LM49350 Datasheet, PDF (42/104 Pages) National Semiconductor (TI) – High Performance Audio Codec Sub-System with a Ground-Referenced Stereo Headphone Amplifier

20.0 Analog Mixer Control Registers
This register is used to control the LM49350's Analog Mixer:
TABLE 18. CLASS_D_OUTPUT (0x10h)
Bits
Field
Description
0
DACR_LS
The right DAC output is added to the loudspeaker output.
1
DACL_LS
The left DAC output is added to the loudspeaker output.
2
MICR_LS
The right MIC input is added to the loudspeaker output. Setting this bit enables MIC BIAS.
3
MICL_LS
The left MIC input is added to the loudspeaker output. Setting this bit enables MIC BIAS.
4
AUXR_LS
The right AUX input is added to the loudspeaker output.
5
AUXL_LS
The left AUX input is added to the loudspeaker output.
20.1 CLASS D LOUDSPEAKER AMPLIFIER
The LM49350 features a filterless modulation scheme. The
differential outputs of the device switch at 300kHz from V_DD
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 LM49350 are differen-
tial, the two signals cancel each other. This results in no net
voltage across the speaker, thus there is no load current dur-
ing an idle state, conserving power.
With an input signal applied, the duty cycle (pulse width) of
the LM49350 outputs changes. For increasing output volt-
ages, the duty cycle of LS+ increases, while the duty cycle
of LS- decreases. For decreasing output voltages, the con-
verse 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.
20.2 SPREAD SPECTRUM MODULATION
The LM49350 features a fitlerless spread spectrum modula-
tion 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 fre-
quency class D exhibits large amounts of spectral energy at
multiples of the switching frequency, the spread spectrum ar-
chitecture of the LM49350 spreads that energy over a larger
bandwidth. The cycle-to-cycle variation of the switching peri-
od does not affect the audio reproduction or efficiency.
20.3 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 pro-
duce it with the difference being the power dissipated, typi-
cally, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered 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 be-
ing transduced is dissipated in the LM49350 and in the trans-
ducer load. The amount of power dissipation in the LM49350's
class D amplifier is very low. This is because the ON resis-
tance of the switches used to form the output waveforms is
typically less than 0.25Ω. This leaves only the transducer load
as a potential "sink" for the small excess of input power over
audio band output power. The LM49350 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 LM49350’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 sen-
sitive 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 loud-
speaker. The ferrite bead that is rated for 1A or greater is
recommended. The DC resistance of the ferrite bead is an-
other important specification that must be taken into consid-
eration. 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 us-
ing 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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