Datasheet

MAX4409
80mW, DirectDrive, Stereo Headphone
Amplifier with Common-Mode Sense
12 ______________________________________________________________________________________
driver. The negative supply voltage appears on the
PV
SS
pin. A typical application is a negative supply to
adjust the contrast of LCD modules.
When considering the use of PV
SS
in this manner, note
that the charge-pump voltage at PV
SS
is roughly pro-
portional to -V
DD
and is not a regulated voltage. The
charge-pump output impedance plot appears in the
Typical Operating Characteristics.
Component Selection
Gain-Setting Resistors
External feedback components set the gain of the
MAX4409. Resistors R
F
and R
IN
(see Typical Application
Circuit) set the gain of each amplifier as follows:
Choose feedback resistor values of 10kΩ. Values other
than 10kΩ increase V
OS
due to the input bias current,
which in turn increases the amount of DC current flow
to the load. Resistors R
IN
, R2, R
F
, and R1 must be of
equal value for best results. Use high-tolerance resis-
tors for best matching and CMRR. For example, the
worst-case CMRR attributed to a 1% resistor mismatch
is -34dB. This is the worst case, and typical resistors do
not affect CMRR as drastically. The effect of resistor
mismatch is shown in Figure 5. If all resistors match
exactly, then any voltage applied to node A should be
duplicated on OUT so no net differential voltage
appears between node A (normally the HP jack socket
GND) and OUT. For resistors with a tolerance of n%,
the worst mismatch is found when R
IN
and R1 are at
+n%, and R
F
and R2 are at -n%. If all four resistors are
nominally the same value, then 2n% of the voltage at A
appears between A and OUT.
Packaged resistor arrays can provide well-matched
components for this type of application. Although their
absolute tolerance is not well controlled, the internal
matching of resistors can be very good. At higher fre-
quencies, the rejection is usually limited by PC board
layout; care should be taken to make sure any stray
capacitance due to PC board traces on node N1 match-
es those on node N2. Ultimately, CMRR performance is
limited by the amplifier itself (see Electrical
Characteristics).
Compensation Capacitor
The stability of the MAX4409 is affected by the value of
the feedback resistor (R
F
). The combination of R
F
and
the input and parasitic trace capacitance introduces an
additional pole. Adding a capacitor in parallel with R
F
compensates for this pole. Under typical conditions
with proper layout, the device is stable without the
additional capacitor.
Input Filtering
The input capacitor (C
IN
), in conjunction with R
IN,
forms a
highpass filter that removes the DC bias from an incom-
ing signal (see Typical Application Circuit). The AC-cou-
pling 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:
f
RC
dB
IN IN
-
2
3
1
=
π
A
V
=−
⎛
⎝
⎜
⎞
⎠
⎟
R
R
F
IN
MAX4409
R1
N2
N1
R2
R
IN
R
F
A
OUT
Figure 5. Common-Mode Sense Equivalent Circuit
100
10
1
0.1
0.01
0.001
0 100 15050 200
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX4409 fig04
OUTPUT POWER (mW)
V
DD
= 3V
A
V
= -1V/V
R
L
= 16Ω
f
IN
= 10kHz
THD+N (%)
OUTPUTS IN
PHASE
ONE
CHANNEL
OUTPUTS
180° OUT OF
PHASE
Figure 4. Output Power vs. THD+N with Inputs In/Out of Phase