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LM4682 10 Watt Stereo CLASS D Audio Power Amplifier
with Stereo Headphone Amplifier and DC Volume Control
Check for Samples: LM4682
1FEATURES DESCRIPTION
The LM4682 is a fully integrated single supply, high
2• Pulse-Width Modulator efficiency audio power amplifier solution. The LM4682
• DC Volume Control utilizes a proprietary balanced pulse-width modulation
• Stereo Headphone Amplifier technique that lowers output noise and THD and
improves PSRR when compared to conventional
• “Click and Pop” Suppression Circuitry pulse width modulators.
• Micropower Shutdown Mode The LM4682 also features a stereo headphone
• 48-Lead WQFN Package (No Heatsink amplifier that delivers 60mW into a 32Ωheadset with
Required) less than 0.5% THD.
APPLICATIONS The LM4682's DC volume control has a +30dB to
–48dB range when speakers are driven and a range
• Flat Panel Displays of +13dB to –65dB when headphones are connected.
• Televisions All amplifiers are protected by thermal shutdown.
Additionally, all amplifiers incorporate output current
• Multimedia Monitors limiting function to protect their outputs from short
circuit.
KEY SPECIFICATIONS The LM4682 features a low-power consumption
• PO at THD+N = 10%, VDD = 14V 10W (typ) shutdown mode. And its efficiency reaches 85% for a
• THD+N at 1kHz at 6W into 8Ω(Power Amp) 10W output power with an 8Ωload. External heatsink
0.2% (typ) is not required when playing music. The IC features
• Efficiency at 7W into 8Ω84% (typ) click and pop reduction circuitry that minimizes
audible popping during device turn-on and turn-off.
• Total Quiescent Power Supply Current 52mA The LM4682 is available in a 48-lead WQFN
(typ) package, ideal for portable and desktop computer
• Total Shutdown Power Supply Current 0.1mA applications.
(typ)
• THD+N 1kHz, 20mW, 32Ω(Headphone) 0.02%
(typ)
• Single Supply Range 8.5V to 15V
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2006–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Block Diagram
Figure 1. Block Diagram for LM4682
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1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
48
47
46
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
PVDD_B
OUT1_B
OUT2_B
PVDD_B
PVDD_B
PGND_B
PGND_B
PVDD_A
OUT1_A
OUT2_A
PVDD_A
PGND_A
PGND_A
BST2_B
NC
MUTEB
SDB
NC
VDD_A
GND_A
VOLVDD
VOL_CTL
HPSEL
NC
BST2_A
BST1_B
NC
IN_B
BYPASS_B
HPOUT_B
HP_VDD
HP_GND
HPOUT_A
BYPASS_A
IN_A
BST1_A
VREF
PVDD_B
OUT1_B
OUT1_B
OUT2_B
OUT2_B
PVDD_A
OUT1_A
OUT1_A
OUT2_A
OUT2_A
PVDD_A
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Connection Diagram
Figure 2. 48-Lead WQFN - Top View
See RHS Package
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+
-
+
-
Click-Pop
Suppression
Over-Current
Protection
Power Management Thermal
Protection
To PWM
To
PWM
Balanced
PWM
A
Balanced
PWM
B
PWM
DC Volume
Control
Preamp
VDD/2
Preamp Attenuator +
-
VREF
Attenuator +
-
VREF
Mute
Shutdown
To HP Sense Pin on
Headphone Jack
To HP Sense Circuit
From Decoupled HP Output A
HP GND
+
+
+
+
+10 PF
4.7 PF
0.1 PF
1 PF
1 PF
1 PF
1 PF
1 PF
10 PF
1 PF
18
19
39
40
38
45
46
15
20
21
16
22
25,26,
35,36 1,2,
11,12 30,316,7 14
Class D
0.22 PF
0.22 PF
0.22 PF
22 PH
22 PH
0.1 PF
0.1 PF
Class D
0.22 PF
0.22 PF
0.22 PF
22 PH
22 PH
0.1 PF
0.1PF
41
37
32,33,34
27,28,29
24
48
3,4,5
8,9,10
13
44
43 42
100 k:
100 k:
VDD
VDD
VDD
5V
Stereo Headphone Jack
+
100 PF1 k:
VDD
+
10 PF
4.7 PF
VDD
To Headphone Jack Output Pin
+
100 PF1 k:
0.1 PF
0.1 PF
HP GND
HP GND
100 k:
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Typical Application
Figure 3. Typical Stereo Audio Amplifier with Headphone Selection Circuit
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Absolute Maximum Ratings(1)(2)
Supply Voltage 15.5V
Input Voltage −0.3V to VDD +0.3V
Power Dissipation(3) Internally Limited
ESD Susceptibility(4) 2000V
ESD Susceptibility(5) 200V
Junction Temperature(6) 150°C
Storage Temperature −65°C ≤TA≤150°C
Soldering Information WQFN Package Vapor Phase (60 sec.) 215°C
Infrared (15 sec.) 220°C
(1) “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. “Operating Ratings” indicate conditions for
which the device is functional, but do not specify performance limits. “Electrical Characteristics” state DC and AC electrical specifications
under particular test conditions which ensure specific performance limits. This assumes that the device is within the Operating Ratings.
Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication of device
performance.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) For operating at case temperatures above 25°C, the device must be derated based on a 150°C maximum junction temperature and a
thermal resistance of θJA = 28°C/W (junction to ambient).
(4) Human body model, 100pF discharged through a 1.5kΩresistor. Device pin 16 has ESD HBM rating = 1500V.
(5) Machine Model 220pF−240pF discharged through all pins.
(6) The operating junction temperature maximum is 150°C.
Operating Ratings(1)
Temperature Range TMIN ≤TA≤TMAX −40°C ≤TA≤+85°C
Supply Voltage 8.5V ≤VDD ≤15V
Thermal Resistance (WQFN Package) θJA 28°C/W
θJC 20°C/W
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
Electrical Characteristics(1)(2)
The following specifications apply for VDD = 12V, VOLVDD = 5V, RL= 8Ω, LC filter values as shown in Figure 3, unless
otherwise specified.
Limits apply for TA= 25°C. LM4682
Symbol Parameter Conditions Units
Typical Max Min
VDD Operating Supply Voltage Range 12 15 8.5 V
Quiescent Power Supply Current, VIN = 0VRMS, VHPSEL = 0V 52 70 mA
Class D Mode
ISQuiescent Power Supply Current, VIN = 0VRMS, VHPSEL = 12V 30 40 mA
Headphone Mode
Quiescent Power Supply Current,
ISD SDB = 0V 0.1 mA
Shutdown Mode
RIN Input Resistance in Both Modes 8 kΩ
VOLVDD DC Reference Supply Voltage 5.5 3 V
VIH Minimum Logic High Input Voltage 0.7xVOLVDD V
SDB, MUTEB pins
VIL Maximum Logic Low Input Voltage 0.3xVOLVDD V
VHPIH HP Sense High Input Voltage VDD-1 V
VHPIL HP Sense Low Input Voltage VDD/2 V
Power Amplifiers
POR Output Power, Per Channel THD+N ≤1%, fIN = 1kHz 6.0 5.5 W
PD1 Power Dissipation PO= 7W/Chan, fIN = 1kHz 2.6 W
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
(2) Limits are specified to AOQL (Average Outgoing Quality Level).
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Electrical Characteristics(1)(2) (continued)
The following specifications apply for VDD = 12V, VOLVDD = 5V, RL= 8Ω, LC filter values as shown in Figure 3, unless
otherwise specified.
Limits apply for TA= 25°C. LM4682
Symbol Parameter Conditions Units
Typical Max Min
EFF1 Efficiency PO= 7W/Chan, fIN = 1kHz 84.4 %
THD+N Harmonic Distortion + Noise PO= 6W/Chan, fIN = 1kHz 0.2 %
RSOURCE = 50Ω, CIN = 1µF,
Output Noise Voltage, RMS.
VNOISE BW = 8Hz to 22kHz 13 µV
A−Weighted A-weighted, input referred
VRIPPLE = 200mVpp, 1kHz,
VIN = 0, input referred
f = 50Hz 94
f = 60Hz 94
PSRR Power Supply Rejection Ratio f 100Hz 93 dB
f = 120Hz 93
f = 1kHz 84
Headphone Amplifiers
POPower Out Per Channel THD+N ≤1%, RL= 32Ω, fIN = 1kHz 80 60 mW
THD+N Distortion + Noise PO= 20mW, RL= 32Ω, fIN = 1kHz 0.02 %
RIN = 50Ω, CIN = 1µF, BW = 20Hz
VNOISE Output Noise Voltage, RMS to 20kHz 9 µV
A-weighted, input referred
Power Supply Rejection Ratio
PSRR 200mV, 1kHz, VIN = 0, RL= 32Ω88 dB
(Referred to Input)
Electrical Characteristics for Volume Control(1)
The following specifications apply for VDD = 12V. Limits apply for TA= 25°C. LM4682 Units
Symbol Parameter Conditions (Limits )
Typical(2) Limit(3)
VOL_CTL voltage = VOLVDD voltage,
No Load
Power Amplifier 30 29 dB (min)
Headphone Amplifier 13 12 dB (min)
CRANGE Gain Range VOL_CTL voltage = 0.069 x VOLVDD
No Load
Power Amplifier –48 –46 dB (min)
Headphone Amplifier –65 –63 dB (min)
VMUTE voltage = 0V, No Load
AMMute Gain Power Amplifier –80 –60 dB (max)
Headphone Amplifier –70 –60 dB (max)
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
(2) Typicals are measured at 25°C and represent the parametric norm.
(3) Limits are specified to AOQL (Average Outgoing Quality Level).
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10m 100m 1 10
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
20
THD+N (%)
0.002
20m 200m 2 20
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
20
THD+N (%)
0.002
10m 100m 1 10
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
20
THD+N (%)
0.002
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
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Typical Performance Characteristics (Power Amplifier)
THD+N THD+N
vs vs
Frequency Frequency
VDD = 9V, RL= 8Ω, PO= 1W VDD = 12V, RL= 8Ω, PO= 1W
Figure 4. Figure 5.
THD+N THD+N
vs vs
Frequency Output Power Per Channel
VDD = 15V, RL= 8Ω, PO= 1W VDD = 9V, fIN = 1kHz, RL= 8Ω
Figure 6. Figure 7.
THD+N THD+N
vs vs
Output Power Per Channel Output Power Per Channel
VDD = 12V, fIN = 1kHz, RL= 8ΩVDD = 15V, fIN = 1kHz, RL= 8Ω
Figure 8. Figure 9.
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20 200 2k 20k
FREQUENCY (Hz)
0
10
20
30
40
GAIN (dB)
Class-D Amplifier Gain
RL = 8:
Po = 1W
HP Amplifier Gain
RL = 32:
Po = 20mW
20 200 2k 20k
FREQUENCY (Hz)
0
10
20
30
40
GAIN (dB)
HP Amplifier Gain
RL = 32:
Po = 20mW
Class-D Amplifier Gain
RL = 8:
Po = 1W
910 11 12 13 14 15
POWER SUPPLY VOLTAGE (V)
0
2
4
6
8
10
12
14
CLASS-D AMPLIFIER OUTPUT POWER (W)
THD+N = 10%
THD+N = 1%
20 200 2k 20k
FREQUENCY (Hz)
0
10
20
30
40
GAIN (dB)
Class-D Amplifier Gain
RL = 8:
Po = 1W
HP Amplifier Gain
RL = 32:
Po = 20mW
10m 100m 1 10
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
20
THD+N (%)
0.002
10m 100m 1 10
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
20
THD+N (%)
0.002
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Typical Performance Characteristics (Power Amplifier) (continued)
THD+N THD+N
vs vs
Output Power Per Channel Output Power Per Channel
VDD = 12V, fIN = 10kHz, RL= 8ΩVDD = 12V, fIN = 20Hz, RL= 8Ω
Figure 10. Figure 11.
Output Power Amplifiers Gain
vs vs
Supply Voltage Frequency
fIN = 1kHz, RL= 8ΩVDD = 9V, RL= 8Ω, PO= 1W
Figure 12. Figure 13.
Amplifiers Gain Amplifiers Gain
vs vs
Frequency Frequency
VDD = 12V, RL= 8Ω, PO= 1W VDD = 15V, RL= 8Ω, PO= 1W
Figure 14. Figure 15.
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02 4 6 8 14 16
LOAD DISSIPATION PER CHANNEL (W)
0
1
2
3
4
5
6
CLASS-D AMPLIFIER DISSIPATION (W)
10 12
0 2 4 6 8 10 12
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
CLASS-D AMPLIFIER DISSIPATION
LOAD DISSIPATION PER CHANNEL (W)
0 1 2 3 4 5 6
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
CLASS-D AMPLIFIER DISSIPATION (W)
LOAD DISSIPATION PER CHANNEL (W)
10 100 1k 10k 100k
FREQUENCY (Hz)
0
20
40
60
80
100
120
PSRR (dB)
10 100 1k 10k 100k
FREQUENCY (Hz)
0
20
40
60
80
100
120
PSRR (dB)
10 100 1k 10k 100k
FREQUENCY (Hz)
0
20
40
60
80
100
120
PSRR (dB)
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Typical Performance Characteristics (Power Amplifier) (continued)
PSRR PSRR
vs vs
Frequency Frequency
VDD = 9V VDD = 12V
Figure 16. Figure 17.
Class-D Amplifier Dissipation
PSRR vs
vs Load Dissipation
Frequency Per Channel, VDD = 9V, RL= 8Ω
VDD = 15V (both channels driven and measured)
Figure 18. Figure 19.
Class-D Amplifier Dissipation Class-D Amplifier Dissipation
vs vs
Load Dissipation Load Dissipation
Per Channel, VDD = 12V, RL= 8ΩPer Channel, VDD = 15V, RL= 8Ω
(both channels driven and measured) (both channels driven and measured)
Figure 20. Figure 21.
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812 16 20 24 28 32
LOAD IMPEDANCE (:
0
2
4
6
8
10
12
14
OUTPUT POWER PER CHANNEL (W)
THD+N = 10%
THD+N = 1%
0 1 2 3 4 5 6
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
OUTPUT POWER PER CHANNEL (W)
0 2 4 6 8 10 12
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
OUTPUT POWER PER CHANNEL (W)
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Typical Performance Characteristics (Power Amplifier) (continued)
Efficiency Efficiency
vs vs
Output Power Output Power
VDD = 9V, RL= 8ΩVDD = 12V, RL= 8Ω
(both channels driven and measured) (both channels driven and measured)
Figure 22. Figure 23.
Efficiency Output Power
vs vs
Output Power Load Resistance
VDD = 15V, RL= 8ΩVDD = 9V, fIN = 1kHz
(both channels driven and measured) (both channels driven and measured)
Figure 24. Figure 25.
Output Power Output Power
vs vs
Load Resistance Load Resistance
VDD = 12V, fIN = 1kHz VDD = 15V, fIN = 1kHz
(both channels driven and measured) (both channels driven and measured)
Figure 26. Figure 27.
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GAIN (dB)
VOLUME CONTROL VOLTAGE (V)
-60
-50
-40
-30
-20
-10
0
10
20
30
40
0 1 2 3 4 5
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Typical Performance Characteristics (Power Amplifier) (continued)
Power Supply Current Class-D Amplifier Gain vs
vs Volume Control Voltage
Power Supply Voltage VDD = 15V
Figure 28. Figure 29.
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10P100P1m 10m 100m
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
THD+N (%)
1
0.02
0.05
0.2
0.5
2
5
10P100P1m 10m 100m
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
THD+N (%)
1
0.02
0.05
0.2
0.5
2
5
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
10P100P1m 10m 100m
OUTPUT POWER PER CHANNEL (W)
0.01
0.1
1
10
THD+N (%)
1
0.02
0.05
0.2
0.5
2
5
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
20 200 2k 20k
FREQUENCY (Hz)
0.001
0.01
0.1
1
THD+N (%)
0.002
0.005
0.02
0.05
0.2
0.5
100
50 500 1k 5k 10k
R
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Typical Performance Characteristics (Headphone Amplifier)
THD+N THD+N
vs vs
Frequency Frequency
VDD = 9V, RL= 32Ω, PO= 20mW VDD = 12V, RL= 32Ω, PO= 20mW
Figure 30. Figure 31.
THD+N THD+N
vs vs
Frequency Output Power
VDD = 15V, RL= 32Ω, PO= 20mW VDD = 9V, RL= 32Ω, fIN = 1kHz
Figure 32. Figure 33.
THD+N THD+N
vs vs
Output Power Output Power
VDD = 12V, RL= 32Ω, fIN = 1kHz VDD = 15V, RL= 32Ω, fIN = 1kHz
Figure 34. Figure 35.
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GAIN (dB)
VOLUME CONTROL VOLTAGE (V)
0 1 2 3 4 5
-70
-60
-50
-40
-30
-20
-10
0
10
20
910 11 12 13 14 15
POWER SUPPLY VOLTAGE (V)
0
12.5
25
37.5
50
62.5
75
87.5
100
HEADPHONE AMPLIFIER OUTPUT
POWER PER CHANNEL (mW)
THD+N = 0.1%
THD+N = 1%
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Typical Performance Characteristics (Headphone Amplifier) (continued)
Output Power
vs Power Supply Current
Supply Voltage Per Channel vs
fIN = 1kHz, RL= 32ΩPower Supply Voltage
Figure 36. Figure 37.
Headphone Amplifier Gain vs
Volume Control Voltage
VDD = 15V
Figure 38.
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GENERAL FEATURES
SYSTEM FUNCTIONAL INFORMATION
Modulation Technique
Unlike typical Class D amplifiers that use single-ended comparators to generate a pulse-width modulated
switching waveform and RC timing circuits to set the switching frequency, the LM4682 uses a balanced
differential floating modulator. Oscillation is a result of injecting complimentary currents onto the respective plates
of a floating, on-die capacitor. The value of the floating capacitor and value of the components in the modulator’s
feedback network set the nominal switching frequency at 450kHz. Modulation results from imbalances in the
injected currents. The amount of current imbalance is directly proportional to the applied input signal’s magnitude
and frequency.
Using a balanced, floating modulator produces a Class D amplifier that is immune to common mode noise
sources such as substrate noise. This noise occurs because of the high frequency, high current switching in the
amplifier’s output stage. The LM4682 is immune to this type of noise because the modulator, the components
that set its switching frequency, and even the load all float with respect to ground.
The balanced modulator’s pulse width modulated output drives the gates of the LM4682’s H-bridge configured
output power MOSFETs. The pulse-train present at the power MOSFETs’ output is applied to an LC low pass
filter that removes the 450kHz energy component. The filter’s output signal, which is applied to the driven load, is
an amplified replica of the audio input signal.
Shutdown Function
The LM4682’s active-low shutdown function allows the user to place the amplifier in a shutdown mode while the
system power supply remains active. Activating shutdown stops the output switching waveform and minimizes
the quiescent current. Applying logic “0” to SDB pin enables the shutdown function. Applying logic “1” to SDB pin
disables the shutdown function and restores full amplifier operation.
Mute Function
The LM4682’s active-low mute function allows the user to place the amplifier outputs in muted mode while the
amplifier’s analog input signals remain active. Activating mute internally removes the analog input signal from the
Class D and headphone amplifier inputs. While muted the amplifier inputs and outputs retain in their VDD/2
operational bias. Applying logic “0” to MUTEB pin activates mute. Applying logic “1” to MUTEB pin deactivates
mute. The MUTEB pin is pull-down internally to put both Class D and headphone amplifier outputs mute.
Stereo Headphone Amplifier
The LM4682’s stereo headphone amplifier operates continuously, even while the Class D amplifiers are active.
When using headphones to listen to program material, it is usually desirable to stop driving external speakers.
This is easily achieved by using the active high HPSEL input. As shown in typical application schematic in
Figure 1, with no headphones connected to the headphone jack, the input voltage applied to the HPSEL pin is a
logic low. In this state, the Class D amplifiers are active and able to drive external speakers. When headphones
are plugged into the headphone jack, the switch inside the jack is opened. This changes the voltage applied to
the HPSEL pin to a logic high, shutting off the LM4682’s Class D amplifiers. The headphone control of the output
configuration is shown in Table 1.
Table 1. Headphone Controls
HP Sense Pin, HPSEL Output Stage Configuration
0 Class D Amps Active
1 Class D Amps Inactive
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Under Voltage Protection
The under voltage protection disables the output driver section of the LM4682 while the supply voltage is below
8V. This condition may occur as power is first applied or during low line conditions, changes in load resistance,
or when power supply sag occurs. The under voltage protection ensures that all of the LM4682’s power
MOSFETs are off. This action eliminates shoot-through current and minimizes output transients during turn-on
and turn-off. The under voltage protection gives the digital logic time to stabilize into known states, further
minimizing turn on output transients.
Power Supply Sequencing
In order to stabilize LM4682 before any operation, a power-up sequence for the power supplies is recommended.
The Power VDD should be applied first. Without deactivating the mute and shutdown function of the amplifiers,
the VOLVDD is then applied. Prior to removing the two supply voltages, activate shutdown and mute.
Turn-On Time
The LM4682 has an internal timer that determines the amplifier’s turn-on time. After power is first applied or the
part returns from shutdown, the nominal turn-on time is 600ms. This delay allows all externally applied capacitors
to charge to a final value of VDD/2. Further, during turn-on, the outputs are muted. This minimizes output
transients that may occur while the part settles into its quiescent operating mode.
Output Stage Current Limit and Fault Detection Protection
The output stage MOSFETs are protected against output conditions that could otherwise compromise their
operational status. The first stage of protection is output current limiting. When conditions that require high
currents to drive a load, the LM4682’s current limit circuitry clamps the output current at a nominal value of 2.5A.
The output waveform is present, but may be clipped or its amplitude reduced. The same 2.5A nominal current
limit also occurs if the amplifier outputs are shorted together or either output is shorted to VDD or GND.
The second stage of protection is an onboard fault detection circuit that continuously monitors the signal on each
output MOSFET’s gate and compares it against the respective drain voltage. When a condition is detected that
violates a MOSFET’s Safe Operating Area (SOA) and the drive signal is disconnected from the output
MOSFETs’ gates. The fault detect circuit maintains this protective condition for approximately 600ms, at which
time the drive signal is reconnected. If the fault condition is no longer present, normal operation resumes. If the
fault condition remains, however, the drive signal is again disconnected.
Thermal Protection
The LM4682 has thermal shutdown circuitry that monitors the die temperature. Once the LM4682 die
temperature reaches 170°C, the LM4682 disables the output switching waveform and remains disabled until the
die temperature falls below 140°C (typ).
Over-Modulation Protection
The LM4682’s over-modulation protection is a result of the preamplifier’s inability to produce signal magnitudes
that equal the power supply voltages. Since the preamplifier’s output magnitude will always be less than the
supply voltage, the duty cycle of the amplifier’s switching output will never reach zero. Peak modulation is limited
to a nominal 95%.
DC Volume Control
The LM4682 has an internal stereo volume control whose setting is a function of the DC voltage applied to the
volume control pin VOLCTL.
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The LM4682 volume control consists of 31 steps, which are individually selected by a variable DC voltage level
on the VOLCTL pin. A linear type 100kΩpotentiometer is used to adjust the VOLCTL voltage in the LM4682
demonstration board as shown in application circuit (see Figure 3). The resistance value of potentiometer fall in
the range from 10kΩto 100kΩis recommended to be used with only small amount of current dissipation and
large enough for the VOLCTL pin to function properly. The Volume Control Characteristics of LM4682 can be
found in the Typical Performance Characteristics (Headphone Amplifier) section. The gain range of Class D
amplifiers are from –48dB to 30dB. The gain range of headphone amplifiers are from –65dB to 13dB. Each gain
step corresponds to specific input voltage of both Class D amplifiers and headphone amplifiers are shown in
Table 2.
To minimize the effect of noise on the volume control VOLCTL pin, which can affect the selected gain level,
hysteresis has been implemented. The amount of hysteresis corresponds to half of the step width. For highest
accuracy, the voltage shown in the “recommended voltage” column of the table is used to select a desired gain.
The recommended voltage is exactly halfway between the two closest transitions to the next highest or next
lowest gain levels.
Table 2. Volume Control Table
Voltage Range (% of VOLVDD) Voltage Range (V), VOLVDD = 5V Gain (dB)
Step Class D Headphone
Low High Recommended Low High Recommended Amplifier Amplifier
1 77.50% 100.00% 100.000% 3.875 5.000 5.000 29.97 13.06
2 75.00% 78.50% 76.875% 3.750 3.925 3.844 28.97 12.07
3 72.50% 76.25% 74.375% 3.625 3.813 3.719 27.97 11.07
4 70.00% 73.75% 71.875% 3.500 3.688 3.594 26.96 10.06
5 67.50% 71.25% 69.375% 3.375 3.563 3.469 25.98 9.07
6 65.00% 68.75% 66.875% 3.250 3.438 3.344 24.97 8.07
7 62.50% 66.25% 64.375% 3.125 3.313 3.219 23.95 7.05
8 60.00% 63.75% 61.875% 3.000 3.188 3.094 21.98 5.08
9 57.50% 61.25% 59.375% 2.875 3.063 2.969 19.95 3.05
10 55.00% 58.75% 56.785% 2.750 2.983 2.844 17.96 1.06
11 52.50% 56.25% 54.375% 2.625 2.813 2.719 15.97 –0.93
12 50.00% 53.75% 51.875% 2.500 2.688 2.594 13.99 –2.91
13 47.50% 51.25% 49.375% 2.375 2.563 2.469 11.99 –4.91
14 45.00% 48.75% 46.875% 2.250 2.438 2.344 9.95 –6.96
15 42.50% 46.25% 44.375% 2.125 2.313 2.219 7.96 –8.94
16 40.00% 43.75% 41.875% 2.000 2.188 2.094 5.96 –10.95
17 37.50% 41.25% 39.375% 1.875 2.063 1.969 3.99 –12.91
18 35.00% 38.75% 36.875% 1.750 1.938 1.844 2.03 –14.87
19 32.50% 36.25% 34.375% 1.625 1.813 1.719 –0.02 –16.92
20 30.00% 33.75% 31.875% 1.500 1.688 1.594 –2.11 –19.02
21 27.50% 31.25% 29.375% 1.375 1.563 1.469 –4.16 –21.06
22 25.00% 28.75% 26.875% 1.250 1.438 1.344 –5.97 –22.87
23 22.50% 26.25% 24.375% 1.125 1.313 1.219 –8.77 –25.68
24 20.00% 23.75% 21.875% 1.000 1.188 1.094 –12.06 –28.96
25 17.50% 21.25% 19.375% 0.875 1.063 0.969 –14.84 –31.75
26 15.00% 18.75% 16.875% 0.750 0.938 0.844 –17.36 –34.26
27 12.50% 16.25% 14.375% 0.625 0.813 0.719 –20.89 –37.79
28 10.00% 13.75% 11.875% 0.500 0.688 0.594 –26.92 –43.83
29 7.50% 11.25% 9.375% 0.375 0.563 0.469 –32.95 –49.85
30 5.00% 8.75% 6.875% 0.250 0.438 0.344 –38.97 –55.88
31 0.00% 6.25% 0.000% 0.000 0.313 0.000 –48.03 –64.94
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Application Hints
SUPPLY BYPASSING
The major source of noises to be taken care and applying bypassing technique in using LM4682 are those
transients response coming from its output stage. During the switching operations of the output stage of LM4682,
the switching frequencies vary when the internal modulator react to the input signals. This creates a band of
switching transients giving back to the power supply terminals of LM4682. A single capacitor may not bypass
those transients well. Two capacitors which values are closed to each other are used to bypass this range of
frequencies to the ground. 10μF tantalum capacitors and 4.7μF ceramic capacitors are needed for this kind of
decoupling of LM4682 switching operation. This results an improvement in terms of both stability and audio
performance of LM4682. In addition, these capacitors should be placed as close as possible to each IC’s supply
pin(s) using leads as short as possible. Apart from the power supply de-coupling capacitors, the four
bootstrapping capacitors (at pins BST1_A, BST2_A, BST1_B and BST2_B) should also be placed close to their
corresponding pins. This could minimize the undesirable switching noise coupled to the supply rail.
The LM4682 has two different sets of VDD pins: a set for power VDD (PVDD_A and P VDD _B) and a set for signal
VDD _A and HP_ VDD. The parallel combination of the low value ceramic (4.7μF) and high value tantalum (10μF)
should be used to bypass the power VDD pins. A small value (1μF) ceramic or tantalum can be used to bypass
the signal VDD _A and HP_ VDD pin.
OUTPUT STAGE FILTERING
The LM4682 requires a low pass filter connected between the amplifier’s bridge output and the load. Figure 3
shows the recommended LC filter. A minimum value of 22µH is recommended. As shown in Figure 3, using the
values of the components connected between the amplifier BTL outputs and the load achieves a 2nd-order
lowpass filter response which optimizes the amplifier's performance within the audio band.
THD+N MEASUREMENTS AND OUT OF AUDIO BAND NOISE
THD+N (Total Harmonic Distortion plus Noise) is a very important parameter by which all audio amplifiers are
measured. Often it is shown as a graph where either the output power or frequency is changed over the
operating range. A very important variable in the measurement of THD+N is the bandwidth-limiting filter at the
input of the test equipment. Class D amplifiers, by design, switch their output power devices at a much higher
frequency than the accepted audio range (20Hz - 20kHz). Alternately switching the output voltage between VDD
and GND allows the LM4682 to operate at much higher efficiency than that achieved by traditional Class AB
amplifiers. Switching the outputs at high frequency also increases the out-of-band noise. Under normal
circumstances the output lowpass filter significantly reduces this out-of-band noise. If the low pass filter is not
optimized for a given switching frequency, there can be significant increase in out-of-band noise. THD+N
measurements can be significantly affected by out-of-band noise, resulting in a higher than expected THD+N
measurement. To achieve a more accurate measurement of THD, the test equipment’s input bandwidth of the
must be limited. Some common upper filter points are 22kHz, 30kHz, and 80kHz. The input filter limits the noise
component of the THD+N measurement to a smaller bandwidth resulting in a more real-world THD+N value.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figure 39 through Figure 43 show the recommended four-layer PCB board layout that is optimized for the 48-pin
WQFN packaged LM4682 and associated external components. This circuit is designed for use with an external
12V supply and 8Ωspeakers (or load resistors). Apply 12V and ground to board’s VDD and GND terminals
respectively. And apply 5V to the VOLVDD (refer to Power Supply Sequencing for details). Connect speakers (or
load resistors) between the board’s OUTA+ and OUTA-, and between the board’s OUTB+ and OUTB-. Apply the
stereo input signals to IN_A and IN_B. When designing the layout of the PCB layout, please pay attention to the
output terminals of LM4682.
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Figure 39. Top Layer
Figure 40. Top Silkscreen Layer
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Figure 41. Upper Middle Layer
Figure 42. Lower Middle Layer
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Figure 43. Bottom Layer
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REVISION HISTORY
Rev Date Description
1.0 02/22/06 Initial WEB release of the document.
1.1 02/24/06 Edited art 201196 71 (changed the y-axis unit from mA to mW.
1.2 03/08/06 Did few texts clean-up and re-released D/S to the WEB (per Kevin H.).
1.3 06/29/06 Added 2 columns on ( Gain dB) Table 2 and re-released the D/S to the WEB
(per Alex CK Wong).
1.4 04/09/08 Added volume control curves and input some text edits.
1.5 04/15/08 Changed the titles on curves 20119628 and 29.
1.6 04/21/08 Text edits.
E 04/10/13 Changed layout of National Data Sheet to TI format.
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