19-3887; Rev 0; 2/06
KIT
ATION
EVALU
E
L
B
AVAILA
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
Features
The MAX9741 stereo Class D audio power amplifier
provides Class AB amplifier performance with Class D
efficiency, conserving board space and eliminating the
need for a bulky heatsink. Using a high-efficiency Class
D architecture, it delivers 12W continuous output power
into 8Ω loads. Proprietary modulation and switching
schemes render the traditional Class D EMI suppression
output filter unnecessary.
The MAX9741 offers two modulation schemes: a fixed-frequency mode (FFM), and a spread-spectrum mode (SSM)
that reduces EMI-radiated emissions. The device utilizes a
fully differential architecture, a full bridged output, and
offers comprehensive click-and-pop suppression.
♦ Low-EMI Class D Amplifier
The MAX9741 features high 80dB PSRR, low 0.1%
THD+N, and SNR in excess of 100dB. Short-circuit and
thermal-overload protection prevent the device from
being damaged during a fault condition. The MAX9741
is available in a 56-pin TQFN (8mm x 8mm x 0.8mm)
package. The MAX9741 is specified over the extended
-40°C to +85°C temperature range.
♦ Industry-Leading Click-and-Pop Suppression
♦ Spread-Spectrum Mode Reduces EMI
♦ Passes FCC EMI Limits with Ferrite Bead Filters
with 0.5m Cables
♦ 12W+12W Continuous Output Power into 8Ω
♦ Low 0.1% THD+N
♦ High PSRR (80dB at 1kHz)
♦ 10V to 25V Single-Supply Operation
♦ Differential Inputs Minimize Common-Mode Noise
♦ Pin-Selectable Gain Reduces Component Count
♦ Short-Circuit and Thermal-Overload Protection
♦ Available in Thermally Efficient, Space-Saving
56-Pin TQFN (8mm x 8mm x 0.8mm) Package
Applications
Ordering Information
LCD/PDP TVs
CRT TVs
PC Speakers
PART
TEMP RANGE
PIN-PACKAGE
MAX9741ETN+
-40°C to +85°C
56 TQFN-EP*
PKG
CODE
T5688-3
+Denotes lead-free package.
*EP = Exposed paddle.
Simplified Block Diagram
INR+
DIFFERENTIAL AUDIO
INPUTS ELIMINATE
NOISE PICKUP
INRGAIN
CONTROL
INL+
INL-
G1
CLASS D
MODULATOR
OUTPUT
PROTECTION
CLASS D
AMPLIFIERS
DRIVE 2 X 12W
INTO 8Ω LOADS
G2
PROGRAMMABLE
SWITCHING
FREQUENCY
FS1, FS2
2
MAX9741
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX9741
General Description
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
ABSOLUTE MAXIMUM RATINGS
(All voltages referenced to GND.)
VDD to PGND, AGND .............................................................30V
OUTR_, OUTL_, C1N..................................-0.3V to (VDD + 0.3V)
C1P............................................(VDD - 0.3V) to (CHOLD + 0.3V)
CHOLD ........................................................(VDD - 0.3V) to +40V
SHDN, FS_, G_ ...........................................................-6.3V to 8V
All Other Pins to GND.............................................-0.3V to +12V
Duration of OUTR_/OUTL_
Short Circuit to GND, VDD ......................................Continuous
Continuous Input Current (VDD, PGND) ..................................2A
Continuous Input Current (all other pins)..........................±20mA
Thermal Limits (Note 1)
Continuous Power Dissipation (TA = +70°C)
Single-Layer PC Board
56-Pin TQFN (derate 28.6mW/°C above +70°C) ............2.29W
θJA ................................................................................ 35°C/W
θJC ............................................................................... 0.6°C/W
Continuous Power Dissipation (TA = +70°C)
Multiple-Layer PC Board
56-Pin TQFN (derate 47.6mW/°C above +70°C) ............3.81W
θJA ................................................................................ 21°C/W
θJC ............................................................................... 0.6°C/W
Junction Temperature ......................................................+150°C
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Note 1: Thermal performance of this device is highly dependant on PC board layout. See the Applications Information for more
detail.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDD = 18V, GND = PGND = 0V, SHDN ≥ VIH, AV = 16dB, CSS = CIN = 0.47µF, CREG = 0.01µF, C1 = 100nF, C2 = 1µF, FS1 = FS2 =
GND (fS = 670kHz), RL connected between OUTL+ and OUTL- and OUTR+ and OUTR-, TA = TMIN to TMAX, unless otherwise noted.
Typical values are at TA = +25°C.) (Notes 1, 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
25
V
26
37
mA
0.2
1.5
µA
GENERAL
Supply Voltage Range
VDD
Inferred from PSRR test
Quiescent Current
IDD
RL = Open
Shutdown Current
ISHDN
Turn-On Time
tON
Amplifier Output Resistance in
Shutdown
Input Impedance
Voltage Gain
RIN
AV
Gain Matching
Output Offset Voltage
CSS = 470nF
100
CSS = 180nF
50
150
320
AV = 13dB
35
53
80
AV = 16dB
30
45
65
AV = 19.1dB
23
36
55
AV = 29.6dB
10
14.3
22
G1 = L, G2 = L
29.4
29.6
29.8
G1 = L, G2 = H
18.9
19.1
19.3
G1 = H, G2 = L
12.8
13
13.2
G1 = H, G2 = H
15.9
16
16.3
Between channels
CMRR
Power-Supply Rejection Ratio
(Note 3)
PSRR
ms
SHDN = GND
±5
fIN = 1kHz, input referred
VDD = 10V to 25V
200mVP-P ripple
kΩ
0.5
VOS
Common-Mode Rejection Ratio
2
10
60
48
kΩ
dB
%
±30
mV
dB
83
fRIPPLE = 1kHz
80
fRIPPLE = 20kHz
60
_______________________________________________________________________________________
dB
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
(VDD = 18V, GND = PGND = 0V, SHDN ≥ VIH, AV = 16dB, CSS = CIN = 0.47µF, CREG = 0.01µF, C1 = 100nF, C2 = 1µF, FS1 = FS2 =
GND (fS = 670kHz), RL connected between OUTL+ and OUTL- and OUTR+ and OUTR-, TA = TMIN to TMAX, unless otherwise noted.
Typical values are at TA = +25°C.) (Notes 1, 2)
PARAMETER
SYMBOL
CONDITIONS
Total Harmonic Distortion Plus
Noise
PCONT
THD+N
Signal-to-Noise Ratio
SNR
Crosstalk
RL = 4Ω
6.5
VDD = 24V, THD+N =
10%, f = 1kHz
RL = 8Ω
11
RL = 4Ω
5
VDD = 12V, THD+N =
10%, f = 1kHz
RL = 8Ω
8
RL = 4Ω
8.5
fIN = 1kHz, either FFM or SSM, RL = 8Ω,
POUT = 4W
RL = 8Ω,
POUT = 4W,
f = 1kHz
BW = 22Hz to 22kHz
Unweighted
A-weighted
FFM
95.8
SSM
91.8
FFM
99.1
SSM
95.7
FS1 = L, FS2 = L
fOSC
η
Efficiency (Note 4)
Regulator Output
MAX
%
dB
65
560
670
FS1 = L, FS2 = H
930
FS1 = H, FS2 = L
470
FS1 = H, FS2 = H (spread-spectrum mode)
670
±7%
VDD = 12V, RL = 8Ω, POUT = 8W
78
VDD = 18V, RL = 8Ω, POUT = 10W
78
UNITS
W
0.1
Left to right, right to left, 8Ω load, fIN = 10kHz
Oscillator Frequency
TYP
12
VDD = 18V, THD+N =
10%, f = 1kHz
Continuous Output Power
(Notes 4, 5)
MIN
RL = 8Ω
dB
800
kHz
%
VREG
6
V
DIGITAL INPUTS (SHDN, FS_, G_)
VIH
Input Thresholds
VIL
Input Leakage Current
2.5
0.8
±1
V
µA
Note 2: All devices are 100% production tested at +25°C. All temperature limits are guaranteed by design.
Note 3: PSRR is specified with the amplifier inputs connected to GND through CIN.
Note 4: Testing performed with a resistive load in series with an inductor to simulate an actual speaker load. For RL = 8Ω, L = 68µH.
For RL = 12Ω, L = 100µH. For RL = 16Ω, L = 120µH.
Note 5: Output power measured at TA = +25°C, with a soak time of 15 minutes.
_______________________________________________________________________________________
3
MAX9741
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VDD = 18V, RL = 8Ω, fIN = 1kHz, 33µH with 4Ω, 68µH with 8Ω, part in SSM mode, 136µH with 16Ω, measurement BW = 22Hz to
22kHz, unless otherwise noted.)
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
1
1
1
THD+N (%)
VDD = 24V
THD+N (%)
VDD = 12V
VDD = 24V
VDD = 18V
0.1
0.1
0.01
0.01
MAX9741 toc03
VDD = 18V
RL = 4Ω
THD+N (%)
VDD = 12V
10
MAX9741 toc02
RL = 8Ω
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
10
MAX9741 toc01
10
POUT = 8W
0.1
POUT = 500mW
15
20
0
5
15
10
OUTPUT POWER (W)
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
THD+N (%)
THD+N (%)
SSM
10k
100
100k
f = 100Hz
f = 10kHz
RL = 8Ω
VDD = 12V
90
80
0.1
0.1
1k
EFFICIENCY vs. OUTPUT POWER
10
1
1
100
FREQUENCY (Hz)
MAX9741 toc05
POUT = 8W
10
MAX9741 toc06
10
OUTPUT POWER (W)
MAX974 toc04
10
5
0.01
EFFICIENCY (%)
0
70
60
VDD = 18V
50
VDD = 24V
40
30
20
f = 1kHz
FFM
10
10
100
1k
10k
0
100k
5
10
15
0
20
4
8
12
FREQUENCY (Hz)
OUTPUT POWER (W)
OUTPUT POWER (W)
OUTPUT POWER vs. SUPPLY VOLTAGE
OUTPUT POWER vs. LOAD RESISTANCE
COMMON-MODE REJECTION RATIO
vs. FREQUENCY
14
OUTPUT POWER (W)
RL = 8Ω
12
RL = 16Ω
10
8
6
-20
12
10
8
2
2
THD+N = 1%
16
19
SUPPLY VOLTAGE (V)
22
25
-40
-60
-70
-80
0
13
-30
-50
6
4
10
-10
14
4
0
THD+N = 10%
16
0
16
MAX9741 toc09
16
18
CMRR (dB)
18
MAX9741 toc08
20
MAX9741 toc07
20
4
0
0.01
0.01
OUTPUT POWER (W)
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
1
10
LOAD RESISTANCE (Ω)
100
10
100
1k
FREQUENCY (Hz)
_______________________________________________________________________________________
10k
100k
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
-80
LEFT TO RIGHT
-60
-80
RIGHT TO LEFT
-100
-100
-120
100
1k
10k
100k
-60
-80
-100
-120
10
100
1k
10k
0
100k
2
4
6
8
10 12 14 16 18 20
FREQUENCY (Hz)
FREQUENCY (Hz)
FREQUENCY (kHz)
OUTPUT FREQUENCY SPECTRUM
OUTPUT FREQUENCY SPECTRUM
WIDEBAND OUTPUT SPECTRUM
(FFM MODE)
-40
-60
-80
-100
-120
0
-20
-40
-60
-80
-100
-140
2
4
6
8
10 12 14 16 18 20
FREQUENCY (kHz)
RBW = 10kHz
-20
-40
-60
-80
-100
-120
-140
0
OUTPUT AMPLITUDE (dBV)
-20
SSM MODE
A-WEIGHTED
fIN = 1kHz
POUT = 5W
MAX9741 toc14
0
20
OUTPUT MAGNITUDE (dB)
SSM MODE
UNWEIGHTED
fIN = 1kHz
POUT = 5W
MAX941 toc13
20
0
-40
-140
-120
10
-20
MAX9741 toc15
-60
-40
FFM MODE
UNWEIGHTED
fIN = 1kHz
POUT = 5W
0
OUTPUT MAGNITUDE (dB)
-20
CROSSTALK (dB)
PSRR (dB)
-40
20
MAX9741 toc11
-20
0
MAX9741 toc10
200mVP-P INPUT
OUTPUT MAGNITUDE (dB)
OUTPUT FREQUENCY SPECTRUM
CROSSTALK vs. FREQUENCY
0
MAX9741 toc12
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
-120
0
2
4
6 8 10 12 14 16 18 20
FREQUENCY (kHz)
100k
1M
10M
100M
FREQUENCY (Hz)
_______________________________________________________________________________________
5
MAX9741
Typical Operating Characteristics (continued)
(VDD = 18V, RL = 8Ω, fIN = 1kHz, 33µH with 4Ω, 68µH with 8Ω, part in SSM mode, 136µH with 16Ω, measurement BW = 22Hz to
22kHz, unless otherwise noted.)
Typical Operating Characteristics (continued)
(VDD = 18V, RL = 8Ω, fIN = 1kHz, 33µH with 4Ω, 68µH with 8Ω, part in SSM mode, 136µH with 16Ω, measurement BW = 22Hz to
22kHz, unless otherwise noted.)
TURN-ON/TURN-OFF RESPONSE
RBW = 10kHz
-20
MAX9741 toc16
MAX9741 toc17
0
10
VDD = 12V
SHDN
5V/div
-40
RL = 8Ω
CSS = 180pF
VDD = 18V
VDD = 24V
1
THD+N (%)
OUTPUT AMPLITUDE (dBV)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
WITH FERRITE BEAD FILTER
MAX9741 toc18
WIDEBAND OUTPUT SPECTRUM
(SSM MODE)
-60
0.1
-80
OUTPUT
1V/div
-100
f = 1kHz
0.01
-120
100k
1M
10M
0
20ms/div
100M
5
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
WITH FERRITE BEAD FILTER
VDD = 18V
VDD = 24V
25
20
15
10
5
10
OUTPUT POWER (W)
6
15
MAX9741 toc21
0.30
0.25
0.20
0.15
0.10
0
0
0
20
0.05
5
0.01
0.35
SUPPLY CURRENT (µA)
30
SUPPLY CURRENT (mA)
VDD = 12V
1
0.1
35
MAX9741 toc20
RL = 4Ω
15
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX9741 toc19
10
10
OUTPUT POWER (W)
FREQUENCY (Hz)
THD+N (%)
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
10
13
16
19
SUPPLY VOLTAGE (V)
22
25
10
12
14
16
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
18
20
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
PIN
NAME
FUNCTION
1, 4, 7, 11–15, 19, 21,
23, 25, 28, 33–36, 39,
42, 43, 44, 49, 50, 55, 56
N.C.
2, 3, 40, 41
PGND
5, 6, 37, 38
VDD
Power-Supply Input
8
C1N
Charge-Pump Flying Capacitor Negative Terminal
9
C1P
Charge-Pump Flying Capacitor Positive Terminal
10
CHOLD
16
INL-
Left-Channel Negative Input
17
INL+
Left-Channel Positive Input
18
SHDN
Active-Low Shutdown. Connect SHDN to GND to disable the device. Connect to VDD for
normal operation.
20
SS
22
AGND
24
REG
Internal Regulator Output. Bypass with a 0.01µF capacitor to PGND.
26
INR-
Right-Channel Negative Input
27
INR+
Right-Channel Positive Input
29
G1
Gain-Select Input 1
30
G2
Gain-Select Input 2
31
FS1
Frequency-Select Input 1
32
FS2
Frequency-Select Input 2
45, 46
OUTR-
Right-Channel Negative Audio Output
47, 48
OUTR+
Right-Channel Positive Audio Output
51, 52
OUTL-
Left-Channel Negative Audio Output
53, 54
OUTL+
Left-Channel Positive Audio Output
—
EP
Exposed Paddle. Connect to GND.
No Connection. Not internally connected.
Power Ground
Charge-Pump Hold Capacitor. Connect a 1µF capacitor from CHOLD to VDD.
Soft-Start. Connect a 0.47µF capacitor from SS to GND to enable soft-start feature.
Analog Ground
Detailed Description
The MAX9741 low-EMI, Class D audio power amplifier
features several improvements to switch-mode amplifier technology. This device offers Class AB performance with Class D efficiency, while occupying
minimal board space. A unique modulation scheme
and spread-spectrum switching mode create a compact, flexible, low-noise, efficient audio power amplifier.
The differential input architecture reduces commonmode noise pickup, and can be used without inputcoupling capacitors. The device can also be
configured as a single-ended input amplifier.
_______________________________________________________________________________________
7
MAX9741
Pin Description
Operating Modes
Fixed-Frequency Modulation (FFM) Mode
The MAX9741 features three FFM modes with different
switching frequencies (Table 1). In FFM mode, the frequency spectrum of the Class D output consists of the
fundamental switching frequency and its associated
harmonics (see the Wideband Output Spectrum graph
in the Typical Operating Characteristics). The MAX9741
allows the switching frequency to be changed by
±35%, should the frequency of one or more of the harmonics fall in a sensitive band. This can be done at any
time and does not affect audio reproduction.
Table 1. Operating Modes
FS1
FS2
L
L
SWITCHING MODE
(kHz)
670
L
H
930
H
L
470
H
H
670 ±7%
Spread-Spectrum Modulation (SSM) Mode
A unique, proprietary spread-spectrum mode flattens
the wideband spectral components, improving EMI
emissions that may be radiated by the speaker and
cables. This mode is enabled by setting FS1 = FS2 =
H. In SSM mode, the switching frequency varies randomly by ±7% around the center frequency (670kHz).
The modulation scheme remains the same, but the
period of the triangle waveform changes from cycle to
cycle. Instead of a large amount of spectral energy present at multiples of the switching frequency, the energy
is now spread over a bandwidth that increases with frequency. Above a few megahertz, the wideband spectrum looks like white noise for EMI purposes.
Efficiency
Efficiency of a Class D amplifier is attributed to the region
of operation of the output stage transistors. In a Class D
amplifier, the output transistors act as current-steering
switches and consume negligible additional power.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9741 still exhibits > 78% efficiency
under the same conditions (Figure 1).
device in low-power (0.2µA) shutdown mode. Connect
SHDN to a logic-high for normal operation.
Click-and-Pop Suppression
Comprehensive click-and-pop suppression eliminates
audible transients on startup and shutdown. While in
shutdown, the H-bridge is pulled to GND through 320kΩ.
During startup, or power-up, the input amplifiers are
muted and an internal loop sets the modulator bias voltages to the correct levels, preventing clicks and pops
when the H-bridge is subsequently enabled. Following
startup, a soft-start function gradually unmutes the input
amplifiers. The value of the soft-start capacitor has an
impact on the click/pop levels. For optimum performance,
CSS should be 470nF with a voltage rating of at least 7V.
Mute Function
The MAX9741 features a clickless/popless mute mode.
When the device is muted, the outputs stop switching,
muting the speaker. Mute only affects the output stage
and does not shut down the device. To mute the
MAX9741, drive SS to GND by using a MOSFET pulldown (Figure 2). Driving SS to GND during the powerup/down or shutdown/turn-on cycle optimizes
click-and-pop suppression.
EFFICIENCY vs. OUTPUT POWER
100
90
MAX9741
80
70
EFFICIENCY (%)
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
60
50
CLASS AB
40
30
20
VDD = 15V
f = 1kHz
RL = 8Ω
10
0
0
2
4
6
8
10 12 14 16 18 20
OUTPUT POWER (W)
Figure 1. MAX9741 Efficiency vs. Class AB Efficiency
SS
GPIO
MUTE SIGNAL
0.47µF
MAX9741
Shutdown
A shutdown mode reduces power consumption and
extends battery life. Driving SHDN low places the
8
Figure 2. MAX9741 Mute Circuit
_______________________________________________________________________________________
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
to pure PWM Class D amplifiers. The outputs will contain
both differential and common-mode noise at the switching frequency and its harmonics. In many applications,
a simple ferrite bead filter (see the Simplified Block
Diagram) will allow the amplifier to pass FCC EMI limits.
Ferrite beads offer significant cost and size reductions
when compared to conventional inductors. The ferrite
bead type and capacitor value can be adjusted to tune
the rejection to match the speaker cable length.
Actual EMI test results for the MAX9741 are shown in
Figure 3. This shows the MAX9741, tested in a 10m anechoic EMC chamber. The MAX9741 test conditions
were: SSM mode, 0.5m cables on each side, 16dB gain,
18V supply voltage, both channels playing pink noise at
4W per channel into 8Ω shielded speakers.
The graph of Figure 3 indicates peak readings. Actual
quasi peak readings per EN55022B specification will
be lower due to Maxim’s proprietary SSM mode. Table
2 lists select values, indicating the peak reading, the
quasi-peak reading, and the actual margin to
EN55022B specification.
Applications Information
Class D Amplifier Outputs
Class D amplifiers differ from analog amplifiers such as
Class AB in that their output waveform is composed of
high-frequency pulses from ground to the supply rail.
When viewed with an oscilloscope the audio signal will
not be seen; instead, the high-frequency pulses dominate. To evaluate the output of a Class D amplifier
requires taking the difference from the positive and
negative outputs, then lowpass filtering the difference
to recover the amplified audio signal.
Ferrite Bead Output Filters
The MAX9741’s low-EMI output switching method
reduces the output filtering requirements when compared
40
AMPLITUDE (dBuV/m)
35
30
25
20
15
10
30
100
200
300
400
500
600
700
800
900
1000
FREQUENCY (MHz)
Figure 3. EMI Measurement of MAX9741 in 10m Anechoic Chamber
Table 2. Peak and Quasi-Peak EMI Readings
FREQUENCY
(MHz)
PRELIMINARY PEAK
READING (dBµV/m)
QUASI PEAK READING
(dBµV/m)
EN55022B LIMIT
(dBµV/m)
ACTUAL MARGIN
(dBµV/m)
75.38
28.1
18.3
30.0
11.7
78.57
28.0
21.9
30.0
-8.1
83.18
26.6
20.6
30.0
-9.4
_______________________________________________________________________________________
9
MAX9741
Internal Regulator
The MAX9741 has an internal linear regulator, REG,
used to power the internal analog circuitry. The voltage
at REG is nominally 6V. Bypass REG to AGND with a
10nF capacitor, rated for at least 10V. REG is turned off
in shutdown.
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
Ferrite beads are available from many manufacturers.
Table 3 lists some manufacturers who make ferrite
beads and other products suitable for use with Class D
amplifiers.
Although they offer a low cost and small size, ferrite
bead filters slightly increase distortion and slightly
reduce efficiency. If the audio performance of the ferrite
bead filters does not meet the system requirements, then
a full inductor/capacitor (LC) filter should be considered.
converting these into power in the audible frequency
range. Filterless operation requires the Class D amplifier to be very close to the speaker. Distances greater
than a few centimeters must be evaluated for EMC
compliance.
Gain Selection
Table 4 shows the suggested gain settings to attain a
maximum output power from a given peak input voltage
and given load.
Inductor/Capacitor Output Filters
Using a full inductor and capacitor (LC) output filter
provides significant attenuation of the fundamental
switching energy.
Select inductors rated for the expected RMS current
load. For example, if using a Class D amplifier up to
10W into 8Ω, the inductor should be rated for 1.25A
RMS or more. Furthermore, the inductor should maintain
a constant inductance value across the expected current range. Inductors which change in value as a function of current will cause harmonic distortion.
The output capacitors can also affect audio performance. Ceramic capacitors are often selected for their
size and cost advantage, but they cause distortion. If
the application constraints dictate ceramic capacitors,
selecting higher voltage rating and larger package size
mitigates some of the shortcomings. Best performance
is obtained with plastic film capacitors, but these are
larger and more expensive.
Filterless Operation
In some cases, a Class D amplifier can be used without
an output filter. The intrinsic inductance of the loudspeaker stores energy from the high-speed PWM pulses,
Table 3. Filter Component Suppliers
SUPPLIER
Murata
PRODUCT
Ferrite beads,
capacitors
WEBSITE
www.murata.com
Taiyo Yuden
Ferrite beads,
capacitors
www.t-yuden.com
TDK
Ferrite beads,
capacitors
www.tdk.co.jp/tetop01
Fairrite
Ferrite beads
www.fair-rite.com
Coilcraft
Inductors
www.coilcraft.com
Sumida
Inductors
www.sumida.com
Panasonic
Inductors
www.panasonic.com/indu
strial/components
10
Output Offset
Unlike a Class AB amplifier, the output offset voltage of
Class D amplifiers does not noticeably increase quiescent current draw when a load is applied. This is due to
the power conversion of the Class D amplifier. For
example, an 8mVDC offset across an 8Ω load results in
1mA extra current consumption in a Class AB device.
In the Class D case, an 8mV offset into 8Ω equates
to an additional power drain of 8µW. Due to the high
efficiency of the Class D amplifier, this represents an
additional quiescent current draw of: 8µW / (VDD / 100 ✕
η), which is in the order of a few microamps.
Input Amplifier
Differential Input
The MAX9741 features a differential input structure, making them compatible with many CODECs, and offering
improved noise immunity over a single-ended input amplifier. In devices such as PCs, noisy digital signals can be
picked up by the amplifier’s input traces. The signals
appear at the amplifiers’ inputs as common-mode noise. A
differential input amplifier amplifies the difference of the
two inputs, any signal common to both inputs is canceled.
Table 4. Gain Settings
G1
G2
GAIN (dB)
0
0
29.6
0
1
19.1
1
0
13
1
1
16
______________________________________________________________________________________
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
0.47µF
SINGLE-ENDED
AUDIO INPUT
IN+
Component Selection
Input Filter
An input capacitor, CIN, in conjunction with the input
impedance of the MAX9741, forms a highpass filter that
removes the DC bias from an incoming signal. The ACcoupling capacitor allows the amplifier to bias the signal to an optimum DC level. Assuming zero-source
impedance, the -3dB point of the highpass filter is
given by:
f -3dB =
1
2π RIN CIN
Choose CIN so f-3dB is well below the lowest frequency
of interest. Setting f-3dB too high affects the low-frequency response of the amplifier. Use capacitors with
dielectrics that have low-voltage coefficients, such as
tantalum or aluminum electrolytic. Capacitors with highvoltage coefficients, such as ceramics, may result in
increased distortion at low frequencies.
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 100mΩ for optimum performance. Low-ESR ceramic capacitors minimize the output resistance of the charge pump. For
best performance over the extended temperature
range, select capacitors with an X7R dielectric.
Flying Capacitor (C1)
The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. A
C1 value that is too small degrades the device’s ability
to provide sufficient current drive. Increasing the value
of C1 improves load regulation and reduces the
charge-pump output resistance to an extent. Above
1µF, the on-resistance of the switches and the ESR of
C1 and C2 dominate.
Hold Capacitor (C2)
The output capacitor value and ESR directly affect the ripple at CHOLD. Increasing C2 reduces output ripple.
Likewise, decreasing the ESR of C2 reduces both ripple
and output resistance. Lower capacitance values can be
used in systems with low maximum output power levels.
Sharing Input Sources
In certain systems, a single audio source can be shared
by multiple devices (speaker and headphone amplifiers).
MAX9741
Single-Ended Input
The MAX9741 can be configured as single-ended input
amplifiers by capacitively coupling either input to GND
and driving the other input (Figure 4).
MAX9741
IN0.47µF
Figure 4. Single-Ended Input
When sharing inputs, it is common to mute the unused
device, rather than completely shutting it down, preventing the unused device inputs from distorting the input
signal. Mute the MAX9741 by driving SS low through an
open-drain output or MOSFET. Driving SS low turns off
the Class D output stage, but does not affect the input
bias levels of the MAX9741. Be aware that during normal
operation, the voltage at SS can be up to 7V, depending
on the MAX9741 supply.
Supply Bypassing/Layout
Proper power-supply bypassing ensures low-distortion
operation. For optimum performance, bypass VDD to
PGND with a 0.1µF or greater capacitor as close to each
V DD pin as possible. In some applications, a 0.1µF
capacitor in parallel with a larger value, low-ESR ceramic
or aluminum electrolytic capacitor provides good results.
A low-impedance, high-current power-supply connection
to VDD is assumed. Additional bulk capacitance should
be added as required depending on the application and
power-supply characteristics. AGND and PGND should
be star connected to system ground. Refer to the
MAX9741 Evaluation Kit for layout guidance.
Class D Amplifier Thermal Considerations
Class D amplifiers provide much better efficiency and
thermal performance than a comparable Class AB
amplifier. However, the system’s thermal performance
must be considered with realistic expectations and
consideration of many parameters. This application
note examines Class D amplifiers using general examples to illustrate good design practices.
Continuous Sine Wave vs. Music
When a Class D amplifier is evaluated in the lab, often
a continuous sine wave is used as the signal source.
While this is convenient for measurement purposes, it
represents a worst-case scenario for thermal loading
on the amplifier. It is not uncommon for a Class D
amplifier to enter thermal shutdown if driven near maximum output power with a continuous sine wave.
______________________________________________________________________________________
11
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
Audio content, both music and voice, has a much lower
RMS value relative to its peak output power. Figure 5
shows a sine wave and an audio signal in the time
domain. Both are measured for RMS value by the oscilloscope. Although the audio signal has a slightly higher
peak value than the sine wave, its RMS value is almost
half that of the sine wave. Therefore, while an audio signal may reach similar peaks as a continuous sine wave,
the actual thermal impact on the Class D amplifier is
highly reduced. If the thermal performance of a system
is being evaluated, it is important to use actual audio
signals instead of sine waves for testing. If sine waves
must be used, the thermal performance will be less
than the system’s actual capability.
PC Board Thermal Considerations
The exposed pad is the primary route of heat away
from the IC. With a bottom-side exposed pad, the PC
board and its copper becomes the primary heatsink for
the Class D amplifier. Solder the exposed pad to a
large copper polygon. Add as much copper as possible from this polygon to any adjacent pin on the Class
D amplifier as well as to any adjacent components, provided these connections are at the same potential.
These copper paths must be as wide as possible. Each
of these paths contributes to the overall thermal capabilities of the system.
The copper polygon to which the exposed pad is
attached should have multiple vias to the opposite side
of the PC board, where they connect to another copper
polygon. Make this polygon as large as possible within
the system’s constraints for signal routing.
Additional improvements are possible if all the traces
from the device are made as wide as possible.
Although the IC pins are not the primary thermal path
out of the package, they do provide a small amount.
The total improvement would not exceed approximately
10%, but it could make the difference between acceptable performance and thermal problems.
With a bottomside exposed pad, the lowest resistance
thermal path is on the bottom of the PC board. The topside
of the IC is not a significant thermal path for the device.
Thermal Calculations
The die temperature of a Class D amplifier can be estimated with some basic calculations. For example, the
die temperature is calculated for the below conditions:
• TA = +40°C
• POUT = 10W (5W + 5W)
• Efficiency (η) = 78%
• θJA = 21°C/W
12
20ms/div
Figure 5. RMS Comparison of Sine Wave vs. Audio Signal
First, the Class D amplifier’s power dissipation must be
calculated.
10W
P
PDISS = OUT − POUT =
− 10W = 2.82W
η
78%
Then the power dissipation is used to calculate the die
temperature, TC, as follows:
TC = TA + PDISS × θ JA = 40°C + 2.82W × 21°C/ W = 99.2°C
Load Impedance
The on-resistance of the MOSFET output stage in Class
D amplifiers affects both the efficiency and the peakcurrent capability. Reducing the peak current into the
load reduces the I2R losses in the MOSFETs, increasing efficiency. To keep the peak currents lower, choose
the highest impedance speaker which can still deliver
the desired output power within the voltage swing limits
of the Class D amplifier and its supply voltage.
Optimize MAX9741 Efficiency with
Load Impedance and Supply Voltage
To optimize efficiency, load the output stage with 12Ω
to 16Ω speakers. The MAX9741 exhibits highest efficiency performance when driving higher load impedance (see the Typical Operating Characteristics). If a
12Ω to 16Ω load is not available, select a lower supply
voltage when driving 4Ω to 10Ω loads.
For best performance, choose a speaker impedance to
complement the required output power and the available
supply voltage. For example, if operating from a 24V supply and a peak output of 10W per channel is desired, using
12Ω speakers provides the best audio performance and
power efficiency. The amplifier outputs are short-circuit
protected at approximately 2A. Selecting a higher impedance driver helps prevent exceeding the current limit.
______________________________________________________________________________________
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
10V TO 25V
33µF
25V
2.2µF
25V*
2
3
PGND
0.47µF
0.47µF
2.2µF
25V*
5
6
37 38
VDD
VDD
40 41
PGND
17 INL+
OUTL+ 54
MODULATOR
16 INL-
OUTL+ 53
OUTL- 52
H-BRIDGE
OUTL- 51
VREG
VREG
0.47µF
31 FS1
32 FS2
OSCILLATOR
27 INR+
OUTR+ 48
MODULATOR
0.47µF
26 INR-
OUTR+ 47
OUTR- 46
H-BRIDGE
OUTR- 45
VIH
VREG
VREG
18 SHDN
29 G1
30 G2
20 SS
0.47µF
VREG
0.01µF
10V
24 REG
GAIN
CONTROL
SHUTDOWN
CONTROL
MAX9741
C1P 9
CHARGE PUMP
8
C1
0.1µF
25V
C1N
22 AGND
CHOLD
10
VDD
C2
1µF
25V
LOGIC INPUTS SHOWN FOR AV = 16dB (SSM).
VIN = LOGIC-HIGH > 2.5V.
*CAPACITOR VOLTAGE RATINGS MAY BE REDUCED WHEN
OPERATING WITH REDUCED SUPPLY VOLTAGES.
______________________________________________________________________________________
13
MAX9741
Application Circuit
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
MAX9741
Pin Configuration
G1
G2
FS1
FS2
N.C.
N.C.
N.C.
N.C.
VDD
VDD
N.C.
PGND
PGND
N.C.
TOP VIEW
42 41 40 39 38 37 36 35 34 33 32 31 30 29
N.C. 43
28 N.C.
N.C. 44
27 INR+
OUTR- 45
26 INR-
OUTR- 46
25 N.C.
OUTR+ 47
24 REG
OUTR+ 48
23 N.C.
22 AGND
N.C. 49
MAX9741
N.C. 50
21 N.C.
OUTL- 51
20 SS
OUTL- 52
19 N.C.
OUTL+ 53
18 SHDN
OUTL+ 54
17 INL+
N.C. 55
16 INL-
+
15 N.C.
10 11 12 13 14
N.C.
VDD
9
N.C.
N.C.
8
N.C.
PGND
7
N.C.
N.C.
6
CHOLD
5
C1P
4
C1N
3
N.C.
2
VDD
1
PGND
N.C. 56
THIN QFN
8mm x 8mm
Chip Information
TRANSISTOR COUNT: 4630
PROCESS: BiCMOS
14
______________________________________________________________________________________
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
56L THIN QFN.EPS
PACKAGE OUTLINE
56L THIN QFN, 8x8x0.8mm
21-0135
E
1
2
______________________________________________________________________________________
15
MAX9741
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
MAX9741
12W+12W, Low-EMI, Spread-Spectrum,
Stereo, Class D Amplifier
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE
56L THIN QFN, 8x8x0.8mm
21-0135
E
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
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Printed USA
is a registered trademark of Maxim Integrated Products, Inc.
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