LM4991
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LM4991
3W Audio Power Amplifier with Shutdown Mode
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FEATURES
DESCRIPTION
•
The LM4991 is a mono bridged audio power amplifier
capable of delivering 3W of continuous average
power into a 3Ω load with less than 10% THD when
powered by a 5V power supply (see Note below). To
conserve power in portable applications, the
LM4991's micropower shutdown mode (ISD = 0.1µA,
typ) is activated when VDD is applied to the
SHUTDOWN pin.
1
2
•
•
•
•
•
•
Available in Space-Saving WSON and SOIC
Packages
Ultra Low Current Shutdown Mode
Can Drive Capacitive Loads up to 500pF
Improved Click and Pop Circuitry Reduces
Noises During Turn-On and Turn-Off
Transitions
2.2 - 5.5V Operation
No Output Coupling Capacitors, Snubber
Networks, Bootstrap Capacitors or GainSetting Resistors Required
Unity-Gain Stable
APPLICATIONS
•
•
•
•
Wireless and Cellular Handsets
PDAs
Portable Computers
Desktop Computers
KEY SPECIFICATIONS
•
•
•
Improved PSRR at 217kHz and 1kHz: 64 dB
(typ)
PO at VDD = 5.0V, 10% THD, 1kHz
– LM4991LD (only), 3Ω, 4Ω: 3W (typ), 2.5 W
(typ)
– All packages, 8Ω load: 1.5 W (typ)
Shutdown current: 0.1µA (typ)
Boomer audio power amplifiers are designed
specifically to provide high power, high fidelity audio
output. They require few external components and
operate on low supply voltages from 2.2V to 5.5V.
Since the LM4991 does not require output coupling
capacitors, bootstrap capacitors, or snubber
networks, it is ideally suited for low-power portable
systems that require minimum volume and weight.
Additional LM4991 features include thermal shutdown
protection, unity-gain stability, and external gain set.
Note: An LM4991LD that has been properly mounted
to a circuit board will deliver 3W into 3Ω (at 10%
THD). The other package options for the LM4991 will
deliver 1.5W into 8Ω (at 10% THD). See the
Application
Information
sections
for
further
information
concerning
the
LM4991LD
and
LM4991M.
Connection Diagrams
Top View
Figure 1. SOIC Package
See Package Number D0008A
Top View
Figure 2. WSON Package
See Package Number NGN0008A
1
2
Please 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.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2004–2013, Texas Instruments Incorporated
LM4991
SNAS217A – MAY 2004 – REVISED APRIL 2013
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Typical Application
Figure 3. Typical Audio Amplifier Application Circuit
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.
Absolute Maximum Ratings (1) (2)
Supply Voltage
6.0V
−65°C to +150°C
Supply Temperature
−0.3V to VDD to +0.3V
Input Voltage
Power Dissipation (3)
Internally Limited
(4)
2000V
ESD Susceptibility
ESD Susceptibility (5)
200V
Junction Temperature
150°C
Thermal Resistance
(1)
(2)
(3)
(4)
(5)
(6)
θJC (LD) (6)
4.3°C/W
θJA (LD)
56°C/W
θJC (MA)
35°C/W
θJA (MA)
140°C/W
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 ensure specific performance limits. Electrical Characteristics VDD= 5V 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 ensure for parameters where no limit is given, however, the typical value is a good
indication of device performance.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature
TA. The maximum allowable power dissipation is PDMAX = (TJMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever
is lower. For the LM4991, TJMAX = 150°C. For the θJA's for different packages, please see theApplication Information section or the
Absolute Maximum Ratings section.
Human body model, 100pF discharged through a 1.5kΩ resistor.
Machine Model, 220pF–240pF discharged through all pins.
The given θJA is for an LM4991 packaged in an LDC08A with the Exposed–DAP soldered to an exposed 1in2 area of 1oz printed circuit
board copper.
Operating Ratings
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ +85°C
2.2V ≤ VDD ≤ 5.5V
Supply Voltage
2
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Electrical Characteristics VDD = 5V
(1) (2)
The following specifications apply for VDD = 5V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4991
Parameter
IDD
Test Conditions
Typ (3)
Quiescent Power Supply Current VIN = 0V, no Load
3
7
VIN = 0V, RL = 8Ω
4
10
VSHUTDOWN = VDD
0.1
2.0
ISD
Shutdown Current
VSDIH
Shutdown Voltage
VSDIL
VOS
THD = 1% (max), f = 1kHz
Output Power
Total Harmonic Distortion+Noise PO = 0.5W, f = 1kHz
PSRR
Power Supply Rejection Ratio
(1)
(2)
(3)
(4)
(5)
µA (max)
V
1.3
V
5
35
mV (max)
(5)
2.38
0.9
W (min)
LM4991LD, RL = 4Ω
(5)
2.1
1.3
LM4991LD, RL = 3Ω
(5)
LM4991LD, RL = 4Ω
(5)
3
2.5
LM4991, RL = 8Ω
THD+N
mA (max)
LM4991LD, RL = 3Ω
LM4991, RL = 8Ω
THD+N = 10%, f = 1kHz
Units
(Limits)
1.5
Output Offset Voltage
Po
Limit (4)
W
1.5
0.2
VRIPPLE = 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
64
%
55
dB (min)
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 ensure specific performance limits. Electrical Characteristics VDD= 5V 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 ensure for parameters where no limit is given, however, the typical value is a good
indication of device performance.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typicals are specified at 25°C and represent the parametric norm.
Limits are ensured to AOQL (Average Outgoing Quality Level).
When driving 3Ω or 4Ω loads from a 5V supply, the LM4991LD must be mounted to a circuit board.
Electrical Characteristics VDD = 3V
(1) (2)
The following specifications apply for VDD = 3V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4991
Parameter
IDD
Test Conditions
Typ (3)
Limit (4)
Units
(Limits)
Quiescent Power Supply
Current
VIN = 0V, no Load
3
7
VIN = 0V, RL = 8Ω
4
7
mA
(max)
ISD
Shutdown Current
VSHUTDOWN = VDD
0.1
2.0
µA (max)
VSDIH
Shutdown Voltage Input High
1.1
V
VSDIL
Shutdown Voltage Input Low
0.9
V
VOS
Output Offset Voltage
Po
Output Power
5
THD = 1% (max), f = 1kHz
RL = 4Ω
RL = 8Ω
THD+N
(1)
(2)
(3)
(4)
Total Harmonic
Distortion+Noise
PO = 0.25W, f = 1kHz
35
600
425
0.1
mV
(max)
mW
%
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 ensure specific performance limits. Electrical Characteristics VDD= 5V 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 ensure for parameters where no limit is given, however, the typical value is a good
indication of device performance.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typicals are specified at 25°C and represent the parametric norm.
Limits are ensured to AOQL (Average Outgoing Quality Level).
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Electrical Characteristics VDD = 3V (1) (2) (continued)
The following specifications apply for VDD = 3V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4991
Parameter
PSRR
Test Conditions
Typ (3)
Power Supply Rejection Ratio VRIPPLE = 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
Electrical Characteristics VDD = 2.6V
Units
(Limits)
Limit (4)
68
dB
(1) (2)
The following specifications apply for VDD = 2.6V and RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4991
Parameter
IDD
Test Conditions
Typ (3)
Quiescent Power Supply Current VIN = 0V, no Load
2
VIN = 0V, RL = 8Ω
3
VSHUTDOWN = VDD
Limits (4)
Units
(Limits)
mA (max)
ISD
Shutdown Current
0.1
µA(max)
VSDIH
Shutdown Voltage Input High
1
V
VSDIL
Shutdown Voltage Input Low
0.9
V
VOS
Output Offset Voltage
5
THD = 1% (max), f = 1kHz
Po
Output Power
THD+N
Total Harmonic Distortion+Noise PO = 0.15W, f = 1kHz
PSRR
Power Supply Rejection Ratio
(1)
(2)
(3)
(4)
35
mV (max)
RL = 4Ω
400
mW
RL = 8Ω
300
%
0.1
VRIPPLE = 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
51
dB
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 ensure specific performance limits. Electrical Characteristics VDD= 5V 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 ensure for parameters where no limit is given, however, the typical value is a good
indication of device performance.
All voltages are measured with respect to the ground pin, unless otherwise specified.
Typicals are specified at 25°C and represent the parametric norm.
Limits are ensured to AOQL (Average Outgoing Quality Level).
External Components Description
(Figure 3)
Components
4
Functional Description
1.
Ri
Inverting input resistance that sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter
with Ci at fC= 1/(2π RiCi).
2.
Ci
Input coupling capacitor that blocks the DC voltage at the amplifiers input terminals. Also creates a highpass filter with
Ri at fc = 1/(2π RiCi). Refer to the section, PROPER SELECTION OF EXTERNAL COMPONENTS, for an explanation
of how to determine the value of Ci.
3.
Rf
Feedback resistance that sets the closed-loop gain in conjunction with Ri.
4.
CS
Supply bypass capacitor that provides power supply filtering. Refer to the POWER SUPPLY BYPASSING section for
information concerning proper placement and selection of the supply bypass capacitor.
5.
CB
Bypass pin capacitor that provides half-supply filtering. Refer to the section, PROPER SELECTION OF EXTERNAL
COMPONENTS, for information concerning proper placement and selection of CB.
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Typical Performance Characteristics
LD and MA Specific Characteristics
THD+N vs Frequency
VDD = 5V, RL = 4Ω, and PO = 1W
THD+N vs Output Power
VDD = 5V, RL = 4Ω, and f = 1 kHz
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.01
20
0.1
100
1k
10k 20k
0.01
10m
100m
FREQUENCY (Hz)
OUTPUT POWER (W)
Figure 4.
Figure 5.
1
3
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Typical Performance Characteristics
THD+N vs Frequency
VDD = 5V, RL = 8Ω, and PO = 500mW
THD+N vs Frequency
VDD = 3V, RL = 4Ω, and PO = 500mW
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.1
0.01
20
100
1k
0.01
20
10k 20k
1k
10k 20k
FREQUENCY (Hz)
Figure 6.
Figure 7.
THD+N vs Frequency
VDD = 3V, RL = 8Ω, and PO = 250mW
THD+N vs Frequency
VDD = 2.6V, RL = 4Ω, and PO = 150mW
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.1
0.01
20
100
1k
0.01
20
10k 20k
100
1k
10k 20k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 8.
Figure 9.
THD+N vs Frequency
VDD = 2.6V, RL = 8Ω, and PO = 150mW
THD+N vs Output Power
VDD = 5V, RL = 8Ω, and f = 1kHz
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.01
20
6
100
FREQUENCY (Hz)
0.1
100
1k
10k 20k
0.01
10m
100m
FREQUENCY (Hz)
OUTPUT POWER (W)
Figure 10.
Figure 11.
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Typical Performance Characteristics (continued)
THD+N vs Output Power
VDD = 3V, RL = 4Ω, and f = 1kHz
THD+N vs Output Power
VDD = 3V, RL = 8Ω, and f = 1kHz
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.1
0.01
10m
100m
0.01
10m
1
100m
1
OUTPUT POWER (W)
OUTPUT POWER (W)
Figure 12.
Figure 13.
THD+N vs Output Power
VDD = 2.6V, RL = 4Ω, and f = 1kHz
THD+N vs Output Power
VDD = 2.6V, RL = 8Ω, and f = 1kHz
1
1
THD+N (%)
10
THD+N (%)
10
0.1
0.1
0.01
10m
100m
0.01
10m
1
100m
500m
OUTPUT POWER (W)
Figure 14.
Figure 15.
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 5V, RL = 8Ω, input 10Ω terminated
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 5V, RL = 8Ω, input floating
0
0
-10
-10
-20
-20
-30
-30
PSRR LEVEL (dB)
PSRR LEVEL (dB)
OUTPUT POWER (W)
-40
-50
-60
-70
-40
-50
-60
-70
-80
-80
-90
-90
-100
20
100
1k
10k 20k
-100
20
FREQUENCY (Hz)
100
1k
10k 20k
FREQUENCY (Hz)
Figure 16.
Figure 17.
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Typical Performance Characteristics (continued)
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 3V, RL = 8Ω, input floating
0
0
-10
-10
-20
-20
-30
-30
PSRR LEVEL (dB)
PSRR LEVEL (dB)
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 3V, RL = 8Ω, input 10Ω terminated
-40
-50
-60
-70
-40
-50
-60
-70
-80
-80
-90
-90
-100
20
100
1k
-100
20
10k 20k
100
FREQUENCY (Hz)
10k 20k
FREQUENCY (Hz)
Figure 19.
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 2.6V, RL = 8Ω, input 10Ω terminated
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 2.6V, RL = 8Ω, Input Floating
0
0
-10
-10
-20
-20
-30
-30
PSRR LEVEL (dB)
PSRR LEVEL (dB)
Figure 18.
-40
-50
-60
-70
-40
-50
-60
-70
-80
-80
-90
-90
-100
20
-100
20
100
1k
10k 20k
FREQUENCY (Hz)
8
1k
100
1k
10k 20k
FREQUENCY (Hz)
Figure 20.
Figure 21.
Open Loop Frequency Response, 5V
Noise Floor, 5V, 8Ω
80kHz Bandwidth, Input to GND
Figure 22.
Figure 23.
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Typical Performance Characteristics (continued)
Power Dissipation vs Output Power, VDD = 5V
Power Dissipation vs Output Power, VDD = 3V
0.5
1.4
0.45
:
POWER DISSIPATION (W)
POWER DISSIPATION (W)
1.2
:
1
0.8
0.6
:
0.4
0.2
0.4
0.35
:
0.3
0.25
0.2
0.15
0.1
0.05
0
0
0
0.5
1
1.5
2
2.5
OUTPUT POWER (W)
0
0.2
0.4
0.6
0.8
1
OUTPUT POWER (W)
Figure 24.
Figure 25.
Power Dissipation vs Output Power, VDD = 2.6V
Shutdown Hysteresis Voltage
VDD = 5V, SD Mode = VDD
0.4
POWER DISSIPATION (W)
0.35
:
0.3
0.25
0.2
:
0.15
0.1
0.05
0
0
0.1
0.2
0.4
0.5
0.3
OUTPUT POWER (W)
0.6
Figure 26.
Figure 27.
Shutdown Hysteresis Voltage
VDD = 3V, SD Mode = VDD
Shutdown Hysteresis Voltage
VDD = 2.6V, SD Mode = VDD
Figure 28.
Figure 29.
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Typical Performance Characteristics (continued)
Output Power vs Supply Voltage, RL = 4Ω
Output Power vs Supply Voltage, RL = 8Ω
3
3.5
f = 1kHz
f = 1kHz
2.5
OUTPUT POWER (W)
OUTPUT POWER (W)
3
2.5
10% THD+N
2
1.5
1% THD+N
1
2
10% THD+N
1.5
1
1% THD+N
500m
500m
0
2.2
3
4
5
0
2.2
5.5
3
5
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
Figure 30.
Figure 31.
Output Power vs Supply Voltage, RL = 16Ω
Output Power vs Supply Voltage, RL = 32Ω
1000
900
1000
900
f=1kHz
800
OUTPUT POWER (mW)
OUTPUT POWER (mW)
4
700
10% THD+N
600
500
1% THD+N
400
300
200
100
0
2.2
f=1kHz
800
700
600
500
10% THD+N
400
300
1% THD+N
200
100
3
4
5
0
2.2
5.5
SUPPLY VOLTAGE (V)
3
4
5
5.5
SUPPLY VOLTAGE (V)
Figure 32.
Figure 33.
Frequency Response vs Input Capacitor Size
Figure 34.
10
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APPLICATION INFORMATION
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4991's exposed-DAP (die attach paddle) package (LD) provides a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the
surrounding PCB copper traces, ground plane, and surrounding air. The result is a low voltage audio power
amplifier that produces 2W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful
consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4991's
high power performance and activate unwanted, though necessary, thermal shutdown protection.
The LD package must have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad is
connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink, and
radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner
layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper
heat sink area with 4(2x2) vias. The via diameter should be 0.012in-0.013in with a 1.27mm pitch. Ensure efficient
thermal conductivity by plating through the vias.
Best thermal performance is achieved with the largest practical heat sink area. If the heatsink and amplifier share
the same PCB layer, a nominal 2.5in2 area is necessary for 5V operation with a 4Ω load. Heatsink areas not
placed on the same PCB layer as the LM4991 should be 5in2 (min) for the same supply voltage and load
resistance. The last two area recommendations apply for 25°C ambient temperature. Increase the area to
compensate for ambient temperatures above 25°C. The LM4991's power de-rating curve in the Typical
Performance Characteristics shows the maximum power dissipation versus temperature. An example PCB layout
for the LD package is shown in the Demonstration Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mounting an LD (WSON) package is available from Texas
Instruments Package Engineering Group under application note AN-1187 (Literature Number SNOA401).
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω
LOADS
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance. As load
impedance decreases, load dissipation becomes increasingly dependant on the interconnect (PCB trace and
wire) resistance between the amplifier output pins and the load's connections. Residual trace resistance causes
a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1Ω
trace resistance reduces the output power dissipated by a 4Ω load from 2.0W to 1.95W. This problem of
decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load
dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide
as possible.
Poor power supply regulation adversely affects maximum output power. A poorly regulated supply's output
voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output
signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the
same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps
maintain full output voltage swing.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 3, the LM4991 has two operational amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier's gain is externally configurable; the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri
while the second amplifier's gain is fixed. Figure 3 shows that the output of amplifier one serves as the input to
amplifier two, which results in both amplifiers producing signals identical in magnitude, but 180° out of phase.
Consequently, the differential gain for the IC is
AVD= 2 *(Rf/Ri)
(1)
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of its load is connected to ground.
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A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides
differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output
power is possible as compared to a single-ended amplifier under the same conditions. This increase in attainable
output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closedloop gain without causing excessive clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN section.
Another advantage of the differential bridge output is no net DC voltage across load. This results from biasing
VO1 and VO2 at the same DC voltage, in this case VDD/2 . This eliminates the coupling capacitor that single
supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration
forces a single supply amplifier's half-supply bias voltage across the load. The current flow created by the halfsupply bias voltage increases internal IC power dissipation and my permanently damage loads such as
speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Equation (2) states the maximum power dissipation point for a bridge
amplifier operating at a given supply voltage and driving a specified output load.
PDMAX = 4*(VDD)2/(2π2RL)
(2)
Since the LM4991 has two operational amplifiers in one package, the maximum internal power dissipation is 4
times that of a single-ended ampifier. Even with this substantial increase in power dissipation, the LM4991 does
not require heatsinking under most operating conditions and output loading. From Equation (2), assuming a 5V
power supply and an 8Ω load, the maximum power dissipation point is 625 mW. The maximum power dissipation
point obtained from Equation (2) must not be greater than the power dissipation that results from Equation (3):
PDMAX = (TJMAX–TA)/θJA
(3)
For the SO package, θJA = 140°C/W. For the LD package soldered to a DAP pad that expands to a copper area
of 1.0in2 on a PCB, the LM4991's θJA is 56°C/W. TJMAX = 150°C for the LM4991. The θJA can be decreased by
using some form of heat sinking. The resultant θJA will be the summation of the θJC, θCS, and θSA. θJC is the
junction to case of the package (or to the exposed DAP, as is the case with the LD package), θCS is the case to
heat sink thermal resistance and θSA is the heat sink to ambient thermal resistance. By adding additional copper
area around the LM4991, the θJA can be reduced from its free air value for the SO package. Increasing the
copper area around the LD package from 1.0in2 to 2.0in2 area results in a θJA decrease to 46°C/W. Depending
on the ambient temperature, TA, and the θJA, Equation (3) can be used to find the maximum internal power
dissipation supported by the IC packaging. If the result of Equation (2) is greater than that of Equation (3), then
either the supply voltage must be decreased, the load impedance increased, the θJA decreased, or the ambient
temperature reduced. For the typical application of a 5V power supply, with an 8Ω load, and no additional
heatsinking, the maximum ambient temperature possible without violating the maximum junction temperature is
approximately 61°C provided that device operation is around the maximum power dissipation point and assuming
surface mount packaging. For the LD package in a typical application of a 5V power supply, with a 4Ω load, and
1.0in2 copper area soldered to the exposed DAP pad, the maximum ambient temperature is approximately 77°C
providing device operation is around the maximum power dissipation point. Internal power dissipation is a
function of output power. If typical operation is not around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the Typical Performance Characteristics curves for power dissipation
information for different output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the LM4991 as
possible. The capacitor connected between the bypass pin and ground improves the internal bias voltage's
stability, producing improved PSRR. The improvements to PSRR increase as the bypass pin capacitor increases.
Typical applications employ a 5V regulator with 10µF and a 0.1µF bypass capacitors which aid in supply stability.
This does not eliminate the need for bypassing the supply nodes of the LM4991 with a 1µF tantalum capacitor.
The selection of bypass capacitors, especially CB, is dependent upon PSRR requirements, click and pop
performance as explained in the section, PROPER SELECTION OF EXTERNAL COMPONENTS, system cost,
and size constraints.
12
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SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4991 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the
shutdown pin. The trigger point between a logic low and logic high level is typically half- supply. It is best to
switch between ground and supply to provide maximum device performance. By switching the shutdown pin to
VDD, the LM4991 supply current draw will be minimized in idle mode. While the device will be disabled with
shutdown pin voltages less then VDD, the idle current may be greater than the typical value of 0.1µA. In either
case, the shutdown pin should be tied to a definite voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which
provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground
and enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4991. This
scheme ensures that the shutdown pin will not float thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using integrated power amplifiers is critical to optimize
device and system performance. While the LM4991 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4991 is unity-gain stable which gives a designer maximum system flexibility. The LM4991 should be used
in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain
configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1
Vrms are available from sources such as audio codecs. Please refer to the section, AUDIO POWER AMPLIFIER
DESIGN, for a more complete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 3. The input coupling capacitor, Ci,
forms a first order high pass filter which limits low frequency response. This value should be chosen based on
needed frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers
used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to
150Hz. Thus, using a large input capacitor may not increase actual system performance.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
1/2 VDD). This charge comes from the output via the feedback and is apt to create pops upon device enable.
Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be
minimized.
Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value.
Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the
LM4991 turns on. The slower the LM4991's outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the
smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci (in the range of 0.1µF to
0.39µF), should produce a virtually clickless and popless shutdown function. While the device will function
properly, (no oscillations or motorboating), with CB equal to 0.1µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of CB equal to 1.0µF is recommended in all but the most cost sensitive
designs.
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13
LM4991
SNAS217A – MAY 2004 – REVISED APRIL 2013
www.ti.com
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Given:
Power Output
1 Wrms
Load Impedance
8Ω
Input Level
1 Vrms
Input Impedance
20 kΩ
Bandwidth
100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from the Output Power vs Supply Voltage graphs in theTypical Performance Characteristics section, the supply
rail can be easily found. A second way to determine the minimum supply rail is to calculate the required Vopeak
using Equation (3) and add the output voltage. Using this method, the minimum supply voltage would be (Vopeak
+ (VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs Supply Voltage curve
in the Typical Performance Characteristics section.
(4)
Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 4.6V. But since 5V is
a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom
that allows the LM4991 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the
designer must make sure that the power supply choice along with the output impedance does not violate the
conditions explained in the POWER DISSIPATION section.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation (4).
(5)
(6)
Rf/Ri = AVD/2
From Equation (4), the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20kΩ, and with a AVD impedance of 2, a ratio of 1.5:1 of Rf to Ri results
in an allocation of Ri = 20kΩ and Rf = 30kΩ. The final design step is to address the bandwidth requirements
which must be stated as a pair of −3dB frequency points. Five times away from a −3dB point is 0.17dB down
from passband response which is better than the required ±0.25dB specified.
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
As stated in the External Components Description section, Ri in conjunction with Ci create a highpass filter.
Ci ≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF
The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain,
AVD. With a AVD = 3 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4991
GBWP of 4MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential
gain, the LM4991 can still be used without running into bandwidth limitations.
14
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LM4991
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SNAS217A – MAY 2004 – REVISED APRIL 2013
REVISION HISTORY
Changes from Original (April 2013) to Revision A
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 14
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15
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM4991LD/NOPB
ACTIVE
WSON
NGN
8
1000
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 85
L4991
LM4991LDX/NOPB
ACTIVE
WSON
NGN
8
4500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 85
L4991
LM4991MA/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 85
LM49
91MA
LM4991MAX/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 85
LM49
91MA
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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