max9744 Maxim Integrated Products, Inc., max9744 Datasheet - Page 15

• Manufacturer Part Number: max9744
• Description: 20w Stereo Class D Speaker Amplifier With Volume Control
• Manufacturer: Maxim Integrated Products, Inc.

The high efficiency of a Class D amplifier is due to the
switching operation of the output stage transistors. In a
Class D amplifier, the output transistors act as current-
steering switches and consume negligible additional
power. Any power loss associated with the Class D out-
put stage is mostly due to the I
on-resistance, and quiescent current overhead.
The theoretical best efficiency of a linear amplifier is
78% at peak output power. Under normal operating lev-
els (typical music reproduction levels), the efficiency
falls below 30%, whereas the MAX9744 exhibits > 80%
efficiency under the same conditions (Figure 2).
When the output current exceeds the current limit, 5.5A
(typ), the MAX9744 disables the outputs and initiates a
220µs startup sequence. The shutdown and startup
sequence is repeated until the output fault is removed.
Since the retry repetition is slow, the average supply
current is low. Most applications do not enter current-
limit mode unless the output is short circuited or incor-
rectly connected.
Table 2. Modulation Scheme Selection
Figure 2. MAX9744 Efficiency vs. Class AB Efficiency
ADDR2
0
0
100
90
80
70
60
50
40
30
20
10
0
0
ADDR1
______________________________________________________________________________________
2
0
0
vs. OUTPUT POWER
20W Stereo Class D Speaker Amplifier
OUTPUT POWER (W)
MAX9744
EFFICIENCY
CLASS AB
4
Analog volume control
Analog volume control
2
6
R loss of the MOSFET
SDA/VOL
PVDD = 12V
f
R
IN
L
Current Limit
8
= 1kHz
= 4Ω
Efficiency
10
SCLK/PWM
0
1
When the die temperature exceeds the thermal-shut-
down threshold, +165°C (typ), the MAX9744 outputs
are disabled. Normal operation resumes when the die
temperature decreases by a factor equal to the
thermal-shutdown threshold minus the thermal-shut-
down hysteresis, (typically below +150°C). The effect of
thermal shutdown is an output signal turning off for
approximately 3s in most applications, depending on
the thermal time constant of the audio system. Most
applications should never enter thermal shutdown.
Some of the possible causes of thermal shutdown are
too low of a load impedance, bad thermal contact
between the MAX9744’s exposed pad and PCB, high
ambient temperature, poor PCB layout and assembly,
or excessive output overdrive.
The MAX9744 features a shutdown mode that reduces
power consumption and extends battery life. Driving
SHDN low places the device in low-power shutdown
mode. Connect SHDN to digital high for normal opera-
tion. In shutdown mode, the outputs are high imped-
ance, SYNCOUT is pulled high, BIAS voltage decays to
zero, and the common-mode input voltage decays to
zero. The I
ing shutdown (MAX9744).
The MAX9744 features a clickless-and-popless mute
mode. When the device is muted, the outputs do not
stop switching; only the volume level is muted to the
speaker. Mute only affects the output stage and does
not shut down the device. To mute the MAX9744, drive
MUTE to logic-high. MUTE should be held high during
system power-up and power-down to ensure that pops
caused by circuits before the MAX9744 are eliminated.
To reduce clicks and pops, the device enters or exits
mute at zero crossing.
with Volume Control
Filterless modulation
Classic PWM (50% duty cycle)
2
C register does not retain its contents dur-
FUNCTION
Thermal Shutdown
Mute Function
Shutdown
15