Datasheet

MAX9791/MAX9792
Windows Vista-Compliant Class D Speaker
Amplifiers with DirectDrive Headphone Amplifiers
______________________________________________________________________________________ 25
Component Selection
Speaker Amplifier Power-Supply Input (PVDD)
PVDD powers the speaker amplifiers. PVDD ranges
from 2.7V to 5.5V. AVDD and PVDD must be tied
together. If LDO is enabled, set AVDD and PVDD as
specified in the Line Regulation row of the Electrical
Characteristics table. Bypass PVDD with a 0.1µF
capacitor to PGND. Apply additional bulk capacitance
at the device if long input traces between PVDD and
the power source are used.
Headphone Amplifier Power-Supply Input
(HPVDD and CPVSS)
The headphone amplifiers are powered from HPVDD
and CPVSS. HPVDD is the positive supply of the head-
phone amplifiers and ranges from 2.7V to 5.5V. Bypass
HPVDD with a 10µF capacitor to PGND. CPVSS is the
negative supply of the headphone amplifiers. Bypass
CPVSS with a 1µF capacitor to PGND. The charge
pump inverts the voltage at HPVDD, and the resulting
voltage appears at CPVSS. A 1µF capacitor should be
connected between C1N and C1P.
Positive Power Supply and LDO Input (AVDD)
The internal LDO and the remainder of the device are
powered by AVDD. AVDD ranges from 2.7V to 5.5V.
AVDD and PVDD must be tied together. If LDO is
enabled, set AVDD and PVDD as specified in LDO line
regulation. Bypass AVDD with a 0.1µF capacitor to
GND and two 1µF capacitors to GND. Note additional
bulk capacitance is required at the device if long input
traces between AVDD and the power source are used.
Input Filtering
The input capacitor (C
IN_
), in conjunction with the ampli-
fier input resistance (R
IN_
), forms a highpass filter that
removes the DC bias from the incoming signal. The AC-
coupling capacitor allows the amplifier to bias the signal
to an optimum DC level. Assuming zero source imped-
ance, the -3dB point of the highpass filter is given by:
R
IN_
is the amplifier’s external input resistance value.
Choose C
IN_
such that f
-3dB
is well below the lowest
frequency of interest. Setting f
-3dB
too high affects
the amplifier’s low frequency response. Use capaci-
tors with adequately low-voltage coefficients (see
Figure 12). Capacitors with higher voltage coeffi-
cients, such as ceramics, result in increased distor-
tion at low frequencies.
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 100mΩ for opti-
mum performance. Low ESR ceramic capacitors mini-
mize the output resistance of the charge pump. For
best performance over the extended temperature
range, select capacitors with an X7R dielectric.
Flying Capacitor (C1)
The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. A
C1 value that is too small degrades the device’s ability
to provide sufficient current drive, which leads to a loss
of output voltage. Connect a 1µF capacitor between
C1P and C1N.
f
RC
dB
IN IN
-3
1
2
=
π
__
INPUT COUPLING CAPACITOR-INDUCED THD+N
vs. FREQUENCY (HEADPHONE MODE)
FREQUENCY (kHz)
THD+N (dBFS)
100
-90
-80
-70
-60
-50
-100
10 1000
0603 10V X5R 10% 1
µ
F
0805 50V X7R 10% 1
µ
F
V
OUT
- -3dBFS
FS = 1V
RMS
R
L
=32
Ω
0603 10V X7R 10% 1
µ
F
0402 6.3V X5R 10% 1
µ
F
Figure 12. Input Coupling Capacitor-Induced THD+N vs.
Frequency
SPEAKER RF IMMUNITY
vs. FREQUENCY
FREQUENCY (MHz)
AMPLITUDE (dBV)
2500200015001000500
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
-130
0 3000
RIGHT
LEFT
Figure 13. Speaker RF Immunity