interest are not attenuated. For a typical application, set
fPOLE equal to or below 20Hz.
Figure 13 shows the frequency response of the circuit
in Figure 12. With RA = 47k
Ω, RB = 22kΩ, and CA =
0.33F, the passband gain is set to 8.8dB.
Layout and Grounding
Proper layout and grounding are essential for optimum
performance. Connect PGND and SGND together at a
single point on the PC board. Connect PVSS to SVSS
and bypass with a 1F capacitor to PGND. Bypass VDD
to PGND with a 1F capacitor. Place the power-supply
bypass capacitor and the charge-pump capacitors as
close to the MAX9723 as possible. Route PGND and all
traces that carry switching transients away from SGND
and the audio signal path. Route digital signal traces
away from the audio signal path. Make traces perpen-
dicular to each other when routing digital signals over
or under audio signals.
The thin QFN package features an exposed paddle
that improves thermal efficiency. Ensure that the
exposed paddle is electrically isolated from PGND,
SGND, and VDD. Connect the exposed paddle to
SVSS when the board layout dictates that the
exposed paddle cannot be left floating.
CA
fRA RB
POLE
()
=
×
1
2
π
MAX9723
Stereo DirectDrive Headphone Amplifier
with BassMax, Volume Control, and I2C
______________________________________________________________________________________
19
CA
RB
RA
R
OUT_
BB_
AUDIO
INPUT
MAX9723
BassMax
ENABLE
Figure 12. Using BassMax to Increase MAX9723’s Maximum
Gain
R2 (k
Ω)AV GAIN (dB)
39
20.6
33
15.1
27
11.3
22
8.8
15
5.7
10
3.7
Table 12. BassMax Gain Examples
(R1 = 47k
Ω)
C3 (nF)
fPOLE (Hz)
fZERO (Hz)
100
38
106
82
47
130
68
56
156
56
68
190
47
81
230
22
174
490
10
384
1060
Table 13. BassMax Pole and Zero
Examples for a Gain Boost of 8.8dB
(R1 = 47k
Ω, R2 = 22kΩ)
FREQUENCY RESPONSE OF FIGURE 12
FREQUENCY (Hz)
A
V(dB)
1k
100
10
1
2
3
4
5
6
7
8
9
10
0
0.1
10k
MAX9723A
CMD REGISTER
CODE = 0xFF
RA = 47k
Ω
RB = 22k
Ω
CA = 0.33
μF
Figure 13. Increasing the Maximum Gain Using BassMax