LM49352 datasheet(39/106 Pages) TI

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LM49352

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SNAS467E – MARCH 2009 – REVISED MARCH 2013

Class D Loudspeaker Amplifier

The LM49352 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz

from VDD to GND. When there is no input signal applied, the two outputs (LS+ and LS-) switch with a 50% duty

cycle, with both outputs in phase. Because the outputs of the LM49352 are differential, the two signals cancel

each other. This results in no net voltage across the speaker, thus there is no load current during an idle state,

conserving power.

With an input signal applied, the duty cycle (pulse width) of the LM49352 outputs changes. For increasing output

voltages, the duty cycle of LS+ increases, while the duty cycle of LS- decreases. For decreasing output voltages,

the converse occurs, the duty cycle of LS- increases while the duty cycle of LS+ decreases. The difference

between the two pulse widths yields the differential output voltage.

Spread Spectrum Modulation

The LM49352 features a fitlerless spread spectrum modulation scheme that eliminates the need for output filters,

ferrite beads or chokes. The switching frequency varies by ±30% about a 300kHz center frequency, reducing the

wideband spectral content, improving EMI emissions radiated by the speaker and associated cables and traces.

Where a fixed frequency class D exhibits large amounts of spectral energy at multiples of the switching

frequency, the spread spectrum architecture of the LM49352 spreads that energy over a larger bandwidth. The

cycle-to-cycle variation of the switching period does not affect the audio reproduction or efficiency.

Class D Power Dissipation and Efficiency

In general terms, efficiency is considered to be the ratio of useful work output divided by the total energy required

to produce it with the difference being the power dissipated, typically, in the IC. The key here is “useful” work. For

audio systems, the energy delivered in the audible bands is considered useful including the distortion products of

the input signal. Sub-sonic (DC) and super-sonic components (>22kHz) are not useful. The difference between

the power flowing from the power supply and the audio band power being transduced is dissipated in the

LM49352 and in the transducer load. The amount of power dissipation in the LM49352's class D amplifier is very

low. This is because the ON resistance of the switches used to form the output waveforms is typically less than

0.25

Ω. This leaves only the transducer load as a potential "sink" for the small excess of input power over audio

band output power. The LM49352 dissipates only a fraction of the excess power requiring no additional PCB

area or copper plane to act as a heat sink.

EMI/RFI Filtering

If system level PCB layout constraints require the LM49352’s Class D output bumps to be placed far away from

the speaker or the Class D output traces to be routed near EMI/RFI sensitive components, an external EMI/RFI

filter should be used. A series ferrite bead placed close to the Class D output bumps along with a shunt capacitor

to ground placed close to the ferrite bead will reduce the EMI/RFI emissions of the Class D amplifier’s switching

outputs. The ferrite bead must be rated with a current rating high enough to properly drive the loudspeaker. The

ferrite bead that is rated for 1A or greater is recommended. The DC resistance of the ferrite bead is another

important specification that must be taken into consideration. A low DC resistance will minimize any power losses

dissipated by the EMI/RFI filter thereby preserving the power efficiency advantages of the Class D amplifier.

Selecting a ferrite bead with high DC resistance will decrease output power delivered to speaker and reduce the

Class D amplifier’s efficiency. The shunt capacitor needs to have low ESR. A 10pF ceramic capacitor with a X7R

dielectric is recommended as a starting point. Care needs to be taken to ensure that the value of the shunt

capacitor does not exceed 47pF when using a low resistance ferrite bead in order to prevent permanent damage

to the low side FETs of the Class D output stage.

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