ETC LM4871

1.1W Audio Power Amplifier with Shutdown Mode
General Description
Key Specifications
The LM4871 is a bridge-connected audio power amplifier capable of delivering typically 1.1W of continuous average
power to an 8Ω load with 0.5% (THD) from a 5V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. Since the LM4871 does not require
output coupling capacitors, bootstrap capacitors, or snubber
networks, it is optionally suited for low-power portable systems.
The LM4871 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.
The unity-gain stable LM4871 can be configured by external
gain-setting resistors.
n THD at 1 kHz at 1W continuous
average output power into 8Ω
0.5% (max)
n Output power at 10% THD+N
at 1 kHz into 8Ω
1.5W (typ)
n Shutdown Current
0.6 µA (typ)
n No output coupling capacitors, bootstrap capacitors, or
snubber circuits are necessary
n Small Outline or DIP packaging
n Unity-gain stable
n External gain configuration capability
n Pin compatible with LM4861
n Portable Computers
n Desktop Computers
n Low Voltage Audio Systems
Typical Application
Connection Diagram
Small Outline and DIP Package
Top View
Order Number LM4871M or LM4871N
See NS Package Number M08A or N08E
FIGURE 1. Typical Audio Amplifier Application Circuit
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LM4871 1.1W Audio Power Amplifier with Shutdown Mode
February 2000
Absolute Maximum Ratings (Note 2)
Infrared (15 sec.)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
θJC (typ) — M08A
−65˚C to +150˚C
θJA (typ) — M08A
−0.3V to VDD to +0.3V
θJC (typ) — N08E
θJA (typ) — N08E
Supply Voltage
Supply Temperature
Input Voltage
See AN-450 ″Surface Mounting and their Effects on
Product Reliability″ for other methods of
soldering surface mount devices.
Power Dissipation (Note 3)
Internally Limited
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Temperature Range
Soldering Information
−40˚C ≤ TA ≤ 85˚C
2.0V ≤ VDD ≤ 5.5V
Supply Voltage
Small Outline Package
Vapor Phase (60 sec.)
Operating Ratings
Electrical Characteristics
(Notes 1, 2)
The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C.
(Note 6)
(Note 7)
Supply Voltage
V (min)
V (max)
mA (max)
Quiescent Power Supply Current
VIN = 0V, Io = 0A
Shutdown Current
µA (max)
Output Offset Voltage
VIN = 0V
mV (max)
Output Power
THD = 0.5% (max); f = 1 kHz
W (min)
THD+N = 10%; f = 1 kHz
Total Harmonic Distortion+Noise
Po = 1 Wrms; AVD = 2; 20 Hz ≤ f
≤ 20 kHz
Power Supply Rejection Ratio
VDD = 4.9V to 5.1V
Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
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: 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 LM4871, TJMAX = 150˚C. The
typical junction-to-ambient thermal resistance is 140˚C/W for package number M08A and is 107˚C/W for package number N08E.
Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 5: Machine Model, 220 pF–240 pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
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External Components Description
(Figure 1)
Functional Description
Inverting input resistance which 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 which 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 which sets the closed-loop gain in conjunction with Ri.
Supply bypass capacitor which 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 which 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
THD+N vs Frequency
THD+N vs Frequency
THD+N vs Output Power
THD+N vs Frequency
THD+N vs Output Power
Output Power vs
Supply Voltage
THD+N vs Output Power
Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
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Typical Performance Characteristics
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
Clipping Voltage vs
Supply Voltage
Power Derating Curve
Frequency Response vs
Input Capacitor Size
Noise Floor
Power Supply
Rejection Ratio
Open Loop
Frequency Response
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Supply Current vs
Supply Voltage
As shown in Figure 1, the LM4871 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally configurable, while 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 by the two internal 40 kΩ
resistors. 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 magnitude, but out of
phase 180˚. 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 configuration 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 conditions. 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 excessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4871,
also creates a second advantage over single-ended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configuration. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
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 device as possible. Typical applications employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the
LM4871. 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.
In order to reduce power consumption while not in use, the
LM4871 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 LM4871 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 0.6 µA. 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 LM4871. This scheme guarantees that
the shutdown pin will not float thus preventing unwanted
state changes.
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or singleended. 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)
Since the LM4871 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 LM4871 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 dissipation that results from Equation 2:
Proper selection of external components in applications using integrated power amplifiers is critical to optimize device
and system performance. While the LM4871 is tolerant of
For package M08A, θJA = 140˚C/W, and for package N08E,
θJA = 107˚C/W assuming free air operation. TJMAX = 150˚C
for the LM4871. 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
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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 LM4871, the θJA can be
reduced from its free air value of 140˚C/W for package
M08A. 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 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. 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.
Application Information
Application Information
external component combinations, consideration to component values must be used to maximize overall system quality.
The LM4871 is unity-gain stable which gives a designer
maximum system flexibility. The LM4871 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 closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components
shown in Figure 1. 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 100 Hz to 150 Hz. 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 LM4871 turns
on. The slower the LM4871’s outputs ramp to their quiescent
DC voltage (nominally 1/2 VDD), the smaller the turn-on pop.
Choosing CB equal to 1.0 µF along with a small value of Ci
(in the range of 0.1 µF to 0.39 µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
CB equal to 0.1 µF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of CB equal to
1.0 µF is recommended in all but the most cost sensitive designs.
Design a 1W/8Ω Audio Amplifier
Power Output
Load Impedance
Input Level
Input Impedance
1 Wrms
1 Vrms
20 kΩ
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 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.
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
LM4871 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.
Rf/Ri = AVD/2
From Equation 4, the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20 kΩ, and with a
AVD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an
allocation of Ri = 20 kΩ and Rf = 30 kΩ. The final design step
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required ± 0.25 dB specified.
fL = 100 Hz/5 = 20 Hz
fH = 20 kHz * 5 = 100 kHz
As stated in the External Components section, Ri in conjunction with Ci create a highpass filter.
Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
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 = 100 kHz, the resulting GBWP =
150 kHz which is much smaller than the LM4871 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4871 can still be used without running into bandwidth limitations.
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inches (millimeters) unless otherwise noted
Order Number LM4871M
NS Package Number M08A
Order Number LM4871N
NS Package Number N08E
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Physical Dimensions