LM4991
LM4991 3W Audio Power Amplifier with Shutdown Mode
Literature Number: SNAS217
LM4991
3W Audio Power Amplifier with Shutdown Mode
General Description
The LM4991 is a mono bridged audio power amplifier ca-
pable of delivering 3W of continuous average power into a
3Ωload with less than 10% THD when powered by a 5V
power supply (Note 1). To conserve power in portable appli-
cations, the LM4991’s micropower shutdown mode (I
SD
=
0.1µA, typ) is activated when V
DD
is applied to the SHUT-
DOWN pin.
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 volt-
ages 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 pro-
tection, unity-gain stability, and external gain set.
Note 1: 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.
Key Specifications
nImproved PSRR at 217kHz and 1kHz 64dB (typ)
nP
O
at V
DD
= 5.0V, 10% THD, 1kHz
nLM4991LD (only), 3Ω,4Ω3W (typ), 2.5W (typ)
nAll packages, 8Ωload 1.5W (typ)
nShutdown current 0.1µA (typ)
Features
nAvailable in space-saving LLP and MA packages
nUltra low current shutdown mode
nCan drive capacitive loads up to 500pF
nImproved pop & click circuitry reduces noises during
turn-on and turn-off transitions
n2.2 - 5.5V operation
nNo output coupling capacitors, snubber networks,
bootstrap capacitors or gain-setting resistors required
nUnity-gain stable
Applications
nWireless and cellular handsets
nPDA’s
nPortable computers
nDesktop computers
Connection Diagrams
Small Outline Package LLP Package
20074002
Top View
Order Number LM4991MA
See NS Package Number M08A
20074039
Top View
Order Number LM4991LD
See NS Package Number LDC08A
Boomer®is a registered trademark of National Semiconductor Corporation.
May 2003
LM4991 3W Audio Power Amplifier with Shutdown Mode
© 2003 National Semiconductor Corporation DS200740 www.national.com
Typical Application
20074001
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4991
www.national.com 2
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 6.0V
Supply Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
to +0.3V
Power Dissipation (Note 4) Internally Limited
ESD Susceptibility (Note 5) 2000V
ESD Susceptibility (Note 6) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JC
(LD) (Note 9) 4.3˚C/W
θ
JA
(LD) 56˚C/W
θ
JC
(MA) 35˚C/W
θ
JA
(MA) 140˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤+85˚C
Supply Voltage 2.2V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
=5V(Notes 2, 3)
The following specifications apply for V
DD
= 5V and R
L
=8Ωunless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4991
Typical
(Note 7)
Limit
(Note 8)
Units
(Limits)
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, no Load 3 7 mA (max)
V
IN
= 0V, R
L
=8Ω410
I
SD
Shutdown Current V
SHUTDOWN
=V
DD
0.1 2.0 µA (max)
V
SDIH
Shutdown Voltage 1.5 V
V
SDIL
1.3 V
V
OS
Output Offset Voltage 5 35 mV (max)
P
o
Output Power
THD = 1% (max), f = 1kHz
LM4991LD, R
L
=3Ω(Note 10)
LM4991LD, R
L
=4Ω(Note 10)
LM4991, R
L
=8Ω
2.38
2.1
1.3 0.9
W (min)
THD+N = 10%, f = 1kHz
LM4991LD, R
L
=3Ω(Note 10)
LM4991LD, R
L
=4Ω(Note 10)
LM4991, R
L
=8Ω
3
2.5
1.5
W
THD+N Total Harmonic
Distortion+Noise P
O
= 0.5W, f = 1kHz 0.2 %
PSRR Power Supply Rejection
Ratio
V
RIPPLE
= 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
64 55 dB (min)
Electrical Characteristics V
DD
=3V(Notes 2, 3)
The following specifications apply for V
DD
= 3V and R
L
=8Ωunless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4991
Typical
(Note 7)
Limit
(Note 8)
Units
(Limits)
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, no Load 3 7 mA (max)
V
IN
= 0V, R
L
=8Ω47
I
SD
Shutdown Current V
SHUTDOWN
=V
DD
0.1 2.0 µA (max)
V
SDIH
Shutdown Voltage Input High 1.1 V
V
SDIL
Shutdown Voltage Input Low 0.9 V
V
OS
Output Offset Voltage 5 35 mV (max)
P
o
Output Power THD = 1% (max), f = 1kHz
R
L
=4Ω
R
L
=8Ω
600
425 mW
THD+N Total Harmonic
Distortion+Noise P
O
= 0.25W, f = 1kHz 0.1 %
LM4991
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Electrical Characteristics V
DD
=3V(Notes 2, 3)
The following specifications apply for V
DD
= 3V and R
L
=8Ωunless otherwise specified. Limits apply for T
A
=
25˚C. (Continued)
Symbol Parameter Conditions
LM4991
Typical
(Note 7)
Limit
(Note 8)
Units
(Limits)
PSRR Power Supply Rejection
Ratio
V
RIPPLE
= 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
68 dB
Electrical Characteristics V
DD
= 2.6V (Notes 2, 3)
The following specifications apply for V
DD
= 2.6V and R
L
=8Ωunless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4991
Typical
(Note 7)
Limit
(Note 8)
Units
(Limits)
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, no Load 2 mA (max)
V
IN
= 0V, R
L
=8Ω3
I
SD
Shutdown Current V
SHUTDOWN
=V
DD
0.1 µA(max)
V
SDIH
Shutdown Voltage Input High 1 V
V
SDIL
Shutdown Voltage Input Low 0.9 V
V
OS
Output Offset Voltage 5 35 mV (max)
P
o
Output Power
THD = 1% (max), f = 1kHz
R
L
=4Ω
R
L
=8Ω
400
300 mW
THD+N Total Harmonic
Distortion+Noise P
O
= 0.15W, f = 1kHz 0.1 %
PSRR Power Supply Rejection
Ratio
V
RIPPLE
= 200mV sine p-p,
Input terminated with 10Ω,
f = 1kHz
51 dB
Note 2: 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 guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 3: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 4: 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 =(T
JMAX–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 the Application Information section or the Absolute Maximum Ratings section.
Note 5: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 6: Machine Model, 220pF–240pF discharged through all pins.
Note 7: Typicals are specified at 25˚C and represent the parametric norm.
Note 8: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 9: The given θJA is for an LM4991 packaged in an LDC08A with the Exposed–DAP soldered to an exposed 1in2area of 1oz printed circuit board copper.
Note 10: When driving 3Ωor 4Ωloads from a 5V supply, the LM4991LD must be mounted to a circuit board.
LM4991
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External Components Description
(Figure 1)
Components Functional Description
1. R
i
Inverting input resistance that sets the closed-loop gain in conjunction with R
f
. This resistor also forms a high
pass filter with C
i
at f
C
= 1/(2πR
i
C
i
).
2. C
i
Input coupling capacitor that blocks the DC voltage at the amplifiers input terminals. Also creates a highpass
filter with R
i
at f
c
= 1/(2πR
i
C
i
). Refer to the section, Proper Selection of External Components, for an
explanation of how to determine the value of C
i
.
3. R
f
Feedback resistance that sets the closed-loop gain in conjunction with R
i
.
4. C
S
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. C
B
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 C
B
.
Typical Performance Characteristics
LD and MH Specific Characteristics
THD+N vs Frequency
V
DD
= 5V, R
L
=4Ω, and P
O
=1W
THD+N vs Output Power
V
DD
= 5V, R
L
=4Ω,andf=1kHz
20074041 20074042
Typical Performance Characteristics
THD+N vs Frequency
V
DD
= 5V, R
L
=8Ω, and P
O
= 500mW
THD+N vs Frequency
V
DD
= 3V, R
L
=4Ω, and P
O
= 500mW
20074043 20074044
LM4991
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Typical Performance Characteristics (Continued)
THD+N vs Frequency
V
DD
= 3V, R
L
=8Ω, and P
O
= 250mW
THD+N vs Frequency
V
DD
= 2.6V, R
L
=4Ω, and P
O
= 150mW
20074045 20074046
THD+N vs Frequency
V
DD
= 2.6V, R
L
=8Ω, and P
O
= 150mW
THD+N vs Output Power
V
DD
= 5V, R
L
=8Ω, and f = 1kHz
20074047 20074048
THD+N vs Output Power
V
DD
= 3V, R
L
=4Ω, and f = 1kHz
THD+N vs Output Power
V
DD
= 3V, R
L
=8Ω, and f = 1kHz
20074049 20074050
LM4991
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 2.6V, R
L
=4Ω, and f = 1kHz
THD+N vs Output Power
V
DD
= 2.6V, R
L
=8Ω, and f = 1kHz
20074051 20074052
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 5V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 5V, R
L
=8Ω, input floating
20074053 20074054
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 3V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 3V, R
L
=8Ω, input floating
20074055 20074056
LM4991
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 2.6V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 2.6V, R
L
=8Ω, Input Floating
20074057 20074058
Open Loop Frequency Response, 5V
Noise Floor, 5V, 8Ω
80kHz Bandwidth, Input to GND
20074059
20074060
Power Dissipation vs
Output Power, V
DD
=5V
Power Dissipation vs
Output Power, V
DD
=3V
20074061 20074062
LM4991
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Typical Performance Characteristics (Continued)
Power Dissipation vs
Output Power, V
DD
= 2.6V
Shutdown Hysteresis Voltage
V
DD
= 5V, SD Mode = V
DD
20074063
20074072
Shutdown Hysteresis Voltage
V
DD
= 3V, SD Mode = V
DD
Shutdown Hysteresis Voltage
V
DD
= 2.6V, SD Mode = V
DD
20074073 20074074
Output Power vs
Supply Voltage, R
L
=4Ω
Output Power vs
Supply Voltage, R
L
=8Ω
20074067 20074068
LM4991
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Typical Performance Characteristics (Continued)
Output Power vs
Supply Voltage, R
L
=16Ω
Output Power vs
Supply Voltage, R
L
=32Ω
20074069 20074070
Frequency Response vs
Input Capacitor Size
20074071
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 4Ωload. 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 prac-
tical heat sink area. If the heatsink and amplifier share the
same PCB layer, a nominal 2.5in
2
area is necessary for 5V
operation with a 4Ωload. Heatsink areas not placed on the
same PCB layer as the LM4991 should be 5in
2
(min) for the
same supply voltage and load resistance. The last two area
recommendations apply for 25˚C ambient temperature. In-
crease 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 lay-
out for the LD package is shown in the Demonstration
Board Layout section. Further detailed and specific infor-
mation concerning PCB layout, fabrication, and mounting an
LM4991
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Application Information (Continued)
LD (LLP) package is available from National Semiconduc-
tor’s Package Engineering Group under application note
AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3ΩAND 4ΩLOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendant on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ωtrace resistance reduces
the output power dissipated by a 4Ωload from 2.0W to
1.95W. This problem of decreased load dissipation is exac-
erbated as load impedance decreases. Therefore, to main-
tain 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 sup-
plies, 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 1, the LM4991 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable; the second amplifier is internally fixed in a unity-gain,
inverting configuration. The closed-loop gain of the first am-
plifier is set by selecting the ratio of R
f
to R
i
while the second
amplifier’s gain is fixed. Figure 1 shows that the output of
amplifier one serves as the input to amplifier two, which
results in both amplifiers producing signals identical in mag-
nitude, but 180˚ out of phase. Consequently, the differential
gain for the IC is
A
VD
= 2 *(R
f
/R
i
)
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 configura-
tion where one side of its load is connected to ground.
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 con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive 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 V
O
1 and
V
O
2 at the same DC voltage, in this case V
DD
/2 . This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single supply
amplifier’s half-supply bias voltage across the load. The
current flow created by the half-supply bias voltage in-
creases 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 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
) (1)
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 1, assuming a 5V power supply and
an 8Ωload, the maximum power dissipation point is
625 mW. The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissi-
pation that results from Equation 2:
P
DMAX
=(T
JMAX
–T
A
)/θ
JA
(2)
For the SO package, θ
JA
= 140˚C/W. For the LD package
soldered to a DAP pad that expands to a copper area of
1.0in
2
on a PCB, the LM4991’s θ
JA
is 56˚C/W. T
JMAX
=
150˚C for the LM4991. The θ
JA
can be decreased by using
some form of heat sinking. The resultant θ
JA
will be the
summation of the θ
JC
,θ
CS
, and θ
SA
.θ
JC
is the junction to
case of the package (or to the exposed DAP, as is the case
with the LD package), θ
CS
is the case to heat sink thermal
resistance and θ
SA
is the heat sink to ambient thermal
resistance. By adding additional copper area around the
LM4991, the θ
JA
can be reduced from its free air value for
the SO package. Increasing the copper area around the LD
package from 1.0in
2
to 2.0in
2
area results in a θ
JA
decrease
to 46˚C/W. Depending on the ambient temperature, T
A
, and
the θ
JA
, Equation 2 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the
result of Equation 1 is greater than that of Equation 2, then
either the supply voltage must be decreased, the load im-
pedance increased, the θ
JA
decreased, or the ambient tem-
perature reduced. For the typical application of a 5V power
supply, with an 8Ωload, and no additional heatsinking, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 61˚C pro-
vided 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 4Ωload, and 1.0in
2
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 Perfor-
mance Characteristics curves for power dissipation infor-
mation 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
LM4991
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Application Information (Continued)
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 regu-
lator with 10µF and a 0.1µF bypass capacitors which aid in
supply stability. This does not eliminate the need for bypass-
ing the supply nodes of the LM4991 with a 1µF tantalum
capacitor. The selection of bypass capacitors, especially C
B
,
is dependent upon PSRR requirements, click and pop per-
formance as explained in the section, Proper Selection of
External Components, system cost, and size constraints.
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 switch-
ing the shutdown pin to V
DD
, the LM4991 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less then V
DD
, 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 pro-
vides a quick, smooth transition into shutdown. Another so-
lution is to use a single-pole, single-throw switch in conjunc-
tion with an external pull-up resistor. When the switch is
closed, the shutdown pin is connected to ground and en-
ables the amplifier. If the switch is open, then the external
pull-up resistor will disable the LM4991. This scheme guar-
antees that the shutdown pin will not float thus preventing
unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4991 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
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 com-
plete 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 band-
width is dictated by the choice of external components
shown in Figure 1. The input coupling capacitor, C
i
, forms a
first order high pass filter which limits low frequency re-
sponse. 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 attenu-
ation. 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 perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i.
A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD
). 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 consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B
, 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 V
DD
), the smaller the turn-on pop.
Choosing C
B
equal to 1.0µF along with a small value of C
i
(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
C
B
equal to 0.1µF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
B
equal to
1.0µF is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8ΩAudio 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 the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required V
opeak
using Equation 3
and add the output voltage. Using this method, the minimum
supply voltage would be (V
opeak
+(V
OD
TOP +V
OD
BOT)), where
V
OD
BOT and V
OD
TOP are extrapolated from the Dropout Volt-
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
(3)
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 al-
lows the LM4991 to reproduce peaks in excess of 1W with-
out producing audible distortion. At this time, the designer
LM4991
www.national.com 12
Application Information (Continued)
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 Equa-
tion 4.
(4)
R
f
/R
i
=A
VD
/2 (5)
From Equation 4, the minimum A
VD
is 2.83; use A
VD
=3.
Since the desired input impedance was 20kΩ, and with a
A
VD
impedance of 2, a ratio of 1.5:1 of R
f
to R
i
results in an
allocation of R
i
= 20kΩand R
f
= 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.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz*5=100kHz
As stated in the External Components section, R
i
in con-
junction with C
i
create a highpass filter.
C
i
≥1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
VD
.
With a A
VD
= 3 and f
H
= 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.
LM4991
www.national.com13
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4991LD
See NS Package Number LDC08A
Order Number LM4991MA
NS Package Number M08A
LM4991
www.national.com 14
Notes
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: ap.support@nsc.com
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Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM4991 3W Audio Power Amplifier with Shutdown Mode
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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