Instruction Manual
MAX9756/MAX9757/MAX9758
The release/attack time ratio that can be achieved by
programming DR is listed in Table 2.
Output Power Threshold
To set the threshold at which speaker output is
clamped, an external resistor must be connected from
PREF to ground. The suggested external resistor range
is from 100kΩ to 200kΩ (for best results use a 1% resis-
tor). Leaving PREF unconnected disables the ALC
function. A constant current of 12µA is sourced at
PREF, so that a 180kΩ resistor results in 1.2W clamp
limit on an 8Ω load and a 200kΩ resistor results in a
1.5W clamp limit on an 8Ω load (Figure 6).
Use the following equation to choose the value for
R
PREF
for the desired maximum output power level
based on a sine wave input:
Gain Selection
The MAX9756/MAX9757/MAX9758 feature an internally
set, selectable gain. The GAIN1, GAIN2, and GAIN3
inputs set the maximum gain for the speaker and head-
phone amplifiers (Table 3). The gain of the device can
vary based upon the voltage at VOL but does not
exceed the maximum gain listed below (see the Analog
Volume (VOL) Control section).
Rk
PR
PREF
OUT L
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
180
1 166 8
Ω
.
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
16 ______________________________________________________________________________________
MAX9756
MAX9757
33nF
180kΩ
VALUES SHOWN FOR AN OUTPUT POWER THRESHOLD OF 1.2W WITH AN
R
L
= 8Ω ATTACK TIME OF 495μs AND A RELEASE TIME OF 990ms
DR
5V
V
DD
CT
PREF
Figure 5. Recommended Output Power Threshold, Attack, and
Release Time Components
OUTPUT POWER THRESHOLD
vs. R
PREF
R
PREF
(kΩ)
OUTPUT POWER THRESHOLD (W)
190180170160150140130120110
0.5
1.0
1.5
2.0
2.5
3.0
0
100 200
R
L
= 4Ω
R
L
= 8Ω
Figure 6. Output Power Threshold vs. R
PREF
Table 2. Release to Attack Ratio
DR RELEASE/ATTACK RATIO
V
DD
200
V
BIAS
633
GND 2000
Table 3. Maximum Gain Settings
GAIN3 GAIN2 GAIN1 SPEAKER MODE GAIN (dB) HEADPHONE MODE GAIN (dB)
0 0 0 +15 0
0 0 1 +16.5 0
0 1 0 +18 +3
0 1 1 +19.5 +3
1 0 0 +21 0
1 0 1 +22.5 0
1 1 0 +24 +3
1 1 1 +25.5 +3