Rainbow Electronics MAX9758 User Manual

Page 14

MAX9756/MAX9757/MAX9758

The MAX9756/MAX9757/MAX9758 feature an under-
voltage lockout that prevents operation from an insuffi-
cient power supply and click-and-pop suppression that
eliminates audible transients on startup and shutdown.
The amplifiers include thermal-overload and short-cir-
cuit protection. An additional feature of the amplifiers is
that there is no phase inversion from input to output.

Automatic Level Control (ALC)

Two-watt amplifiers are commonly used in notebook
PCs (almost always powered from a 5V supply). With
an 8

Ω speaker driven from a BTL amplifier, the maxi-

theoretical continuous power available is:

See Figure 5 for suggested ALC component values.
The ALC feature offers two benefits:

1) To limit amplifier power to protect a loudspeaker.

2) To make input signals with a wide dynamic range

more intelligible by boosting low-level signals with-
out distorting the high-level signals.

A device without ALC experiences clipping at the output
when too much gain is applied to the input. ALC pre-
vents clipping at the output when too much gain is

applied to the input, eliminating output clipping. Figure 3
shows a comparison of an overgained speaker input with
and without ALC.

The MAX9756/MAX9758 control the gain to the speakers
by first detecting that the output voltage to the speaker
has exceeded a preset limit. The speaker amplifier gain
is rapidly reduced to correct for the excessive output
power. This process is known as the attack time. When
the signal subsequently lowers in amplitude, the gain is
held at the reduced state for a short period before slowly
increasing to the normal value. This process is known as
the hold and release time. The speed at which the ampli-
fiers adjust to changing input signals is set by the exter-
nal timing capacitor C

and the setting of logic input

DR. The output power limit can be set by adjusting the
value of the external resistor connected to PREF. Gain
reduction is a function of input signal amplitude with a
maximum ALC attenuation of 6dB. Figure 4 shows the
effect of an input burst exceeding the preset limit, output
attack, hold and release times.

This process (referred to as “limiting” in audio) limits the
amplifier output power so loudspeaker overload can be
prevented. If the attack and release times are configured
to respond too fast, audible artifacts often, described as
“pumping” or “breathing,” can occur as the gain is rapid-
ly adjusted to follow the dynamics of the signal. For best
results, adjust the time constant of the ALC to accommo-
date the source material. Notebook applications in which
music CDs and DVDs are the main audio source, a
495µs attack time with a 990ms release time is recom-
mended with a 1.2W output into an 8

Ω load.

OUT

PEAK

SPEAKER

⎛
⎝⎜

⎞
⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

⎛
⎝⎜

⎞
⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

1 56

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

______________________________________________________________________________________

ALC DISABLE, CLIPPING AT THE OUTPUT

INPUT

SIGNAL

10ms/div

OUTPUT

SIGNAL

ALC ENABLE, NO CLIPPING AT THE OUTPUT

Figure 3. ALC Disabled vs. ALC Enabled