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PDF LM4991MA Data sheet ( Hoja de datos )
| Número de pieza | LM4991MA | |
| Descripción | 3W Audio Power Amplifier with Shutdown Mode | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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May 2003
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
3Ω load 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 (ISD =
0.1µA, typ) is activated when VDD 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
n Improved PSRR at 217kHz and 1kHz
64dB (typ)
n PO at VDD = 5.0V, 10% THD, 1kHz
n LM4991LD (only), 3Ω, 4Ω
3W (typ), 2.5W (typ)
n All packages, 8Ω load
1.5W (typ)
n Shutdown current
0.1µA (typ)
Features
n Available in space-saving LLP and MA packages
n Ultra low current shutdown mode
n Can drive capacitive loads up to 500pF
n Improved pop & click circuitry reduces noises during
turn-on and turn-off transitions
n 2.2 - 5.5V operation
n No output coupling capacitors, snubber networks,
bootstrap capacitors or gain-setting resistors required
n Unity-gain stable
Applications
n Wireless and cellular handsets
n PDA’s
n Portable computers
n Desktop 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.
© 2003 National Semiconductor Corporation DS200740
www.national.com
1 page  
External Components Description
(Figure 1)
Components
1. Ri
2. Ci
3. Rf
4. CS
5. CB
Functional Description
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).
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.
Feedback resistance that sets the closed-loop gain in conjunction with Ri.
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.
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.
Typical Performance Characteristics
LD and MH 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
20074041
Typical Performance Characteristics
THD+N vs Frequency
VDD = 5V, RL = 8Ω, and PO = 500mW
20074042
THD+N vs Frequency
VDD = 3V, RL = 4Ω, and PO = 500mW
20074043
5
20074044
www.national.com
5 Page 
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 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 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, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.0W to
1.95W. This problem of decreased load dissipation is 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 Rf to Ri 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
AVD= 2 *(Rf/Ri)
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 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 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.
PDMAX = 4*(VDD)2/(2π2RL) (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 8Ω load, 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:
PDMAX = (TJMAX–TA)/θJA
(2)
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 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 θJA decreased, or the ambient tem-
perature 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 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 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 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
11 www.national.com
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