Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
20 ______________________________________________________________________________________
To guarantee stability and minimize distortion, select
the external feedback resistor (R
F_
) and capacitor
(C
FB_
) so that the following conditions are met:
where f
SW
is the output switching frequency deter-
mined by R
REF
(see the Setting the Switching
Frequency and Output Current Limit (R
REF
) section).
Setting the Switching Frequency and
Output Current Limit (R
REF
)
Resistor R
REF
determines the output switching frequency
(f
SW
) and the output short-circuit current-limit value (I
SC
).
Set f
SW
and I
SC
with the following equations:
For example, selecting a 68kΩ resistor for R
REF
results
in a switching frequency of 303kHz and an output
short-circuit current limit of 4.5A.
To prevent damage to the MAX9742 during output
short-circuit conditions and to utilize its full output
power capabilities, use resistor values greater than or
equal to 58kΩ and less than or equal to 75kΩ for R
REF
.
Input-Coupling Capacitor
The AC-coupling capacitors (C
IN
) and input resistors
(R
IN_
) form highpass filters that remove any DC bias
from an input signal (see the Typical Application
Circuits/Functional Diagrams ). C
IN
prevents any DC
components from the input-signal source from appear-
ing at the amplifier outputs. The -3dB point of the high-
pass filter, assuming zero source impedance due to the
input signal source, is given by:
Choose C
IN
so that f
-3dB
is well below the lowest frequen-
cy of interest. Setting f
-3dB
too high affects the amplifier’s
low-frequency response. Use capacitors with low-voltage
coefficient dielectrics. Aluminum electrolytic, tantalum, or
film dielectric capacitors are good choices for AC-cou-
pling capacitors. Capacitors with high-voltage coeffi-
cients, such as ceramics (non-C0G dielectrics), can
result in increased distortion at low frequencies.
Single-Ended LC Output Filter Design (L
F
and C
F
)
An LC output filter is needed to extract the amplified
audio signal from the PWM output (see Figure 8). The LC
circuit forms an LCR lowpass filter (neglecting voice coil
inductance) with the impedance of the speaker. To pro-
vide a maximally flat-frequency response, the LCR filter
should be designed to have a Butterworth response and
should be optimized for a specific speaker load. Table 1
provides some recommended standard L
F
and C
F
com-
ponent values for 4Ω, 6Ω, and 8Ω speaker loads. The
component values given in Table 1 provide an approxi-
mate -3dB cutoff frequency (f
C
) of 40kHz. The following
paragraph provides information on calculating filter com-
ponent values for cutoff frequencies other than 40kHz
and speaker loads not listed in Table 1.
The LCR filter has the following 2nd order transfer function:
where L
F
is the value of the filter inductor, C
F
is the
value of the filter capacitor, and R
SPKR
is the DC resis-
tance of the speaker. The voice coil inductance of the
speaker has been neglected to simplify filter calcula-
tions (see the Zobel Network section). The above trans-
fer function is presented in the general 2nd order
transfer function format given below:
where w
n
is the natural frequency in radians/s and ζ is
the damping ratio of the 2nd order system. For an ideal
Butterworth response, ζ is equal to 0.707 and ω
C
is
equal to the -3dB cutoff frequency, ω
c
. Using the above
transfer functions and converting to Hertz, the -3dB cut-
off frequency of the filter is:
f
1
2 L C
C
FF
=
×× ×
()
π
Hz
H
s2 s
(s)
n
2
2
nn
2
=
+×× ×+
ω
ζω ω
H
1
L C
s
1
R C
s
1
L C
(s)
FF
2
SPKR F F F
=
×
+
×
+
×
f
1
2 R C
3dB
IN IN
−
=
××
()
π
Hz
f
1
3.3 s
68k
R
I 3.6A
68k
R
SW
REF
SC
REF
=
×
=×
()
()
µ
Ω
Ω
Hz
A
R C
21.5
f
F_ FB_
SW
× ≥ >
_
and R k
F
400 Ω