20W/40W, Filterless, Spread-Spectrum,

Mono/Stereo, Class D Amplifier

16

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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 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 MAX9708.

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:

• Efficiency (

η) = 87%

•

First, the Class D amplifier’s power dissipation must be

calculated:

Then the power dissipation is used to calculate the die

Load Impedance

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

efficiency. To keep the peak currents lower, choose the

highest impedance speaker that can still deliver the

desired output power within the voltage swing limits of

the Class D amplifier and its supply voltage.

Although most loudspeakers fall either 4

Ω or 8Ω, there

are other impedances available that can provide a

more thermally efficient solution.

Another consideration is the load impedance across

the audio frequency band. A loudspeaker is a complex

electro-mechanical system with a variety of resonance.

In other words, an 8

Ω speaker usually has 8Ω imped-

ance within a very narrow range. This often extends

well below 8

Ω, 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.

Systems Application Circuit

The MAX9708 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

MAX9708 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 MAX9708 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.

The circuit in Figure 6 uses inverting amplifiers for their

ease in biasing. Note the phase labeling at the outputs

has been reversed. The resistors should be 1% or better

in tolerance and the capacitors 5% tolerance or better.

TT

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C

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C

CA

DISS

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θ

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P

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.

η

16

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