MAX13301 Datasheet, PDF (33/37 Pages) Maxim Integrated Products – 4-Channel, Automotive Class D Audio Amplifier Feedback After the Filter

4-Channel, Automotive Class D Audio Amplifier
External Filters
The Class D amplifiers work with external filters. The filter
requirement is due to the unshielded wiring harness. This
is common for longer speaker lead lengths, and to gain
increased margin to EMC limits. See Figure 8 for the cor-
rect connections of these components.
The component selection is based on the load imped-
ance of the speaker. Table 32 lists suggested values for
a variety of load impedances. Inductors L1 and L2 and
capacitor C2 form the primary output filter. In addition to
these primary filter components, other components in the
filter improve its functionality. RC networks R1-C4 and
R2-C5 form Zobels at the output. A Zobel corrects the
output loading to compensate for the rising impedance
of the loudspeaker. Without a Zobel, the filter has a peak
in its response near the cutoff frequency. Capacitors
C1 and C3 provide additional high-frequency bypass to
reduce radiated emissions.
Power Supplies
The devices use different supplies for each portion of
the device, allowing for the optimum combination of
headroom, power dissipation, and noise immunity. The
speaker amplifiers are powered from PVDD and can
range from 6V to 25.5V. The remainder of the device
is powered by VDD (analog and digital blocks) and
VDD5 (gate drivers and charge-pump circuit). Power
supplies are independent of each other so sequencing
is not necessary. Power can be supplied by separate
sources or derived from a single higher source using
a linear regulator to reduce the voltage, as shown in
Figure 9.
Component Selection
Input Filter
The device has four positive inputs and one common
negative input. The input resistance of each positive
input is 20kI, while the input resistance of the negative
input is 5kI. An input capacitor, CIN, in conjunction
with the input resistance forms a highpass filter that
removes the DC bias from an incoming signal. The
DC-blocking capacitor allows the amplifier to automati-
cally bias the signal to an optimum DC level. Assuming
zero-source impedance, the -3dB point of the highpass
filter is given by:
f0 = 1/2GRINCIN
C1
MAX13301
OUT_+
L1
OUT_-
L2
C2
C3
C4
R1
RL
R2
C5
24V
SHDN IN
OUT
5V
1µF
MAX16910
2I
GND
V_DD5
0.1µF
VDD
22µF
PVDD
MAX13301
GND
PGND
4.7µF
Figure 8. Output Filter
Figure 9. Using a Linear Regulator to Produce 5V from a 24V
Power Supply
Table 32. Suggested Values for LC Filters
RL (I)
8
4
2*
1*
L1, L2 (µH)
10
10
10
10
C1, C3 (µF)
0.15
0.15
0.15
0.15
C2 (µF)
0.47
0.47
0.68
1
*Parallel channel operation: at 14.4V (for RL = 1I), at 24V (for RL = 2I).
C4, C5 (nF)
220
220
220
220
R1, R2 (I)
10
10
10
10
33