LM49350RLX/NOPB National Semiconductor, LM49350RLX/NOPB Datasheet - Page 62

• Manufacturer Part Number: LM49350RLX/NOPB
• Description: IC AUDIO SUBSYSTM .8W D 36USMDXT
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class Dr

Specifications of LM49350RLX/NOPB

• Output Type: 1-Channel (Mono) with Mono and Stereo Headphones
• Max Output Power X Channels @ Load: 2W x 1 @ 4 Ohm; 69mW x 2 @ 32 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: 3D, DAC, Depop, I²C, I²S, Mute, Short-Circuit and Thermal Protection, Shutdown, Volume Control
• Mounting Type: Surface Mount
• Package / Case: 36-MicroSMDxt
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM49350RLX

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25.2 ALC OVERVIEW
The Automatic Level Control (ALC) system can be used to
regulate the audio output level to a user defined target level.
The ALC feature is especially useful whenever the level of the
audio input is unknown, unpredictable, or has a large dynamic
range. The main purpose of the ALC is to optimize the dy-
namic range of the audio input to audio output path.
There are two separate and independent ALC circuits in the
LM49350. One of the ALC circuits is located within the DAC
DSP effects block. The other ALC circuit is integrated into the
ADC DSP effects block. The DAC ALC controls the DAC dig-
ital gain. The ADC ALC controls the auxiliary input amplifier
gain or microphone preamplifier gain. The dual ALCs can be
used to regulate the level of the analog (Stereo Auxiliary,
mono differential, Stereo MIC/LINE) and digital (Port1 Data
In, Port2 Data In) audio inputs. The ALC regulated output can
be routed to any of the LM49350’s amplifier outputs for play-
back. The ALC regulated output can also be routed to Audio
Port1 or Audio Port2 for digital data transmission via I
PCM.
Only audio inputs that are considered signals (rather than
noise) are sent to the ALC’s peak detector block. The peak
detector compares the level of the audio input versus the ALC
target level (TARGET_LEVEL). Signals lower than the target
level will be amplified and signals higher than the target level
will be attenuated. Any audio input that is lower than the level
specified by the noise floor level (NOISE_FLOOR) will be
considered as noise and will be gated from the ALC’s peak
Bits
2:0
SAMPLE_RATE
Field
This programs the timers on the ALC with the closest sample rate of the ADC.
TABLE 48. ADC_ALC_1 (0x81h)
FIGURE 23. ALC Example
SAMPLE_RATE
2
S or
000
001
010
011
100
101
110
111
62
detector in order to avoid noise pumping. So it is important to
set NOISE_FLOOR to correlate with the signal to noise ratio
of the corresponding audio path. In some instances (ie. Con-
ference calls), it may be desirable to mute audio input signals
that consist solely of background noise from the audio output.
This is accomplished by enabling the ALC’s noise gate
(NG_ENB). When the noise gate is enabled, signals lower
than the noise floor level will be muted from the audio output.
If the audio input signal is below the target level, the ALC will
increase the gain of the corresponding volume control until
the signal reaches the target level. The rate at which the ALC
performs gain increases is known as decay rate (DECAY
RATE). But before each ALC gain increase the ALC must wait
a predetermined amount of time (HOLD TIME). If the audio
input signal is above the target level, the ALC will decrease
the gain of the corresponding volume control until the signal
reaches the target level. The rate at which the ALC performs
attenuation is known as attack rate (ATTACK RATE). The
ALC’s peak detector tracks increases in audio input signal
amplitude instantaneously, but tracks decreases in audio in-
put signal amplitude at programmable rate (PEAK DECAY
TIME). ATTACK RATE, DECAY RATE, HOLD TIME, and
PEAK DECAY TIME are fully adjustable which allows flexible
operation of the ALC circuit. The ALC’s timers are based on
the sample rate of the DAC or ADC, so the closest corre-
sponding sample rate must be programmed into the ALC
SAMPLE RATE setting (for DAC ALC) or the ALC MODE set-
ting (for ADC ALC).
Description
ADC Fs
192kHz
12kHz
16kHz
24kHz
32kHz
48kHz
96kHz
8kHz
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