MAX4411
80mW, Fixed-Gain, DirectDrive, Stereo
Headphone Amplifier with Shutdown
12
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output, supply, and ground traces improve the maxi-
mum power dissipation in the package.
Thermal-overload protection limits total power dissipa-
tion in the MAX4411. When the junction temperature
exceeds +140°C, the thermal protection circuitry dis-
ables the amplifier output stage. The amplifiers are
enabled once the junction temperature cools by 15°C.
This results in a pulsing output under continuous thermal-
overload conditions.
Output Power
The device has been specified for the worst-case sce-
nario—when both inputs are in phase. Under this con-
dition, the drivers simultaneously draw current from the
charge pump, leading to a slight loss in headroom of
VSS. In typical stereo audio applications, the left and
right signals have differences in both magnitude and
phase, subsequently leading to an increase in the max-
imum attainable output power. Figure 5 shows the two
extreme cases for in and out of phase. In reality, the
available power lies between these extremes.
Powering Other Circuits from a
Negative Supply
An additional benefit of the MAX4411 is the internally
generated, negative supply voltage (PVSS). This volt-
age provides the ground-referenced output level. PVSS
can, however, also be used to power other devices
within a design limit current drawn from PVSS to 5mA;
exceeding this affects the headphone driver operation.
A typical application is a negative supply to adjust the
contrast of LCD modules.
PVSS is roughly proportional to PVDD and is not a regu-
lated voltage. The charge-pump output impedance
must be taken into account when powering other
devices from PVSS. The charge-pump output imped-
ance plot appears in the Typical Operating
Characteristics. For best results, use 2.2F charge-
pump capacitors.
Component Selection
Input Filtering
The input capacitor (CIN), in conjunction with the inter-
nal RIN, forms a highpass filter that removes the DC
bias from an incoming signal (see 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:
RIN is the amplifier’s internal input resistance value
given in the Electrical Characteristics. Choose the CIN
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 capacitors whose dielectrics
have low-voltage coefficients, such as tantalum or
aluminum electrolytic ones. Capacitors with high-voltage
coefficients, such as ceramics, may result in increased
distortion at low frequencies.
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 1 lists sug-
gested manufacturers.
Flying Capacitor (C1)
The value of the flying capacitor (C1) affects the charge
pump’s load regulation and output resistance. 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. Increasing the value of C1 improves
load regulation and reduces the charge-pump output
resistance to an extent. See the Output Power vs.
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics. Above
2.2F, the on-resistance of the switches and the ESR of
C1 and C2 dominate.
Hold Capacitor (C2)
The hold capacitor value and ESR directly affect the
ripple at PVSS. Increasing the value of C2 reduces
f
RC
dB
IN IN
=
3
1
2
π
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX4411
fig05
SUPPLY VOLTAGE (V)
OUTPUT
POWER
(mW)
3.3
3.0
2.7
2.4
2.1
50
100
150
200
250
300
0
1.8
3.6
fIN = 1kHz
RL = 16
Ω
THD+N = 10%
INPUTS
IN PHASE
INPUTS 180
°
OUT OF PHASE
Figure 5. Output Power vs. Supply Voltage with Inputs In/Out of
Phase