LM4666
LM4666 Filterless High Efficiency Stereo 1.2W Switching Audio Amplifier
Literature Number: SNAS189
LM4666
Filterless High Efficiency Stereo 1.2W Switching Audio
Amplifier
General Description
The LM4666 is a fully integrated single-supply high efficiency
switching audio amplifier. It features an innovative modulator
that eliminates the LC output filter used with typical switching
amplifiers. Eliminating the output filter reduces parts count,
simplifies circuit design, and reduces board area. The
LM4666 processes analog inputs with a delta-sigma modu-
lation technique that lowers output noise and THD when
compared to conventional pulse width modulators.
The LM4666 is designed to meet the demands of mobile
phones and other portable communication devices. Operat-
ing on a single 3V supply, it is capable of driving 8Ωtrans-
ducer loads at a continuous average output of 450mW with
less than 1%THD+N. Its flexible power supply requirements
allow operation from 2.8V to 5.5V.
The LM4666 has high efficiency with an 8Ωtransducer load
compared to a typical Class AB amplifier. With a 3V supply,
the IC’s efficiency for a 100mW power level is 79%, reaching
84% at 450mW output power.
The LM4666 features a low-power consumption shutdown
mode. Shutdown may be enabled by driving the Shutdown
pin to a logic low (GND).
The LM4666 has fixed selectable gain of either 6dB or 12dB.
The LM4666 has short circuit protection against a short from
the outputs to V
DD
or GND.
Key Specifications
jEfficiency at 3V, 100mW into 8Ωtransducer 79% (typ)
jEfficiency at 3V, 450mW into 8Ωtransducer 84% (typ)
jEfficiency at 5V, 1W into 8Ωtransducer 85% (typ)
jTotal quiescent power supply current 7.0mA (typ)
jTotal shutdown power supply current 0.02µA (typ)
jSingle supply range 2.8V to 5.5V
Features
nNo output filter required for inductive transducers
nSelectable gain of 6dB or 12dB
nVery fast turn on time: 6ms (typ)
nMinimum external components
n"Click and pop" suppression circuitry
nMicro-power shutdown mode
nShort circuit protection
nAvailable in space-saving SDA package
Applications
nMobile phones
nPDAs
nPortable electronic devices
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
200558E6
FIGURE 1. Typical Audio Amplifier Application Circuit
May 2004
LM4666 Filterless High Efficiency Stereo 1.2W Switching Audio Amplifier
© 2004 National Semiconductor Corporation DS200558 www.national.com
Connection Diagrams
SDA Package
200558I8
Top View
Order Number LM4666SDA
See NS Package Number SDA14A
SDA Marking
200558I9
Top View
Z — Plant Code
XY — Date Code
TT — Die Traceability
L4666 — LM4666
LM4666
www.national.com 2
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (Note1) 6.0V
Storage Temperature −65˚C to +150˚C
Voltage at Any Input Pin V
DD
+ 0.3V ≥V≥GND - 0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility,
pins 4, 7, 11, 14 (Note 4) 1kV
ESD Susceptibility,
all other pins (Note 4) 2.0kV
ESD Susceptibility (Note 5) 200V
Junction Temperature (T
J
) 150˚C
Thermal Resistance
θ
JA
(SDA) 63˚C/W
θ
JC
(SDA) 12˚C/W
Soldering Information
See AN-1112 "microSMD Wafers Level Chip Scale
Package."
Operating Ratings (Notes 1, 2)
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2.8V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
=5V (Notes 1, 2)
The following specifications apply for V
DD
= 5V and R
L
=15µH+8Ω+ 15µH unless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions
LM4666 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 8)
I
DD
Quiescent Power Supply Current V
IN
= 0V, No Load
V
IN
= 0V, R
L
=15µH+8Ω+ 15µH
15
16
mA
mA
I
SD
Shutdown Current V
SD
= GND (Note 9) 0.02 µA
V
SDIH
Shutdown Voltage Input High 1.2 V
V
SDIL
Shutdown Voltage Input Low 1.1 V
V
GSIH
Gain Select Input High 1.2 V
V
GSIL
Gain Select Input Low 1.1 V
A
V
Closed Loop Gain V
Gain Select
=V
DD
6dB
A
V
Closed Loop Gain V
Gain Select
= GND 12 dB
V
OS
Output Offset Voltage 10 mV
T
WU
Wake-up Time 6 ms
P
o
Output Power THD = 1% (max), f = 1kHz,
22kHz BW 1.2 W
THD+N Total Harmonic Distortion+Noise
P
O
= 100mW
RMS
/Channel,
f
IN
= 1kHz, 22kHz BW,
Both channels in phase
0.65 %
X
TALK
Channel Separation P
O
= 100mW
RMS
, f = 1kHz 57 dB
R
IN
Differential Input Resistance V
Gain Select
=V
DD
90 kΩ
V
Gain Select
= GND 60 kΩ
PSRR Power Supply Rejection Ratio
V
Ripple
= 100mV
RMS
sine wave,
f
RIPPLE
= 217Hz
Inputs terminated to AC GND
60 dB
V
Ripple
= 100mV
RMS
sine wave,
f
RIPPLE
= 217Hz
P
OUT
= 10mW,1kHz
65 dB
CMRR Common Mode Rejection Ratio V
Ripple
= 100mV
RMS
,
f
Ripple
= 217Hz, Input referred 48 dB
SNR Signal to Noise Ratio P
O
=1W
RMS
; A-Weighted Filter 83 dB
e
OUT
Output Noise A-Weighted filter, V
in
= 0V 200 µV
LM4666
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Electrical Characteristics V
DD
=3V (Notes 1, 2)
The following specifications apply for V
DD
= 3V and R
L
=15µH+8Ω+ 15µH unless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions
LM4666 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 8)
I
DD
Quiescent Power Supply Current V
IN
= 0V, No Load
V
IN
= 0V, R
L
=15µH+8Ω+ 15µH
6.5
7.0 10 mA (max)
I
SD
Shutdown Current V
SD
= GND (Note 9) 0.02 2.0 µA (max)
V
SDIH
Shutdown Voltage Input High 1.0 1.4 V (min)
V
SDIL
Shutdown Voltage Input Low 0.8 0.4 V (max)
V
GSIH
Gain Select Input High 1.0 1.4 V (min)
V
GSIL
Gain Select Input Low 0.8 0.4 V (max)
A
V
Closed Loop Gain V
Gain Select
=V
DD
65.25
6.75
dB (min)
dB (max)
A
V
Closed Loop Gain V
Gain Select
= GND 12 11.25
12.75
dB (min)
dB (max)
V
OS
Output Offset Voltage 10 35 mV (max)
T
WU
Wake-up Time 6 ms
P
o
Output Power THD = 1% (max); f = 1kHz,
22kHz BW 450 400 mW (min)
THD+N Total Harmonic Distortion+Noise
P
O
= 100mW
RMS
/Channel,
f
IN
= 1kHz, 22kHz BW,
Both channels in phase
0.65 %
X
TALK
Channel Separation P
O
= 100mW
RMS
, f = 1kHz 57 dB
R
IN
Differential Input Resistance V
Gain Select
=V
DD
90 kΩ
V
Gain Select
= GND 60 kΩ
PSRR Power Supply Rejection Ratio
V
ripple
= 100mV
RMS
sine wave,
f
RIPPLE
= 217Hz,
Inputs terminated to AC GND
60 dB
V
Ripple
= 100mV
RMS
sine wave,
f
RIPPLE
= 217Hz,
P
OUT
= 10mW,1kHz
65 dB
CMRR Common Mode Rejection Ratio V
Ripple
= 100mV
RMS
,
f
Ripple
= 217Hz, Input referred 48 dB
SNR Signal to Noise Ratio P
O
= 400mW
RMS
, A-Weighted Filter 83 dB
e
OUT
Output Noise A-Weighted filter, V
in
= 0V 125 µV
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 =(T
JMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4666, TJMAX = 150˚C.
The typical θJA is 63˚C/W and the typical θJC is 12˚C/W for the SDA package.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model, 220pF – 240pF discharged through all pins.
Note 6: Typical specifications are specified at 25˚C and represent the parametric norm.
Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: Shutdown current is measured in a normal room environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA. The Shutdown pin should
be driven as close as possible to GND for minimal shutdown current and to VDD for the best THD performance in PLAY mode. See the Application Information
section under SHUTDOWN FUNCTION for more information.
Note 10: The performance graphs were taken using the Audio Precision AUX–0025 Switching Amplifier Measurement Filter in series with the LC filter on the demo
board.
LM4666
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External Components Description
(Figure 1)
Components Functional Description
1. C
S
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.
2. C
I
Input AC coupling capacitor which blocks the DC voltage at the amplifier’s input terminals.
Typical Performance Characteristics (Note 10)
THD+N vs Frequency
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH
P
OUT
= 100mW/Channel, 30kHz BW
THD+N vs Frequency
V
DD
= 3V, R
L
=15µH+8Ω+ 15µH
P
OUT
= 100mW/Channel, 30kHz BW
20055827 20055826
THD+N vs Frequency
V
DD
= 3V, R
L
=15µH+4Ω+ 15µH
P
OUT
= 100mW/Channel, 30kHz BW
THD+N vs Output Power/Channel
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH
f = 1kHz, 22kHz BW
20055825 20055830
LM4666
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Typical Performance Characteristics (Note 10) (Continued)
THD+N vs Output Power/Channel
V
DD
= 3V, R
L
=15µH+4Ω+ 15µH
f = 1kHz, 22kHz BW
THD+N vs Output Power/Channel
V
DD
= 3V, R
L
=15µH+8Ω+ 15µH
f = 1kHz, 22kHz BW
20055828 20055829
CMRR vs Frequency
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH
V
CM
= 100mV
RMS
Sine Wave, 30kHz BW
CMRR vs Frequency
V
DD
= 3V, R
L
=15µH+8Ω+ 15µH
V
CM
= 100mV
RMS
Sine Wave, 30kHz BW
20055811 20055810
PSRR vs Frequency
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH
V
Ripple
= 100mV
RMS
Sine Wave, 22kHz BW
PSRR vs Frequency
V
DD
= 3V, R
L
=15µH+8Ω+ 15µH
V
Ripple
= 100mV
RMS
Sine Wave, 22kHz BW
20055819 20055818
LM4666
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Typical Performance Characteristics (Note 10) (Continued)
Efficiency and Power Dissipation
vs Output Power
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH, f = 1kHz, THD ≤1%
Efficiency and Power Dissipation
vs Output Power
V
DD
= 3V, R
L
=15µH+8Ω+ 15µH, f = 1kHz, THD ≤1%
20055814 20055813
Efficiency and Power Dissipation
vs Output Power
V
DD
= 3V, R
L
=15µH+4Ω+ 15µH, f = 1kHz, THD ≤1%
Gain Select Threshold
V
DD
=3V
20055812 200558H6
Gain Select Threshold
V
DD
=5V
Gain Select Threshold
vs Supply Voltage
R
L
=15µH+8Ω+ 15µH
200558H1 20055815
LM4666
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Typical Performance Characteristics (Note 10) (Continued)
Output Power/Channel vs Supply Voltage
R
L
=15µH+8Ω+ 15µH, f = 1kHz
22kHz BW
Output Power/Channel vs Supply Voltage
R
L
=15µH+4Ω+ 15µH, f = 1kHz
22kHz BW
20055817 20055816
Shutdown Threshold
V
DD
=5V
Shutdown Threshold
V
DD
=3V
20055822 20055821
Shutdown Threshold
vs Supply Voltage
R
L
=15µH+8Ω+ 15µH
Supply Current
vs Shutdown Voltage
R
L
=15µH+8Ω+ 15µH
20055820 20055823
LM4666
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Application Information
GENERAL AMPLIFIER FUNCTION
The output signals generated by the LM4666 consist of two,
BTL connected, output signals that pulse momentarily from
near ground potential to V
DD
on each channel. The two
outputs on a given channel can pulse independently with the
exception that they both may never pulse simultaneously as
this would result in zero volts across the BTL connected
load. The minimum width of each pulse is approximately
160ns. However, pulses on the same output can occur se-
quentially, in which case they are concatenated and appear
as a single wider pulse to achieve an effective 100% duty
cycle. This results in maximum audio output power for a
given supply voltage and load impedance. The LM4666 can
achieve much higher efficiencies than class AB amplifiers
while maintaining acceptable THD performance.
The short (160ns) drive pulses emitted at the LM4666 out-
puts means that good efficiency can be obtained with mini-
mal load inductance. The typical transducer load on an audio
amplifier is quite reactive (inductive). For this reason, the
load can act as it’s own filter, so to speak. This "filter-less"
switching amplifier/transducer load combination is much
more attractive economically due to savings in board space
and external component cost by eliminating the need for a
filter.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of
useful work output divided by the total energy required to
produce it with the difference being the power dissipated,
typically, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered useful including the distortion products of the input
signal. Sub-sonic (DC) and super-sonic components
(>22kHz) are not useful. The difference between the power
flowing from the power supply and the audio band power
being transduced is dissipated in the LM4666 and in the
transducer load. The amount of power dissipation in the
LM4666 is very low. This is because the ON resistance of the
switches used to form the output waveforms is typically less
than 0.25Ω. This leaves only the transducer load as a po-
tential "sink" for the small excess of input power over audio
band output power. The LM4666 dissipates only a fraction of
the excess power requiring no additional PCB area or cop-
per plane to act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are
increasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage swing.
The LM4666 is a fully differential amplifier that features
differential input and output stages. A differential amplifier
amplifies the difference between the two input signals. Tra-
ditional audio power amplifiers have typically offered only
single-ended inputs resulting in a 6dB reduction in signal to
noise ratio relative to differential inputs. The LM4666 also
offers the possibility of DC input coupling which eliminates
the two external AC coupling, DC blocking capacitors. The
LM4666 can be used, however, as a single ended input
amplifier while still retaining it’s fully differential benefits. In
fact, completely unrelated signals may be placed on the
input pins. The LM4666 simply amplifies the difference be-
tween the signals. A major benefit of a differential amplifier is
the improved common mode rejection ratio (CMRR) over
single input amplifiers. The common-mode rejection charac-
teristic of the differential amplifier reduces sensitivity to
ground offset related noise injection, especially important in
high noise applications.
PCB LAYOUT CONSIDERATIONS
As output power increases, interconnect resistance (PCB
traces and wires) between the amplifier, load and power
supply create a voltage drop. The voltage loss on the traces
between the LM4666 and the load results is lower output
power and decreased efficiency. Higher trace resistance
between the supply and the LM4666 has the same effect as
a poorly regulated supply, increase ripple on the supply line
also reducing the peak output power. The effects of residual
trace resistance increases as output current increases due
to higher output power, decreased load impedance or both.
To maintain the highest output voltage swing and corre-
sponding peak output power, the PCB traces that connect
the output pins to the load and the supply pins to the power
supply should be as wide as possible to minimize trace
resistance.
The rising and falling edges are necessarily short in relation
to the minimum pulse width (160ns), having approximately
2ns rise and fall times, typical, depending on parasitic output
capacitance. The inductive nature of the transducer load can
also result in overshoot on one or both edges, clamped by
the parasitic diodes to GND and V
DD
in each case. From an
EMI standpoint, this is an aggressive waveform that can
radiate or conduct to other components in the system and
cause interference. It is essential to keep the power and
output traces short and well shielded if possible. Use of
ground planes, beads, and micro-strip layout techniques are
all useful in preventing unwanted interference.
As the distance from the LM4666 and the speakers increase
the amount of EMI radiation will increase since the output
wires or traces acting as antenna become more efficient with
length. What is acceptable EMI is highly application specific.
Ferrite chip inductors placed close to the LM4666 may be
needed to reduce EMI radiation. The value of the ferrite chip
is very application specific.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor (C
S
) location should be
as close as possible to the LM4666. Typical applications
employ a voltage regulator with a 10µF and a 0.1µF bypass
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing on the supply pin of the
LM4666. A 1µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4666 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics
Tables and in the Shutdown Hysteresis Voltage graphs
found in the Typical Performance Characteristics section.
It is best to switch between ground and supply for minimum
current usage while in the shutdown state. While the
LM4666 may be disabled with shutdown voltages in between
ground and supply, the idle current will be greater than the
LM4666
www.national.com 10
Application Information (Continued)
typical value. Increased THD may also be observed with
voltages less than V
DD
on the Shutdown pin when in PLAY
mode.
The LM4666 has an internal resistor connected between
GND and Shutdown pins. The purpose of this resistor is to
eliminate any unwanted state changes when the Shutdown
pin is floating. The LM4666 will enter the shutdown state
when the Shutdown pin is left floating or if not floating, when
the shutdown voltage has crossed the threshold. To mini-
mize the supply current while in the shutdown state, the
Shutdown pin should be driven to GND or left floating. If the
Shutdown pin is not driven to GND, the amount of additional
resistor current due to the internal shutdown resistor can be
found by Equation (1) below.
(V
SD
- GND) / 60kΩ(1)
With only a 0.5V difference, an additional 8.3µA of current
will be drawn while in the shutdown state.
GAIN SELECTION FUNCTION
The LM4666 has fixed selectable gain to minimize external
components, increase flexibility and simplify design. For a
differential gain of 6dB, the Gain Select pin should be per-
manently connected to V
DD
or driven to a logic high level.
For a differential gain of 12dB, the Gain Select pin should be
permanently connected to GND or driven to a logic low level.
The gain of the LM4666 can be switched while the amplifier
is in PLAY mode driving a load with a signal without damage
to the IC. The voltage on the Gain Select pin should be
switched quickly between GND (logic low) and V
DD
(logic
high) to eliminate any possible audible artifacts from appear-
ing at the output. For typical threshold voltages for the Gain
Select function, refer to the Gain Threshold Voltages graph
in the Typical Performance Characteristics section.
LM4666
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Application Information (Continued)
CIRCUIT CONFIGURATIONS
20055803
FIGURE 2. Single-Ended input with low gain selection configuration
20055802
FIGURE 3. Differential input with low gain selection configuration
LM4666
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Application Information (Continued)
REFERENCE DESIGN BOARD SCHEMATIC
In addition to the minimal parts required for the application
circuit, a measurement filter is provided on the evaluation
circuit board so that conventional audio measurements can
be conveniently made without additional equipment. This is a
balanced input, grounded differential output low pass filter
with a 3dB frequency of approximately 35kHz and an on
board termination resistor of 300Ω(see schematic). Note
that the capacitive load elements are returned to ground.
This is not optimal for common mode rejection purposes, but
due to the independent pulse format at each output there is
a significant amount of high frequency common mode com-
ponent on the outputs. The grounded capacitive filter ele-
ments attenuate this component at the board to reduce the
high frequency CMRR requirement placed on the analysis
instruments.
Even with the grounded filter the audio signal is still differ-
ential necessitating a differential input on any analysis instru-
ment connected to it. Most lab instruments that feature BNC
connectors on their inputs are NOT differential responding
because the ring of the BNC is usually grounded.
The commonly used Audio Precision analyzer is differential
but its ability to accurately reject fast pulses of 160ns width is
questionable necessitating the on board measurement filter.
When the signal needs to be single-ended, use an audio
signal transformer to convert the differential output to a
single ended output. Depending on the audio transformer’s
characteristics, there may be some attenuation of the audio
signal which needs to be taken into account for correct
measurement of performance.
Measurements made at the output of the measurement filter
suffer attenuation relative to the primary, unfiltered outputs
even at audio frequencies. This is due to the resistance of
the inductors interacting with the termination resistor (300Ω)
and is typically about -0.35dB (4%). In other words, the
voltage levels and corresponding power levels indicated
through the measurement filter are slightly lower than those
that actually occur at the load placed on the unfiltered out-
puts. This small loss in the filter for measurement gives a
lower output power reading than what is really occurring on
the unfiltered outputs and its load.
The AUX-0025 Switching Amplifier Measurement Filter from
Audio Precision may be used instead of the on board mea-
surement filter. The AUX-0025 filter should be connected to
the high current direct outputs on the evaluation board and in
series with the measurement equipment. Attaching oscillo-
scope probes on the outputs of the AUX-0025 filter will
display the audio waveforms. The AUX-0025 filter may also
be connected to the on board filter without any adverse
effects.
20055801
FIGURE 4.
LM4666
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Application Information (Continued)
LM4666 SDA BOARD ARTWORK
Composite View Silk Screen
20055805 20055808
Top Layer Internal Layer 1, GND
20055809 20055806
Internal Layer 2, V
DD
Bottom Layer
20055807 20055804
LM4666
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Physical Dimensions inches (millimeters) unless otherwise noted
LLP Package
Order NumberLM4666SD
NS Package Number SDA14A
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LM4666 Filterless High Efficiency Stereo 1.2W Switching Audio Amplifier
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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