Table 2. release to attack ratio, Table 3. maximum gain settings – Rainbow Electronics MAX9758 User Manual
Page 16
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 100k
Ω 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 180k
Ω resistor results in 1.2W clamp
limit on an 8
Ω load and a 200kΩ resistor results in a
1.5W clamp limit on an 8
Ω 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).
R
k
P
R
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
180k
Ω
VALUES SHOWN FOR AN OUTPUT POWER THRESHOLD OF 1.2W WITH AN
R
L
= 8
Ω 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
(k
Ω)
OUTPUT POWER THRESHOLD (W)
190
180
170
160
150
140
130
120
110
0.5
1.0
1.5
2.0
2.5
3.0
0
100
200
R
L
= 4
Ω
R
L
= 8
Ω
Figure 6. Output Power Threshold vs. RPREF
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