1.1W Audio Power Amplifier with Shutdown Mode
General Description
Key Specifications
The LM4861 is a bridge-connected audio power amplifier capable of delivering 1.1W of continuous average power to an
8Ω load with 1% (THD) using a 5V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components using surface mount packaging. Since
the LM4861 does not require output coupling capacitors,
bootstrap capacitors, or snubber networks, it is optimally
suited for low-power portable systems.
The LM4861 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.
The unity-gain stable LM4861 can be configured by external
gain-setting resistors.
n THD at 1 kHz at 1W continuous
average output power into 8Ω
1.0% (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 (SO) packaging
n Compatible with PC power supplies
n Thermal shutdown protection circuitry
n Unity-gain stable
n External Gain Configuration Capability
Typical Application
Personal computers
Portable consumer products
Self-powered speakers
Toys and games
Connection Diagram
Small Outline Package
Top View
Order Number LM4861M
See NS Package Number M08A
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 1999 National Semiconductor Corporation
LM4861 1.1W Audio Power Amplifier with Shutdown Mode
May 1997
Absolute Maximum Ratings (Note 2)
See AN-450 “Surface Mounting and their Effects on
Product Reliability” for other methods of soldering surface
mount devices.
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.)
Infrared (15 sec.)
Operating Ratings
−65˚C to +150˚C
−0.3V to VDD + 0.3V
Internally limited
Temperature Range
Supply Voltage
Thermal Resistance
θJC (typ) — M08A
θJA (typ) — M08A
θJC (typ) — N08E
θJA (typ) — N08E
−40˚C ≤ TA ≤ +85˚C
2.0V ≤ 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
V (min)
V (max)
mA (max)
Quiescent Power Supply Current
VIN = 0V, IO = 0A (Note 8)
Shutdown Current
Vpin1 = VDD
µA (max)
Output Offset Voltage
VIN = 0V
mV (max)
Output Power
THD = 1% (max); f = 1 kHz
W (min)
Total Harmonic Distortion +
PO = 1Wrms; 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 the Absolute Maximum Ratings, whichever is lower. For the LM4861, TJMAX = 150˚C,
and the typical junction-to-ambient thermal resistance, when board mounted, is 140˚C/W.
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 Nationai’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.
High Gain Application Circuit
FIGURE 2. Audio Ampiifier with AVD = 20
Single Ended Application Circuit
*CS and CB size depend on specific application requirements and constraints. Typical vaiues of CS and CB are 0.1 µF.
**Pin 1 should be connected to VDD to disable the amplifier or to GND to enable the amplifier. This pin should not be left floating.
***These components create a “dummy” load for pin 8 for stability purposes.
FIGURE 3. Single-Ended Amplifier with AV = −1
External Components Description
(Figures 1, 2)
Functional Description
1. Ri
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 Ri Ci).
2. Ci
Input coupling capacitor which blocks DC voltage at the amplifier’s input terminals. Also creates a highpass
filter with Ri at fC = 1/(2π Ri Ci).
3. Rf
Feedback resistance which sets closed-loop gain in conjunction with Ri.
4. CS
Supply bypass capacitor which provides power supply filtering. Refer to the Application Information
section for proper placement and selection of supply bypass capacitor.
5. CB
Bypass pin capacitor which provides half supply filtering. Refer to the Application Information section for
proper placement and selection of bypass capacitor.
6. Cf (Note 9)
Cf in conjunction with Rf creates a low-pass filter which bandwidth limits the amplifier and prevents possible
high frequency oscillation bursts. fC = 1/(2π Rf Cf)
Note 9: Optional component dependent upon specific design requirements. Refer to the Application Information section for more information.
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
THD+N vs Output Power
THD+N vs Output Power
THD+N vs Frequency
THD+N vs Output Power
Typical Performance Characteristics
Output Power vs
Load Resistance
Output Power vs
Supply Voltage
Power Dissipation vs
Output Power
Supply Current Distribution
vs Temperature
Noise Floor vs Frequency
Supply Current vs
Supply Voltage
Power Supply
Rejection Ratio
Power Derating Curve
Open Loop
Frequency Response
625 mW.The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissipation that results from Equation 2:
Application Information
As shown in Figure 1 , the LM4861 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)
PDMAX = (TJMAX − TA)/θJA (2)
For the LM4861 surface mount package, θJA = 140˚C/W and
TJMAX = 150˚C. Depending on the ambient temperature, TA,
of the system surroundings, 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 or the load impedance increased. 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 62.5˚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.
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. 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 Boomer Audio Power Amplifiers, 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, singleended amplifier configuration. Without an output coupling capacitor in a single supply, single-ended amplifier, the halfsupply bias across the load would result in both increased
internal IC power dissipation and also permanent loudspeaker damage. An output coupling capacitor forms a high
pass filter with the load requiring that a large value such as
470 µF be used with an 8Ω load to preserve low frequency
response. This combination does not produce a flat response down to 20 Hz, but does offer a compromise between printed circuit board size and system cost, versus low
frequency response.
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 low frequency THD + N 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 LM4861. The selection of bypass capacitors, especially CB, is thus dependant upon desired low frequency THD + N, system cost, and size constraints.
In order to reduce power consumption while not in use, the
LM4861 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.
Upon going into shutdown, the output is immediately disconnected from the speaker. A typical quiescent current of
0.6 µA results when the supply voltage is applied to the shutdown pin. 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 LM4861. There are no soft pull-down resistors inside the LM4861, 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.
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) (1)
The LM4861 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor may be needed, as
shown in Figure 2, to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be taken when
calculating the −3 dB frequency in that an incorrect combina-
Since the LM4861 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 LM4861 does not require
heatsinking. From Equation 1, assuming a 5V power supply
and an 8Ω load, the maximum power dissipation point is
Application Information
Design a 1W / 8Ω Audio Amplifier
tion of Rf and Cf will cause rolloff before 20 kHz. A typical
combination of feedback resistor and capacitor that will not
produce audio band high frequency rolloff is Rf = 100 kΩ and
Cf = 5 pF. These components result in a −3 dB point of approximately 320 kHz. Once the differential gain of the amplifier has been calculated, a choice of Rf will result, and Cf can
then be calculated from the formula stated in the External
Components Description section.
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 needed supply rail to obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graph 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 + VOD , where VOD is
typically 0.6V.
Many applications, such as telephony, only require a voiceband frequency response. Such an application usually requires a flat frequency response from 300 Hz to 3.5 kHz. By
adjusting the component values of Figure 2, this common
application requirement can be implemented. The combination of Ri and Ci form a highpass filter while Rf and Cf form a
lowpass filter. Using the typical voice-band frequency range,
with a passband differential gain of approximately 100, the
following values of Ri, Ci, Rf, and Cf follow from the equations stated in the External Components Description section.
Ri = 10 kΩ, Rf = 510k ,Ci = 0.22 µF, and Cf = 15 pF
For 1W of output power into an 8Ω load, the required Vopeak
is 4.0V. A minumum supply rail of 4.6V results from adding
Vopeak and Vod. But 4.6V is not a standard voltage that exists
in many applications and for this reason, a supply rail of 5V
is designated. Extra supply voltage creates dynamic headroom that allows the LM4861 to reproduce peaks in excess
of 1Wwithout clipping the signal. 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.
Five times away from a −3 dB point is 0.17 dB down from the
flatband response. With this selection of components, the resulting −3 dB points, fL and fH, are 72 Hz and 20 kHz, respectively, resulting in a flatband frequency response of better than ± 0.25 dB with a rolloff of 6 dB/octave outside of the
passband. If a steeper rolloff is required, other common
bandpass filtering techniques can be used to achieve higher
order filters.
Although the typical application for the LM4861 is a bridged
monoaural amp, it can also be used to drive a load singleendedly in applications, such as PC cards, which require that
one side of the load is tied to ground. Figure 3 shows a common single-ended application, where VO1 is used to drive the
speaker. This output is coupled through a 470 µF capacitor,
which blocks the half-supply DC bias that exists in all singlesupply amplifier configurations. This capacitor, designated
CO in Figure 3, in conjunction with RL, forms a highpass filter.
The −3 dB point of this high pass filter is 1/(2πRLCO), so care
should be taken to make sure that the product of RL and CO
is large enough to pass low frequencies to the load. When
driving an 8Ω load, and if a full audio spectrum reproduction
is required, CO should be at least 470 µF. VO2, the output
that is not used, is connected through a 0.1 µF capacitor to
a 2 kΩ load to prevent instability. While such an instability will
not affect the waveform of VO1, it is good design practice to
load the second output.
Rf/Ri = AVD/2
From equation 4, the minimum Avd is 2.83: Avd = 3
Since the desired input impedance was 20 kΩ, and with a
Avd of 3, a ratio of 1:1.5 of Rf to Ri results in an allocation of
Ri = 20 kΩ, 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. 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 a Avd = 2 and fH = 100 kHz, the resulting GBWP =
100 kHz which is much smaller than the LM4861 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4861 can still be used without running into bandwidth
LM4861 1.1W Audio Power Amplifier with Shutdown Mode
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outllne Package, JEDEC (M)
Order Number LM4861
NS Package Number M08A
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