NSC LM4860

Series 1W Audio Power Amplifier with Shutdown Mode
General Description
The LM4860 is a bridge-connected audio power amplifier capable of delivering 1W of continuous average power to an
8Ω load with less than 1% (THD+N) over the audio spectrum
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 using surface mount packaging. Since
the LM4860 does not require output coupling capacitors,
bootstrap capacitors or snubber networks, it is optimally
suited for low-power portable systems.
The LM4860 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism. It also includes two headphone control inputs and a headphone sense output for external monitoring.
The unity-gain stable LM4860 can be configured by external
gain setting resistors for differential gains of 1 to 10 without
the use of external compensation components.
Key Specifications
n THD+N at 1W continuous average
Typical Application
output power into 8Ω: 1% (max)
n Instantaneous peak output power:
n Shutdown current: 0.6 µA (typ)
> 2W
n No output coupling capacitors, bootstrap capacitors, or
snubber circuits are necessary
n Small Outline (SO) power packaging
n Compatible with PC power supplies
n Thermal shutdown protection circuitry
n Unity-gain stable
n External gain configuration capability
n Two headphone control inputs and headphone sensing
Personal computers
Portable consumer products
Cellular phones
Self-powered speakers
Toys and games
Connection Diagram
Small Outline Package
Top View
Order Number LM4860M
See NS Package Number M16A
FIGURE 1. Typical Audio Amplifier Application Circuit
The Boomer ® registered trademark is licensed to National Semiconductor for audio integrated circuits by Rockford Corporation.
Patents pending.
© 1999 National Semiconductor Corporation
LM4860 1W Audio Power Amplifier with Shutdown Mode
August 1994
Absolute Maximum Ratings (Note 2)
Small Outline Package
Vapor Phase (60 sec.)
Infrared (15 sec.)
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
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Soldering Information
−65˚C to +150˚C
−0.3V to VDD + 0.3V
Internally limited
Operating Ratings
Temperature Range
Supply Voltage
−20˚C ≤ TA ≤ +85˚C
2.7V ≤ VDD ≤ 5.5V
Electrical Characteristics
(Notes 1, 2) The following specifications apply for VDD = 5V, RL = 8Ω 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 + Noise
Power Supply Rejection Ratio
V (min)
V (max)
mA (max)
VO = 0V, IO = 0A (Note 8)
Vpin2 = VDD (Note 9)
VIN = 0V
THD+N = 1% (max); f = 1 kHz
PO = 1 Wrms; 20 Hz ≤ f ≤ 20 kHz
VDD = 4.9V to 5.1V
mV (max)
W (min)
VIN = 0V to 5V, Vod = (Vo1 − Vo2)
HP-SENSE = 0V to 4V
HP-SENSE High Output Voltage
HP-SENSE = 4V to 0V
IO = 500 µA
V (min)
HP-SENSE Low Output Voltage
IO = −500 µA
V (max)
Output Dropout Voltage
HP-IN High Input Voltage
HP-IN Low Input Voltage
V (max)
Note 1: All voltages are measured with respect to the ground pins, 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 LM4860, TJMAX =
+150˚C, and the typical junction-to-ambient thermal resistance, when board mounted, is 100˚C/W.
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.
Note 9: Shutdown current has a wide distribution. For Power Management sensitive designs, contact your local National Semiconductor Sales Office.
High Gain Application Circuit
FIGURE 2. Stereo Amplifier with AVD = 20
Single Ended Application Circuit
*CS and CB size depend on specific application requirements and constraints. Typical values of CS and CB are 0.1 µF.
**Pin 2, 6, or 7 should be connected to VDD to disable the amplifier or to GND to enable the amplifier. These pins 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
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).
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).
Feedback resistance which sets closed-loop gain in conjunction with Ri.
Supply bypass capacitor which provides power supply filtering. Refer to the Application Information
section for proper placement and selection of supply bypass capacitor.
Single Ended Application Circuit
External Components Description
(Figures 1, 2)
6. Cf (Note 10)
Functional Description
Bypass pin capacitor which provides half supply filtering. Refer to Application Information section for
proper placement and selection of bypass capacitor.
Used when a differential gain of over 10 is desired. Cf in conjunction with Rf creates a low-pass filter
which bandwidth limits the amplifier and prevents high frequency oscillation bursts. fC = 1/(2π Rf Cf)
Note 10: Optional component dependent upon specific design requirements. Refer to the Application Information section for more in formation.
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
Supply Current vs Time
in Shutdown Mode
THD+N vs Output Power
Supply Current vs
Supply Voltage
Power Derating Curve
Typical Performance Characteristics
LM4860 Noise Floor
vs Frequency
Supply Current Distribution
vs Temperature
Output Power vs
Load Resistance
Power Dissipation
vs Output Power
Output Power vs
Supply Voltage
Open Loop
Frequency Response
Power Supply
Rejection Ratio
Application Information
plifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for the IC is:
Avd = 2 * (Rf/Ri)
As shown in Figure 1, the LM4860 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 am-
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
Application Information
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 LM4860. The selection of bypass capacitors, especially CB, is thus dependant upon desired low frequency THD+N, system cost, and size constraints.
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 Deslgn 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,
single-ended amplifier configuration. Without an output coupling capacitor in a single supply single-ended amplifier, the
half-supply 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.
In order to reduce power consumption while not in use, the
LM4860 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. There is a built-in threshold which
produces a drop in quiescent current to 500 µA typically. For
a 5V power supply, this threshold occurs when 2V–3V is applied to the shutdown pin. 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 LM4860. There are no soft pull-down resistors inside the LM4860, so a definite shutdown pin voltage
must be appliied 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
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)
The LM4860 possesses two headphone control inputs that
disable the amplifier and reduce IDD to less than 1 mA when
either one or both of these inputs have a logic-high voltage
placed on their pins.
Unlike the shutdown function, the headphone control function does not provide the level of current conservation that is
required for battery powered systems. Since the quiescent
current resulting from the headphone control function is
1000 times more than the shutdown function, the residual
currents in the device may create a pop at the output when
coming out of the headphone control mode. The pop effect
may be eliminated by connecting the headphone sensing
output to the shutdown pin input as shown in Figure 4. This
solution will not only eliminate the output pop, but will also
utilize the full current conservation of the shutdown function
by reducing IDD to 0.6 µA. The amplifier will then be fully
shutdown. This configuration also allows the designer to use
the control inputs as either two headphone control pins or a
headphone control pin and a shutdown pin where the lowest
level of current consumption is obtained from either function.
Since the LM4860 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 LM4860 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 the LM4860 surface mount package, θJA = 100˚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 88˚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.
Figure 5 shows the implementation of the LM4860’s headphone control function using a single-supply headphone amplifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN1 pin to be approximately 50 mV when there are
no headphones plugged into the system. This logic-low voltage at the HP-IN1 pin enables the LM4860 to amplify AC signals. Resistor R3 limits the amount of current flowing out of
the HP-IN1 pin when the voltage at that pin goes below
Application Information
to indicate to control inputs that the user has inserted a plug
into a jack and that another mode of operation is desired.
For a system implementation where the headphone amplifier
is designed using a split supply, the output coupling cap, CC
and resistor R2 of Figure 5, can be eliminated. The functionality described earlier remains the same, however.
In addition, the HP-SENSE pin, although it may be connected to the SHUTDOWN pin as shown in Figure 4, may
still be used as a control flag. It is capable of driving the input
to another logic gate or approximately 2 mA without serious
ground resulting from the music coming from the headphone
amplifier. The output coupling cap protects the headphones
by blocking the amplifier’s half-supply DC voltage. The capacitor also protects the headphone amplifier from the low
voltage set up by resistors R1 and R2 when there aren’t any
headphones plugged into the system. The tricky point to this
setup is that the AC output voltage of the headphone amplifier cannot exceed the 2.0V HP-IN1 voltage threshold when
there aren’t any headphones plugged into the system, assuming that R1 and R2 are 100k and 1k, respectively. The
LM4860 may not be fully shutdown when this level is exceeded momentarily, due to the discharging time constant of
the bias-pin voltage. This time constant is established by the
two 50k resistors (in parallel) with the series bypass capacitor value.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, enabling the headphone function and disabling the LM4860
amplifier. The headphone amplifier then drives the headphones, whose impedance is in parallel with resistor R2.
Since the typical impedance of headphones are 32Ω, resistor R2 has negligible effect on the output drive capability.
Also shown in Figure 5 are the electrical connections for the
headphone jack and plug. A 3-wire plug consists of a Tip,
Ring, and Sleave, where the Tip and Ring are signal carrying
conductors and the Sleave is the common ground return.
One control pin contact for each headphone jack is sufficient
SHUTDOWN Pin Connection
FIGURE 5. Typical Headphone Control Input Circuitry
Application Information
ity. While such an instability will not affect the waveform of
VO1, it is good design practice to load the second output.
The LM4860 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, then a feedback capacitor is needed, as
shown in Figure 2, to bandwidth limit the amplifier. The feedback capacitor creates a low pass filter that eliminates unwanted high frequency oscillations. Care should be taken
when calculating the −3 dB frequency in that an incorrect
combination 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.
Design a 500 mW/8Ω Audio Amplifier
Power Output: 500 mWrms
Load Impedance: 8Ω
Input Level: 1 Vrms(max)
Input Impedance: 20 kΩ
Bandwidth: 20 Hz-20 kHz ± 0.25 dB
A designer must first determine the needed supply rail to obtain the specified output power. Calculating the required supply rail involves knowing two parameters, Vopeak and also the
dropout voltage. The latter is typically 0.7V. Vopeak can be
determined from equation 3.
Many applications, such as telephony, only require a
voice-band 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 500 mW of output power into an 8Ω load, the required
Vopeak is 2.83V. A minimum supply rail of 3.53V results from
adding Vopeak and Vod. But 3.53V 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 LM4860 to reproduce peaks
in excess of 500 mW without 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.
From equation 4, the minimum Avd is: Avd = 2
Since the desired input impedance was 20 kΩ, and with an
Avd of 2, a ratio of 1:1 of Rf to Riresults in an allocation of
Ri = Rf = 20 kΩ. Since the Avd was less than 10, a feedback
capacitor is not needed. 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 4 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 * 4 Hz) = 1.98 µF; use 2.2 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 LM4860 GBWP of
7 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4860 can still be used without running into bandwidth
Although the typical application for the LM4860 is a bridged
monoaural amp, it can also be used to drive a load
single-endedly 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 single-supply amplifier configurations. This capacitor, designated CO in Figure 3, in conjunction with RL,
forms a highpass filter. The −3 dB point of this highpass 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 instabil-
LM4860 1W Audio Power Amplifier with Shutdown Mode
Physical Dimensions
inches (millimeters) unless otherwise noted
Small Outline Package (M)
Order Number LM4860M
NS Package Number M16A
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