MAX9725 Maxim Integrated Products, MAX9725 Datasheet - Page 9

• Manufacturer Part Number: MAX9725
• Description: Stereo Headphone Amplifier
• Manufacturer: Maxim Integrated Products

Figure 4. Output Power vs. Supply Voltage with Inputs In-/Out-
of-Phase
The MAX9725’s output power increases when the left
and right audio signals differ in magnitude and/or
phase. Figure 4 shows the two extreme cases for in-
and out-of-phase input signals. The output power of a
typical stereo application lies between the two extremes
shown in Figure 4. The MAX9725 is specified to output
20mW per channel when both inputs are in-phase.
The MAX9725 internally generates a negative supply
voltage (PV
signal. Other devices can be powered from PV
vided the current drawn from the charge pump does
not exceed 1mA. Headphone driver output power and
THD+N will be adversely affected if more than 1mA is
drawn from PV
cal application for the negative supply.
PV
a 1µF capacitor from C1P to C1N for best charge-pump
operation.
1V, Low-Power, DirectDrive, Stereo Headphone
SS
is unregulated and proportional to V
SS
50
45
40
35
30
25
20
15
10
5
0
) to provide the ground-referenced output
0.9
SS
OUTPUT POWER vs. SUPPLY VOLTAGE
WITH INPUTS IN- AND OUT-OF-PHASE
f
R
THD+N = 1%
IN
L
= 1kHz
= 16Ω
. Using PV
Powering Other Circuits from
_______________________________________________________________________________________
SUPPLY VOLTAGE (V)
INPUTS 180 ° OUT-OF-PHASE
1.1
INPUTS IN-PHASE
SS
the Negative Supply
as an LCD bias is a typi-
1.3
Output Power
1.5
DD
. Connect
SS
pro-
Amplifier with Shutdown
The AC-coupling capacitor (C
setting resistor form a highpass filter that removes any
DC bias from an input signal (see the Functional
Diagram). C
an optimum DC level. The -3dB point of the highpass
filter, assuming zero source impedance, is given by:
Choose C
interest. Setting f
frequency response. Use capacitors with low-voltage
coefficient dielectrics. Film or C0G dielectric capacitors
are good choices for AC-coupling capacitors. Capacitors
with high-voltage coefficients, such as ceramics, can
result in increased distortion at low frequencies.
Use capacitors with less than 100mΩ of ESR. Low-ESR
ceramic capacitors minimize the output impedance of the
charge pump. Capacitors with an X7R dielectric provide
the best performance over the extended temperature
range. Table 1 lists suggested capacitor manufacturers.
The value of C1 affects the charge pump’s load regula-
tion and output impedance. Choosing C1 too small
degrades the MAX9725’s ability to provide sufficient
current drive and leads to a loss of output voltage.
Increasing the value of C1 improves load regulation
and reduces the charge-pump output impedance. See
the Output Power vs. Charge-Pump Capacitance and
Load Impedance graph in the Typical Operating
Characteristics.
The hold capacitor’s value and ESR directly affect the
ripple at PV
ple. Choosing a capacitor with lower ESR reduces rip-
ple and output impedance. Lower capacitance values
can be used in systems with low maximum output
power levels. See the Output Power vs. Charge-Pump
Capacitance and Load Impedance graph in the Typical
Operating Characteristics.
IN
SS
IN
so f
f
. Increasing the value of C2 reduces rip-
-3dB
allows the MAX9725 to bias the signal to
-3dB
-3dB
=
Charge-Pump Capacitor Selection
is well below the lowest frequency of
too high affects the amplifier’s low-
2
π
×
25k
Component Selection
1
IN
Ω
) and an internal gain-
Flying Capacitor (C1)
×
Hold Capacitor (C2)
C
IN
Input Filtering
9