Rainbow Electronics MAX9758 - Manual (Page 15)

Attack Time
The attack time is the time it takes to reduce the gain
after the input signal has exceeded the threshold level.
Suggested attack time range is from 150µs to 50ms.
The gain attenuation in attack is exponential and the
attack time is defined as one time constant. The time
constant of the attack is given by 15,000 x C
CT sec-
onds (where C CT is the external timing capacitor).
• Use a short attack time for the ALC to react quickly
to transient signals, such as snare drum beats
(music) or gun shots (DVD). Fast attack times can
lead to gain “pumping” where rapid ALC action can
be heard reacting to dynamic material.
• Use a longer attack time to allow the ALC to ignore
short-duration peaks and only reduce the gain when
a noticeable increase in loudness occurs. Short-dura-
tion peaks are not reduced, but louder passages are.
This allows the louder passages to be reduced in vol-
ume, thereby maximizing output dynamic range.
Having the attack time too long can possibly result in
some damage to the loudspeaker under harsh condi-
tions.
Hold Time
Hold time is the delay after the signal falls below the
threshold level before the release phase is initiated.
Hold time is internally set to 50ms and nonadjustable.
The hold time is cancelled by any signal exceeding the
set threshold level and attack is reinitiated.
Release Time
The release time is how long it takes for the gain to
return to its normal level after the input signal has fallen
below the threshold level and 50ms hold time has
expired. Release time is defined as release from a 6dB
gain compression to 10% of the nominal gain setting
after the input signal has fallen below PREF threshold
and the 50ms hold time has expired. Release time is
adjustable between 95ms and 10s. The release time is
set by picking an attack time using C
CT and setting the
attack to release time ratio by configuring DR as shown
in Table 2. Release time is linear in dB with time and is
inversely proportional to the magnitude of gain com-
pression:
• Use a small ratio to maximize the speed of the ALC.
• Use a large ratio to maximize the sound quality and
prevent repeated excursions above the threshold
from being independently adjusted by the ALC.
Release and attack times are set by selecting the
capacitance value between CT and GND, and by set-
ting the logic state of DR (Table 1). DR is a tristate logic
input that sets the attack-to-release time ratio. A fixed
hold time of 50ms is internally added to the release time.
MAX9756/MAX9757/MAX9758
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
______________________________________________________________________________________ 15
OUTPUT
2V/div
CT
1V/div
10ms/div
Figure 4. Attack, Hold, and Release Time
Table 1. Attack and Release Time
ATTACK TIME RELEASE TIME TIMING CAPACITOR
(C CT ) DR = ‘X’ DR = V
DD
DR = V
BIAS
DR = GND
10nF 150µs 30ms 95ms 300ms
33nF 495µs 99ms 313ms 990ms
100nF 1.5ms 300ms 950ms 3s
330nF 4.95ms 990ms 3.1s 9.9s
1µF 15ms 3s 9.5s —
2.2µF 33ms 6.6s — —
3.3µF 49.5ms 10s — —