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Maxim > Design
support > App notes
> Audio
Circuits > APP 3673

Dec 22, 2005

Keywords: MAX9756, ALC, Automatic Level Control,
AGC, Automatic Gain Control, Maxim
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APPLICATION NOTE 3673

An Overview of Automatic Level Control

Abstract: Automatic
Level Control (ALC) is a technology that automatically
controls output power to the speaker. ALC prevents loudspeaker
overload and optimizes dynamic range. This application note
presents ALC technology and demonstrates its use in the
MAX9756/MAX9757/MAX9758 stereo speaker amplifiers.

Attend
this brief webcast by Maxim on
TechOnline

Maxim's
Automatic Level Control (ALC) offers two key benefits
(Figures 1 and 2).

It protects the loudspeaker by limiting the amplifier
output power.

It boosts low-level signals without distorting the
high-level signals.

ALC is implemented in the MAX9756, MAX9757, and
MAX9758 2.3W stereo speaker amplifiers and DirectDrive
headphone amplifiers.

Figures
1 and 2. The MAX9756's automatic level control (ALC) function
protects the speakers without adding distortion.

ALC vs. Output Limiting
ALC differs from traditional
output limiting. An output-limiter function limits the output
swing at a predetermined level so that the transducer is
protected from overvoltage peaks. Clipping (distortion) is
added at the output signal as a result (Figure 3). An
ALC function, however, reduces the gain so that the transducer
is protected. No distortion is added (Figure
4).

Figure
3. The output limiter clips the output signal in overvoltage
conditions and, thus, produces audible
distortion.

Figure
4. The MAX9756's ALC reduces the amplifier gain in overvoltage
conditions so that no distortion is added to the output
signal.

How Does ALC Work?
There are three key timing
specifications in Maxim's ALC technology (Figures 5 and
6): Attack Time, Hold Time, and Release
Time.

Figure
5. Key timing specifications of the ALC
function.

Figure
6. Data plotted for the three key timing specifications of the
ALC function. Data were generated on the
MAX9756.

ATTACK TIME
This
is the time it takes to reduce the gain after the output
signal has exceeded the threshold level. The time constant of
the attack is given by 15,000 X CCT seconds,
where CCT is the external timing capacitor. The
selection of the time constant is application specific. For
example, in applications where transient signals such as snare
drum beats (music) or gun shots (DVD) must be reduced quickly,
shorter attack times should be selected. For other
applications with longer attack times, the ALC will ignore
short-duration peaks and will only reduce the gain when
loudness increases noticeably.

HOLD
TIME
This is the delay after the signal falls below
the threshold level, but before the release phase is
initiated. The hold time is not adjustable; it is internally
set to 50ms. The hold time prevents gain 'pumping,' where ALC
action can be heard responding to low-frequency material. The
hold time is cancelled by any signal exceeding the set
threshold level, at which point the attack is
reinitiated.

RELEASE
TIME
This is the time it takes for the gain to
return to its normal level after the input signal fell below
the threshold level and the hold time expired. Release time is
further defined as release from a 6dB gain compression to 10%
of the nominal gain setting, after the input signal has fallen
below the PREF (pin 29) threshold and the 50ms hold time has
expired. The release time is determined by an
attack-/release-time ratio that can be selected by setting the
logic state of DR (pin 25) (Table 1). Release time is
adjustable between 95ms and 10s.

Table 1.
Attack/Release Ratio Is Set with a Logic Level at Pin DR

DR

ATTACK/RELEASE RATIO

VDD

1:200

VBIAS

1:633

GND

1:2000

Output Power Threshold
An external resistor connected
from PREF to ground sets the threshold at which the speaker
output is clamped (Figure 7). Equation 1 sets the
resistor value at PREF for a desired maximum output power and
a selected speaker impedance.

Figure
7. The output power threshold is set by an external
resistor.

Maxim Application Recommendation
For notebook
applications in which music CDs and DVDs are the main audio source,
an attack time of 495ms and a release time of 990ms are
recommended with an output power threshold of 1.2W with an RL
= 8Ω (Figure 8).

Figure
8. Recommended ALC values for a notebook application.

Related parts

MAX9756

2.3W Stereo Speaker
Amplifiers and DirectDrive Headphone Amplifiers
with Automatic Level Control

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Download,
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APP 3673: Dec 22, 2005

AN3673, AN 3673, APP3673,
Appnote3673, Appnote 3673

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