cutoff frequency, a measurement of sound
pressure level (SPL) vs. frequency would have a peak
at the crossover frequency.
Systems Application Circuit