675 mW Audio Power Amplifier with Shutdown Mode
General Description
Key Specifications
The LM4862 is a bridge-connected audio power amplifier capable of delivering typically 675 mW of continuous average
power to an 8Ω load with 1% (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 LM4862 does not require
output coupling capacitors, bootstrap capacitors, or snubber
networks, it is optimally suited for low-power portable systems.
The LM4862 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.
The unity-gain stable LM4862 can be configured by external
gain-setting resistors.
n THD at 500 mW continuous average
output power at 1 kHz into 8Ω
n Output power at 10% THD+N at
1 kHz into 8Ω
1% (max)
825 mW (typ)
n Shutdown Current
0.7 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 Cellular Phones
n Toys and Games
Typical Application
Connection Diagram
Small Outline and DIP Package
Top View
Order Number LM4862M, LM4862N
See NS Package Number M08A or N08E
*Refer to the Application Information section for information
concerning proper selection of the input coupling capacitor.
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 1999 National Semiconductor Corporation
LM4862 675 mW Audio Power Amplifier with Shutdown Mode
May 1997
Absolute Maximum Ratings (Note 2)
Infrared (15 sec.)
See AN-450 “Surface Mounting and their Effects on
Product Reliability” for other methods of soldering surface
mount devices.
Thermal Resistance
θJC (typ) — M08A
θJA (typ) — M08A
θJC (typ) — N08E
θJA (typ) — N08E
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Storage Temperature
Input Voltage
Power Dissipation (Note 3)
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Soldering Information
Small Outline Package
Vapor Phase (60 sec.)
−65˚C to +150˚C
−0.3V to VDD + 0.3V
Internally limited
Operating Ratings
Temperature Range
Supply Voltage
−40˚C ≤ TA ≤ 85˚C
2.7V ≤ VDD ≤ 5.5V
Electrical Characteristics(Note 1) (Note 2)
The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C.
(Note 6)
(Note 7)
Supply Voltage
Quiescent Power Supply Current
Shutdown Current
Output Offset Voltage
Output Power
Total Harmonic Distortion +
Power Supply Rejection Ratio
VIN = 0V, IO = 0A (Note 8)
VIN = 0V
THD = 1% (max); f = 1 kHz; RL = 8Ω
THD + N = 10%; f = 1 kHz; RL = 8Ω
PO = 500 mWrms; RL = 8Ω
AVD = 2; 20 Hz ≤ f ≤ 20 kHz
VDD = 4.9V to 5.1V
V (min)
V (max)
mA (max)
µA (max)
mV (max)
mW (min)
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 = (TMAX − TA)/θJA. For the LM4862, TJMAX = 150˚C. The typical junction-to-ambient thermal resistance, when board mounted,
is 170˚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, 200 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).
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Automatic Switching Circuit
FIGURE 2. Automatic Switching Circuit
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/(2πRiCI).
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
highpass filter with Ri at fc = 1/(2πRiCi). 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 proper placement and selection of the supply bypass capacitor.
Bypass pin capacitor which provides half-supply filtering. Refer to the Proper Selection of External
Components section for proper placement and selection of the half-supply bypass capacitor.
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
THD+N vs Output Power
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
THD+N vs Frequency
Typical Performance Characteristics
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
Dropout Voltage vs
Power Supply
Power Derating Curve
Frequency Response vs
Input Capacitor Size
Noise Floor
Power Supply
Rejection Ratio
Open Loop
Frequency Response
Supply Current vs
Supply Voltage
Application Information
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, or the ambient temperature reduced. For the typical
application of a 5V power supply, with an 8Ω load, the maximum ambient temperature possible without violating the
maximum junction temperature is approximately 44˚C provided that device operation is around the maximum power
dissipation point. Power dissipation is a function of output
power and thus, 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 lower output powers.
As shown in Figure 1, the LM4862 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 10 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 the 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. Consequently, 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 which will damage high frequency
transducers used in loudspeaker systems, please refer to
the Audio Power Amplifier Design section.
A bridge configuration, such as the one used in LM4862,
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
lC power dissipation and also permanent loudspeaker damage.
As with any power 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. As
displayed in the Typical Performance Characteristics section, the effect of a larger half supply bypass capacitor is improved PSSR due to increased half-supply stability. Typical
applications employ a 5V regulator with 10 µF and a 0.1 µF
bypass capacitors which aid in supply stability, but do not
eliminate the need for bypassing the supply nodes of the
LM4862. The selection of bypass capacitors, especially CB,
is thus dependant upon desired PSSR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
In order to reduce power consumption while not in use, the
LM4862 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. The 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 LM4862 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less than VDD, the idle
current may be greater than the typical value of 0.7 µA. In either case, the shutdown pin should be tied to a definite voltage because leaving the pin floating may result in an unwanted shutdown condition.
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)
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 that when
closed, is connected to ground and enables the amplifier. If
the switch is open, then a soft pull-up resistor of 47 kΩ will
disable the LM4862. There are no soft pull-down resistors inside the LM4862, so a definite shutdown pin voltage must be
applied externally, or the internal logic gate will be left floating which could disable the amplifier unexpectedly.
Since the LM4862 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4862 does not require
heatsinking. 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:
For package M08A, θJA = 170˚C/W and for package N08E,
θJA = 107˚C/W. TJMAX = 150˚C for the LM4862. Depending
on the ambient temperature, TA, of the system surroundings,
Equation 2 can be used to find the maximum internal power
As shown in Figure 2, the LM4862 and the LM4880 can be
set up to automatically switch on and off depending on
whether headphones are plugged in. The LM4880 is used to
drive a stereo single ended load, while the LM4862 drives a
bridged internal speaker.
Application Information
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 LM4862 turns
on. The slower the LM4862’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 or larger is recommended in all but the most cost sensitive designs.
The Automatic Switching Circuit is based upon a single
control pin common in many headphone jacks which forms a
normally closed switch with one of the output pins. The output of this circuit (the voltage on pin 5 of the LM4880) has
two states based on the position of the switch. When the
switch inside the headphone jack is open, the LM4880 is enabled and the LM4862 is disabled since the NMOS inverter
is on. If a headphone jack is not present, it is assumed that
the internal speakers should be on and the external speakers should be off. Thus the voltage on the LM4862 shutdown
pin is low and the voltage on the LM4880 shutdown pin is
The operation of this circuit is rather simple. With the switch
closed, RP and RO form a resistor divider which produces a
gate voltage of less than 50 mV. The gate voltage keeps the
NMOS inverter off and RSD pulls the shutdown pin of the
LM4880 to the supply voltage. This shuts down the LM4880
and places the LM4862 in its normal mode of operation.
When the switch is open, the opposite condition is produced.
Resistor RP pulls the gate of the NMOS high which turns on
the inverter and produces a logic low signal on the shutdown
pin of the LM4880. This state enables the LM4880 and
places the LM4862 in shutdown mode.
Only one channel of this circuit is shown in Figure 2 to keep
the drawing simple but a typical application would be a
LM4880 driving a stereo headphone jack and two LM4862’s
driving a pair of internal speakers. If a single internal speaker
is required, one LM4862 can be used as a summer to mix
the left and right inputs into a mono channel.
Proper selection of external components in applications using integrated power amplifiers is critical to optimize device
and system performance. While the LM4862 is tolerant of
external component combinations, consideration to component values must be used to maximize overall system quality.
The LM4862 is unity-gain stable which gives a designer
maximum system flexibility. The LM4862 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
band-width 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 100–150 Hz. Thus using a large input
capacitor may not increase system performance.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling capacitor,
Application Information
make sure that the power supply choice along with the output impedance does not violate the conditions explained in
the Power Dissipation section.
Design a 500 mW/8Ω Audio Amplifier
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equation 4.
Power Output
Load Impedance
Input Level
Input Impedance
500 mWrms
1 Vrms
Rf/Ri = AVD/2:
From equation 4, the minimum AVD is 2; use AVD = 2.
20 kΩ
100 Hz–20 kHz ± 0.25 dB
Since the desired input impedance was 20 kΩ, and with a
AVD of 2, a ratio of 1:1 of Rf to Ri results in an allocation of Ri
= Rf = 20 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. This fact results in a low
and high frequency pole of 20 Hz and 100 kHz respectively.
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 high frequency pole, fH, and the differential gain,
AVD. With an AVD = 2 and fH = 100 kHz, the resulting GBWP
= 100 kHz which is much smaller than the LM4862 GBWP of
12.5 MHz. This figure displays that if a designer has a need
to design an amplifier with a higher differential gain, the
LM4862 can still be used without running into bandwidth
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 dropout voltage. Using this method, the minimum supply voltage would be (Vopeak + (2*VOD)), where
VOD is 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 4.3V. But since 5V is a standard supply voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that allows the LM4862 to reproduce peaks in excess of 500 mW
without clipping the signal. At this time, the designer must
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC
Order Number LM4862M
NS Package Number M08A
8-Lead (0.300" Wide) Molded Dual-In-Line Package
Order Number LM4862N
NS Package Number N08E
LM4862 675 mW Audio Power Amplifier with Shutdown Mode
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
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