MAX9742 Datasheet, PDF (22/36 Pages) Maxim Integrated Products – Single-/Dual-Supply, Stereo 16W, Class D Amplifier with Differential Inputs

Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
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:
R_ZBL = 1.2 × R_SPKR (Ω)
C_ZBL
=
1
2π × R_SPKR
× f_C
(F)
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.
Table 3. Recommended Differential LC
Filter Component Values for an 8Ω BTL
Speaker Load (f_C = 40kHz)
DC Resistance of Speaker (Ω)
8
L_F (µH)
22
C_F (µF)
0.68
OUT_
L_F
R_ZBL
C_F
C_ZBL
L_SPKR
R_SPKR
SPEAKER
LOAD
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
BRIDGE-TIED-LOAD (BTL) OUTPUT FILTER
L_F
OUTL
C_F
OUTR
C_F
L_F
R_SPKR
Figure 9. BTL LC Output Filter
L_F
OUTL
OUTR
C_F
C_F
L_F
R_ZBL
C_ZBL
C_ZBL
R_ZBL
L_SPKR
R_SPKR
SPEAKER
LOAD
NOTE: AN OUTPUT-COUPLING CAPACITOR (C_OUT) IS NEEDED FOR SINGLE-SUPPLY,
SINGLE-ENDED OUTPUT CONFIGURATION.
Figure 10. Zobel Network Connections for High-Inductance
Speakers
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