MAX13301 Datasheet, PDF (13/37 Pages) Maxim Integrated Products – 4-Channel, Automotive Class D Audio Amplifier Feedback After the Filter

4-Channel, Automotive Class D Audio Amplifier
amplifier ramp. The design equation that must be met to
ensure constant frequency is as follows:
Error-Amplifier Ramp < 2/3 Triangle-Wave Ramp
The error-amplifier ramp is fixed by the gain of the dif-
ferential amplifiers used in the feedback loop and by
the error-amplifier compensation capacitor programmed
through I²C. To ensure the design equation for fixed fre-
quency is met, the error-amplifier compensation capaci-
tor tracks the integrator capacitor used to generate the
triangle wave.
For optimal noise shaping, the error-amplifier capaci-
tor should be set to a small value. This results in a
broadband spectrum where the error amplifier pushes
the noise created by the clock jitter and PWM sam-
pling above the audio range. However, there is a limit.
Because the triangle-wave capacitor tracks the error-
amplifier capacitor, small capacitor values can clip
the triangle wave as it runs out of supply. This effect is
aggravated at high PVDD voltages by the ADC action
that decreases the integrator resistor at higher supply
voltages. Tables 20 and 21 are lookup tables to facili-
tate choosing the optimal setting for the error-amplifier
capacitance (MAP.COMP[2:0]).
It is possible to change this setting instantaneously while
playing music, but if there is no music, a slight audible
click is heard at the speakers. Systems that monitor
the input voltage can take advantage of this instanta-
neous programmability and use a smaller error-amplifier
capacitor at lower PVDD voltages. Higher switching
frequencies also allow the use of a smaller integrator
capacitor, and thus help improve the noise performance
of the amplifier.
Do not set the error-amplifier capacitor to a value less
than 18pF. Doing so results in extreme distortion, as the
triangle wave clips. The MAP.COMP[2:0] settings that
result in this behavior are listed as reserved (Table 19).
Advantage of Feedback After the Filter
High-fidelity audio amplifiers require very low output
impedance. The device achieves this by using a dual-
feedback approach. The digital feedback (feedback
from OUT__ outputs) emulates current-mode enabling on
chip compensation. The analog feedback (FB__ inputs)
significantly reduces the output impedance of the ampli-
fier and at the same time, compensates for the nonideal
characteristics of the output filter. If the characteristics
of the speaker and/or output filter change with age or
temperature, the analog feedback compensates accord-
ingly. Further inductor matching is less critical because
the inductors are inside the feedback loop. Because the
inductors are inside the feedback loop, the loop can
dampen out any LC ringing that might occur when the
amplifier is used as a line driver. The analog feedback is
differential so it does not help with common-mode ring-
ing. Thus, the Zobel (RC) networks are required on each
speaker connection to damp any common-mode ringing
associated with the LC output filter and speaker.
Operating Modes
Configure the device for one of three states of activity:
normal, standby, or shutdown.
Normal
In normal mode, the device is ready for play. Placing
the device in standby reduces power consumption while
keeping fault monitors and the I²C interface on to com-
municate fault conditions. In shutdown, the device is
completely disabled and draws minimal current from the
battery.
To reset the device and clear all register contents to their
reset values, set CTRL5.RST to 1. After reset, this bit is
automatically cleared back to 0.
Standby
In standby, all circuitry is disabled except the fault moni-
tors and the I²C interface. The I²C registers retain their
content and are still interactive. To place the device in
standby, set the CTRL2.STBY bit to 1. In standby, the
device draws 11mA from all power-supply inputs.
Before exiting standby, always set the CTRL1.CL_TH (cur-
rent-limit threshold setting) bit to 1. After exiting standby,
clear CTRL1.CL_TH back to 0.
Table 1. Operating Modes
MODE
Normal
Standby
Shutdown
X = Don’t care
EN
High
High
Low
CTRL2.STBY
0
1
X
I²C
Enabled
Enabled
Off
FAULT MONITORS
On
On
Off
ALL OTHER CIRCUITRY
On
Off
Off
13