Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
24 ______________________________________________________________________________________
Multiple-Pole MID Network vs.
Single-Pole VMID Network for Increased PSRR
Performance (Single-Supply Operation)
A multiple-pole MID network improves PSRR perfor-
mance over a single-pole network. Since the input
amplifiers of the MAX9742 are biased at V
MID
, any
noise coupled into the MID input using the MID bias
network supply appears at the outputs of the MAX9742.
Increasing the number of poles in the MID network pro-
vides further attenuation of low-frequency noise at the
MID input, and therefore, improving the AC PSRR per-
formance of the MAX9742. Figure 11 shows the recom-
mended single-pole and two-pole MID input bias
networks. Figure 12 illustrates the differences of the
MAX9742’s low-frequency AC PSRR performance with
the single-pole and two-pole networks shown in Figure
11.
Soft-Start Capacitor (C
SFT
)
The soft-start capacitor determines the timing for the
soft-start power-up sequencing that minimizes audible
clicks-and-pops during power-up/power-down transi-
tions and when entering/exiting shutdown mode.
Connect a capacitor between SFT and ground for
proper operation. For optimum performance, this
capacitor should equal 0.22µF. Using capacitor values
much smaller than these values degrade click-and-
pop performance and values much greater lengthen
startup time.
Startup Time Considerations
At the beginning of the soft-start sequence, the
MAX9742 ensures V
OUT_
is approximately equal to
V
MID
before continuing the soft-start sequence. For sin-
gle-supply operation with single-ended outputs, the
output-coupling capacitors (C
OUT
) are first gradually
charged up to V
MID
before continuing soft-start
sequencing. This gradual charging up of C
OUT
mini-
mizes audible transients that may appear across the
R1
10kΩ
R2
10kΩ
V
DD
SINGLE-POLE NETWORK
TO
MID
C
MID1
22µF
C
MID2
1µF
R1
10kΩ
R2
10kΩ
V
DD
TWO-POLE NETWORK
TO
MID
C
MID1
10µF
C
MID2
10µF
R3
10kΩ
Figure 11. Recommended MID Input Bias Networks
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
MAX9742 fg12
FREQUENCY (Hz)
PSRR (dB)
10k1k100
-100
-80
-60
-40
-20
0
20
-120
10 100k
1-POLE MID NETWORK
SINGLE SUPPLY
V
DD
= 24V + 500mV
P-P
R
L
= 8Ω
2-POLE MID NETWORK
Figure 12. Comparison of MAX9742 AC PSRR with Single-Pole
and Two-Pole MID Networks