Datasheet
MAX9742
To maximize the performance of the differential output
filter and minimize EMI radiation, keep the ground con-
nections of the C
F
capacitors close together on the
PCB and place the filter near the MAX9742.
The component ratings for C
F
and L
F
follow the same
requirements mentioned in the
Single-Ended LC Output
Filter Design (L
F
and C
F
)
section.
Zobel Network
For speaker loads that have appreciable amounts of
voice coil inductance (> 33µH), peaking in the frequen-
cy response of the output may occur near the cutoff fre-
quency of the LC filter, which may cause the device to
go into current limit at high output powers. This peaking
is due to the resonant circuit formed by the LC output
filter and complex impedance of the speaker. To nullify
the peaking in the frequency response, connect a
Zobel network (series RC circuit) in parallel with the
speaker load as shown in Figure 10. The Zobel circuit
reduces the peaking by dampening the reactive behav-
ior of the speaker. For the single-ended output configu-
ration, use the following equations to calculate the
component values for the Zobel network:
where R
ZBL
is the value of the Zobel resistor, C
ZBL
is
the value of the Zobel capacitor, R
SPKR
is the DC resis-
tance of the speaker, and f
C
is the cutoff frequency of
the LC filter. For the BTL configuration, use half of the
BTL resistance for the Zobel network calculations.
Connect a ground-terminated Zobel network on each
side of the BTL resistance to prevent excessive peak-
ing in the common-mode response of the output filter.
For most applications, R
ZBL
should have a minimum
power rating of 1/4W or greater. C
ZBL
should have a
voltage rating greater than or equal to V
DD
.
R 1.2 R
C
1
2R f
ZBL
SPKR
ZBL
SPKR C
=×
=
××
()
()
Ω
π
F
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
22 ______________________________________________________________________________________
Table 3. Recommended Differential LC
Filter Component Values for an 8Ω BTL
Speaker Load (f
C
= 40kHz)
DC Resistance of Speaker (Ω)L
F
(µH) C
F
(µF)
8 22 0.68
L
F
C
F
C
F
OUTL
L
F
OUTR
R
SPKR
BRIDGE-TIED-LOAD (BTL) OUTPUT FILTER
Figure 9. BTL LC Output Filter
L
F
C
F
C
F
OUTL
L
F
OUTR
R
SPKR
NOTE: AN OUTPUT-COUPLING CAPACITOR (C
OUT
) IS NEEDED FOR SINGLE-SUPPLY,
SINGLE-ENDED OUTPUT CONFIGURATION.
SPEAKER
LOAD
L
SPKR
R
ZBL
R
ZBL
C
ZBL
C
ZBL
L
F
C
F
OUT_
R
SPKR
SPEAKER
LOAD
L
SPKR
R
ZBL
C
ZBL
Figure 10. Zobel Network Connections for High-Inductance
Speakers