lm49350rl National Semiconductor Corporation, lm49350rl Datasheet - Page 41

• Manufacturer Part Number: lm49350rl
• Description: High Performance Audio Codec Sub-system With A Ground-referenced Stereo Headphone Amplifier & An Ultra Low Emi Class D Loudspeaker Amplifier With Dual I2s/pcm Digital Audio Interfaces
• Manufacturer: National Semiconductor Corporation

20.0 Analog Mixer Control Registers
This register is used to control the LM49350's Analog Mixer:
Class D Loudspeaker Amplifier
The LM49350 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz from V
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 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 LM49350 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 LM49350 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 LM49350 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 LM49350 and in the transducer load. The amount of power dissipation in the LM49350'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 0.25Ω. 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.
Bits
Bits
0
1
2
3
4
5
0
1
2
3
4
5
DACR_HPL
AUXR_HPL
DACL_HPL
MICR_HPL
AUXL_HPL
MICL_HPL
DACR_LS
DACL_LS
AUXR_LS
MICR_LS
AUXL_LS
MICL_LS
Field
Field
TABLE 18. LEFT HEADPHONE_OUTPUT (0x11h)
The right DAC output is added to the loudspeaker output.
The left DAC output is added to the loudspeaker output.
The right MIC input is added to the loudspeaker output. Setting this bit enables MIC BIAS.
The left MIC input is added to the loudspeaker output. Setting this bit enables MIC BIAS.
The right AUX input is added to the loudspeaker output.
The left AUX input is added to the loudspeaker output.
The right DAC output is added to the left headphone output.
The left DAC output is added to the left headphone output.
The right MIC input is added to the left headphone output. Setting this bit enables MIC BIAS.
The left MIC input is added to the left headphone output. Setting this bit enables MIC BIAS.
The right AUX input is added to the left headphone output.
The left AUX input is added to the left headphone output.
TABLE 17. CLASS_D_OUTPUT (0x10h)
41
Description
Description
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DD
to GND.