Rainbow Electronics MAX9758 - Manual (Page 22)

MAX9756/MAX9757/MAX9758
Output Power (Headphone Amplifier)
The headphone amplifiers have been specified for the
worst-case scenario—when both inputs are in phase.
Under this condition, the drivers simultaneously draw
current from the charge pump, leading to a slight loss
in headroom of V
SS . In typical stereo audio applica-
tions, the left and right signals have differences in both
magnitude and phase, subsequently leading to an
increase in the maximum attainable output power.
Figure 14 shows the two extreme cases for in and out
of phase. In reality, the available power lies between
these extremes.
Power Supplies
The MAX9756/MAX9757/MAX9758 have different sup-
plies for each portion of the device, allowing for the opti-
mum combination of headroom and power dissipation
and noise immunity. The speaker amplifiers are pow-
ered from PV
DD . PV DD ranges from 4.5V to 5.5V. The
headphone amplifiers are powered from HPV DD and
V
SS
. HPV
DD
is the positive supply of the headphone
amplifiers and ranges from 3V to 5.5V. V
SS
is the nega-
tive supply of the headphone amplifiers. Connect V SS to
CPV SS . The charge pump is powered by CPV DD .
CPV DD ranges from 3V to 5.5V and should be the same
potential as HPV DD . The charge pump inverts the volt-
age at CPV DD , and the resulting voltage appears at
CPV
SS
. The remainder of the device is powered by V
DD
.
Component Selection
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 an incoming signal (see the
Typical Application Circuit). The AC-coupling capacitor
allows the amplifier to bias the signal to an optimum DC
level. Assuming zero source impedance, the -3dB point
of the highpass filter is given by:
R IN is the amplifier’s internal input resistance value
given in the Electrical Characteristics. 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-fre-
quency response. Use capacitors with low-voltage
coefficient dielectrics, such as tantalum or aluminum
electrolytic. Capacitors with high-voltage coefficients,
such as ceramics, may result in increased distortion at
low frequencies.
BIAS Capacitor
BIAS is the output of the internally generated DC bias
voltage. The BIAS bypass capacitor, C
BIAS
, improves
PSRR and THD+N by reducing power supply and other
noise sources at the common-mode bias node, and
also generates the startup/shutdown DC bias wave-
forms for the speaker amplifiers. Bypass BIAS with a
1µF capacitor to GND.
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 100m Ω 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. Table 6
lists suggested manufacturers.
f
R C
dB
IN IN
−
=
3
1
2
π
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
22 ______________________________________________________________________________________
OUTPUT POWER (mW)
THD+N (%)
160 140 100 120 60 80 20 40
0.01
0.1
1
10
100
0.001
0 200 180
HPV
DD
= 5V
R
L
= 16 Ω
OUTPUTS 180 ° OUT OF PHASE
OUTPUTS IN PHASE
Figure 14. Total Harmonic Distortion Plus Noise vs. Output
Power with Inputs In/Out of Phase (Headphone Mode)
Table 6. Suggested Capacitor Manufacturers
SUPPLIER PHONE FAX WEBSITE
Taiyo Yuden 800-384-2496 800-925-0899 www.t-yuden.com
TDK 807-803-6100 847-390-4405 www.component.tdk.com