Datasheet

Using the transfer functions and the equation for f
c
, the
following expressions for L
F
and C
F
can be derived:
Since the frequency response of the output filter is
dependent on the speaker resistance, it is best to opti-
mize the LC filter for a particular load resistance. To
calculate the component values of the LC filter for a
given speaker load resistance, first select an appropri-
ate cutoff frequency for the filter. The cutoff frequency
should be high enough so that upper audio frequency
band attenuation is kept to a minimum while providing
sufficient attenuation at the switching frequency (f
SW
) of
the MAX9742. Once the cutoff frequency is determined,
calculate C
F
using the DC resistance of the speaker
(R
SPKR
) and a damping ratio (ζ) equal to 0.707. Finally,
calculate L
F
using the resulting C
F
value.
When selecting C
F
, use capacitors with DC voltage rat-
ings greater than V
DD
.
When selecting L
F
, it is important to take into account
the DC resistance, current capabilities, and upper fre-
quency limitations of the inductor. Choosing an induc-
tor with minimum DC resistance minimizes I
2
R losses
due to the filter inductor and therefore preserves power
efficiency. The inductor current rating should be
greater than the maximum peak output current to pre-
vent the inductor from going into saturation. Output
inductor saturation introduces nonlinearities into the
output signal and therefore increases distortion. The
upper frequency limit of the inductor should also be
taken into account. The load connected to the output of
the half-bridge (LC filter and speaker) should remain
inductive at the switching frequency of the MAX9742. If
not, a significant amount of high-frequency energy is
dissipated in the resistive load, therefore, increasing
the supply current to excessive levels. To prevent this
from occurring, select an output inductor whose self-
resonant frequency is substantially higher than the
switching frequency of the MAX9742.
To minimize possible EMI radiation, place the LC filter
near the MAX9742 on the PCB.
Table 2 provides some suggested inductor manufac-
turers.
BTL LC Output Filter Design
When using the BTL configuration, optimize the output fil-
ter for fully differential operation (see Figure 9 and Table
3). Follow the design criteria provided for the single-
ended filter except use half the value of the BTL resis-
tance for the output filter calculations. This is because
each half-bridge output sees half of the BTL resistance.
For example, with a BTL resistance of 8Ω the ideal filter
component values are C
F
= 0.7µF and L
F
= 22.5µH for a
maximally flat differential filter response with an approxi-
mate cutoff frequency of 40kHz. Rounding to the nearest
standard component values yields C
F
= 0.68µF and L
F
=
22µH. Also connect ground-terminated Zobel networks
on each side of the speaker load (see the Zobel Network
section). Ground terminating the Zobel networks pre-
vents excessive peaking in the common-mode frequen-
cy response of the filter.
C
1
4 f R
L
1
4 f C
F
C
SPKR
F
2
C
2
F
=
×× × ×
=
×× ×
()
()
πζ
π
F
H
MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
______________________________________________________________________________________ 21
Table 1. Recommended LC Filter
Component Values for Various Speaker
Loads (f
C
= 40kHz)
DC RESISTANCE OF SPEAKER (Ω)L
F
(µH) C
F
(µF)
4 22 0.68
6 33 0.47
8 47 0.33
Table 2. Suggested Inductor Manufacturers
MODEL MANUFACTURER
DIMENSIONS
WEBSITE
DO3340P Coilcraft 12.95mm x 9.4mm x 11.43mm www.coilcraft.com
CDRH127 Sumida 12.3mm x 12.3mm x 8mm www.sumida.com
11RHBP Toko 11mm x 11mm x 13.75mm www.tokoam.com
SLF12575 TDK 12.5mm x 12.5mm x 7.5mm www.component.tdk.com
L
F
C
F
OUT_
R
SPKR
SINGLE-ENDED OUTPUT FILTER
NOTE: AN OUTPUT-COUPLING CAPACITOR (C
OUT
) IS NEEDED FOR SINGLE-SUPPLY,
SINGLE-ENDED OUTPUT CONFIGURATION.
Figure 8. Single-Ended LC Output Filter