LM4866
LM4866 2.2W Stereo Audio Amplifier
Literature Number: SNAS137D
LM4866
2.2W Stereo Audio Amplifier
General Description
The LM4866 is a bridge-connected (BTL) stereo audio
power amplifier which, when connected to a 5V supply,
delivers 2.2W to a 4Ωload (Note 1) or 2.5W to a 3Ωload
(Note 2) with less than 1.0% THD+N.
With the LM4866 packaged in the LLP, the customer benefits
include low thermal impedance, low profile, and small size.
This package minimizes PCB area and maximizes output
power.
The LM4866 features an externally controlled, low-power
consumption shutdown mode, and thermal shutdown protec-
tion. It also utilizes circuitry to reduce “clicks and pops”
during device turn-on.
Boomer audio power amplifiers are designed specifically to
use few external components and provide high quality output
power in a surface mount package.
Note 1: An LM4866MTE or LM4866LQ that has been properly mounted to
a circuit board will deliver 2.2W into 4Ω. The other package options for the
LM4866 will deliver 1.1W into 8Ω. See the Application Information sections
for further information concerning the LM4866MTE and LM4866LQ.
Note 2: An LM4866MTE or LM4866LQ that has been properly mounted to a
circuit board will deliver 2.5W into 3Ω.
Key Specifications
nP
O
at 1% THD+N
nLM4866LQ, 3Ω,4Ωloads 2.5W(typ), 2.2W(typ)
nLM4866MTE, 3Ω,4Ωloads 2.5W(typ), 2.2W(typ)
nLM4866MTE, 8Ωload 1.1W(typ)
nLM4866MT, 8Ωload 1.1W(typ)
nShutdown current 0.7µA(typ)
nSupply voltage range 2.0V to 5.5V
Features
nStereo BTL amplifier mode
n“Click and pop” suppression circuitry
nUnity-gain stable
nThermal shutdown protection circuitry
nTSSOP and Exposed-DAP LLP packages
Applications
nMultimedia monitors
nPortable and desktop computers
nPortable televisions
Typical Application
20018601
Note: Pin out shown for LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package.
Boomer®is a registered trademark of National Semiconductor Corporation.
April 2005
LM4866 2.2W Stereo Audio Amplifier
© 2005 National Semiconductor Corporation DS200186 www.national.com
Connection Diagrams
20018629
Top View
Order Number LM4866MT
See NS Package Number MTC20 for TSSOP
20018630
Top View
Order Number LM4866LQ
See NS Package Number LQA24A for Exposed-DAP LLP
20018643
Top View
Order Number LM4866MTE
See NS Package Number MXA20A for Exposed-DAP TSSOP
LM4866
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Absolute Maximum Ratings (Note 3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
+0.3V
Power Dissipation (Note 4) Internally limited
ESD Susceptibility(Note 5) 2000V
ESD Susceptibility (Note 6) 200V
Junction Temperature 150˚C
Solder Information
Small Outline Package
Vapor Phase (60 sec.) 215˚C
Infrared (15 sec.) 220˚C
See AN-450 “Surface Mounting and their Effects on
Product Reliablilty” for other methods of soldering
surface mount devices.
Thermal Resistance
θ
JC
(typ) — MTC20 20˚C/W
θ
JA
(typ) — MTC20 80˚C/W
θ
JC
(typ) — LQ24A 3.0˚C/W
θ
JA
(typ) — LQ24A 42˚C/W (Note 7)
θ
JC
(typ) — MXA20A 2˚C/W
θ
JA
(typ) — MXA20A 41˚C/W (Note 8)
θ
JA
(typ) — MXA20A 51˚C/W (Note 9)
θ
JA
(typ) — MXA20A 90˚C/W (Note 10)
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2.0V ≤V
DD
≤5.5V
Electrical Characteristics for Entire IC (Notes 3, 11)
The following specifications apply for V
DD
= 5V unless otherwise noted. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4866 Units
(Limits)
Typical Limit
(Note 12) (Note 13)
V
DD
Supply Voltage 2 V (min)
5.5 V (max)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A (Note 14) 11.5 20
6
mA (max)
mA (min)
I
SD
Shutdown Current V
DD
applied to the SHUTDOWN pin 0.7 2 µA (max)
Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11)
The following specifications apply for V
DD
= 5V unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4866 Units
(Limits)
Typical Limit
(Note 12) (Note 13)
V
OS
Output Offset Voltage V
IN
= 0V 5 50 mV (max)
P
O
Output Power (Note 15) THD+N = 1%, f = 1kHz (Note 16)
LM4866MTE, R
L
=3Ω2.5 W
LM4866LQ, R
L
=3Ω2.5 W
LM4866MTE, R
L
=4Ω2.2 W
LM4866LQ, R
L
=4Ω2.2 W
LM4866MT, R
L
=8Ω1.1 1.0 W (min)
THD+N = 10%, f = 1kHz
LM4866MTE, R
L
=3Ω3.2 W
LM4866LQ, R
L
=3Ω3.2 W
LM4866MTE, R
L
=4Ω2.7 W
LM4866LQ, R
L
=4Ω2.7 W
LM4866MT, R
L
=8Ω1.5 W
LM4866
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Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11) (Continued)
The following specifications apply for V
DD
= 5V unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4866 Units
(Limits)
Typical Limit
(Note 12) (Note 13)
THD+N Total Harmonic Distortion+Noise 20Hz ≤f≤20kHz, A
VD
=2
LM4866MTE, R
L
=4Ω,P
O
=2W
LM4866LQ, R
L
=4Ω,P
O
=2W
LM4866MT, R
L
=4Ω,P
O
=1W
0.3
0.3
0.3
LM4866MT, R
L
=8Ω,P
O
= 1W 0.3 %
PSRR Power Supply Rejection Ratio V
DD
= 5V, V
RIPPLE
= 200mV
RMS
,R
L
=8Ω,
C
B
= 1.0µF
67 dB
X
TALK
Channel Separation f = 1kHz, C
B
= 1.0µF 90 dB
SNR Signal To Noise Ratio V
DD
= 5V, P
O
= 1.1W, R
L
=8Ω98 dB
Note 3: 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 4: The maximum power dissipation is dictated by TJMAX,θJA, and the ambient temperature TAand must be derated at elevated temperatures. The maximum
allowable power dissipation is PDMAX =(T
JMAX −T
A)/θJA. For the LM4866, TJMAX = 150˚C. For the θJAs for different packages, please see the Application
Information section or the Absolute Maximum Ratings section.
Note 5: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 6: Machine model, 220pF–240pF discharged through all pins.
Note 7: The given θJA is for an LM4866 packaged in an LQA24A with the exposed−DAP soldered to an exposed 2in2area of 1oz printed circuit board copper.
Note 8: The given θJA is for an LM4866 packaged in an MXA20A with the exposed−DAP soldered to an exposed 2in2area of 1oz printed circuit board copper.
Note 9: The given θJA is for an LM4866 packaged in an MXA20A with the exposed−DAP soldered to an exposed 1in2area of 1oz printed circuit board copper.
Note 10: The given θJA is for an LM4866 packaged in an MXA20A with the exposed−DAP not soldered to prinbted circuit board copper.
Note 11: All voltages are measured with respect to the ground (GND) pins unless otherwise specified.
Note 12: Typicals are measured at 25˚C and represent the parametric norm.
Note 13: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 14: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Note 15: Output power is measured at the device terminals.
Note 16: When driving 3Ωor 4Ωloads and operating on a 5V supply, the LM4866LQ and LM4866MTE must be mounted to a circuit board that has a minimum of
2.5in2of exposed, uninterrupted copper area connected to the package’s exposed DAP.
LM4866
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Typical Performance Characteristics
LQ Specific Characteristics
LM4866LQ
THD+N vs Output Power
LM4866LQ
THD+N vs Frequency
20018666 20018668
LM4866LQ
THD+N vs Output Power
LM4866LQ
THD+N vs Frequency
20018665 20018667
LM4866LQ
Power Dissipation vs Power Output
LM4866LQ (Note 17)
Power Derating Curve
20018664 20018695
Note 17: This curve shows the LM4866LQ’s thermal dissipation ability at different ambient temperatures given this condition:
The LLP package’s DAP is soldered to a 2.5in2, 1oz. copper plane.
LM4866
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Typical Performance Characteristics
MTE Specific Characteristics
LM4866MTE
THD+N vs Output Power
LM4866MTE
THD+N vs Frequency
20018637 20018638
LM4866MTE
THD+N vs Output Power
LM4866MTE
THD+N vs Frequency
20018639 20018640
LM4866MTE
Power Dissipation vs Power Output
LM4866MTE(Note 18)
Power Derating Curve
20018641 20018642
Note 18: This curve shows the LM4866MTE’s thermal dissipation ability at different ambient temperatures given these conditions:
500LFPM + JEDEC board: The part is soldered to a 1S2P 20-lead exposed-DAP TSSOP test board with 500 linear feet per minute of forced-air flow across
it. Board information - copper dimensions: 74x74mm, copper coverage: 100% (buried layer) and 12% (top/bottom layers), 16 vias under the exposed-DAP.
500LFPM + 2.5in2:The part is soldered to a 2.5in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.
2.5in2:The part is soldered to a 2.5in2, 1oz. copper plane.
Not Attached: The part is not soldered down and is not forced-air cooled.
LM4866
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Typical Performance Characteristics
THD+N vs Frequency THD+N vs Output Power
20018603 20018606
THD+N vs Output Power THD+N vs Frequency
20018661 20018663
THD+N vs Output Power THD+N vs Frequency
20018660 20018662
LM4866
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Typical Performance Characteristics (Continued)
Output Power vs
Supply Voltage
Output Power vs
Load Resistance
20018609 20018612
Power Dissipation vs
Output Power
Dropout Voltage vs
Supply Voltage
20018614
20018615
Power Derating Curve Noise Floor
20018616 20018618
LM4866
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Typical Performance Characteristics (Continued)
Channel Separation
Power Supply
Rejection Ratio
20018619 20018621
Open Loop
Frequency Response
Supply Current vs
Supply Voltage
20018622 20018623
External Components Description
(Refer to Figure 1.)
Components Functional Description
1. R
i
The Inverting input resistance, along with R
f
, set the closed-loop gain. R
i
, along with C
i
, form a high pass
filter with f
c
= 1/(2πR
i
C
i
).
2. C
i
The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. C
i
, along with R
i
, create a
highpass filter with f
c
= 1/(2πR
i
C
i
). Refer to the section, SELECTING PROPER EXTERNAL
COMPONENTS, for an explanation of determining the value of C
i
.
3. R
f
The feedback resistance, along with R
i
, set the closed-loop gain.
4. C
s
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of, this capacitor.
5. C
B
The capacitor, C
B
, filters the half-supply voltage present on the BYPASS pin. Refer to the SELECTING
PROPER EXTERNAL COMPONENTS section for information concerning proper placement and selecting
C
B
’s value.
LM4866
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Application Information
EXPOSED-DAP PACKAGE (LLP) PCB MOUNTING
CONSIDERATIONS
The LM4866’s exposed-DAP (die attach paddle) packages
(MTE and LQ) provide a low thermal resistance between the
die and the PCB to which the part is mounted and soldered.
This allows rapid heat transfer from the die to the surround-
ing PCB copper traces, ground plane and, finally, surround-
ing air. The result is a low voltage audio power amplifier that
produces 2.2W at ≤1% THD with a 4Ωload. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4866’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MTE and LQ packages must have their DAPs soldered
to a copper pad on the PCB. The DAP’s PCB copper pad is
connected to a large plane of continuous unbroken copper.
This plane forms a thermal mass and heat sink and radiation
area. Place the heat sink area on either outside plane in the
case of a two-sided PCB, or on an inner layer of a board with
more than two layers. Connect the DAP copper pad to the
inner layer or backside copper heat sink area with 32(4x8)
(MTE) or 6(3x2) (LQ) vias. The via diameter should be
0.012in - 0.013in with a 1.27mm pitch. Ensure efficient ther-
mal conductivity by plating-through and solder-filling the
vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
2
(min) area is
necessary for 5V operation with a 4Ωload. Heatsink areas
not placed on the same PCB layer as the LM4866 should be
5in
2
(min) for the same supply voltage and load resistance.
The last two area recommendations apply for 25˚c ambient
temperature. Increase the area to compensate for ambient
temperatures above 25˚c. In systems using cooling fans, the
LM4866MTE can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in
2
exposed copper or 5.0in
2
inner layer copper plane heatsink,
the LM4866MTE can continuously drive a 3Ωload to full
power. The LM4866LQ achieves the same output power
level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4866’s thermal shutdown
protection. The LM4866’s power de-rating curve in the Typi-
cal Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP and LLP packages are shown
in the Demonstration Board Layout section.
Further detailed and specific information concerning PCB
layout, fabrication, and mounting an LLP package is avail-
able from National Semiconductor’s AN-1187.
PCB LAYOUT AND SUPPLY REGULATION CONSIDER-
ATIONS FOR DRIVING 3ΩAND 4ΩLOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ωtrace resistance reduces
the output power dissipated by a 4Ωload from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
LM4866
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Application Information (Continued)
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4866 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
R
f
and R
i
set the closed-loop gain of Amp1A, whereas two
internal 20kΩresistors set Amp2A’s gain at -1. The LM4866
drives a load, such as a speaker, connected between the two
amplifier outputs, -OUTA and +OUTA.
Figure 1 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals iden-
tical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially (commonly referred to
as "bridge mode"). This results in a differential gain of
A
VD
=2x(R
f
/R
i
) (1)
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage swing
across the load. This produces four times the output power
when compared to a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load
P
DMAX
=(V
DD
)
2
/(2π
2
R
L
) Single-Ended (2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4866 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
20018601
* Refer to the section Proper Selection of External Components, for a detailed discussion of CBsize.
FIGURE 1. Typical Audio Amplifier Application Circuit
Pin out shown for the LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package.
LM4866
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Application Information (Continued)
fier. From Equation (3), assuming a 5V power supply and an
4Ωload, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
P
DMAX
=4x(V
DD
)
2
/(2π
2
R
L
) Bridge Mode (3)
The LM4973’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
P
DMAX
’=(T
JMAX
−T
A
)/θ
JA
(4)
The LM4866’s T
JMAX
= 150˚C. In the LQ (LLP) package
soldered to a DAP pad that expands to a copper area of 5in
2
on a PCB, the LM4866’s θ
JA
is 20˚C/W. In the MTE package
soldered to a DAP pad that expands to a copper area of 2in
2
on a PCB , the LM4866’s θ
JA
is 41˚C/W. At any given
ambient temperature T
J\A
, use Equation (4) to find the maxi-
mum internal power dissipation supported by the IC packag-
ing. Rearranging Equation (4) and substituting PDMAX for
PDMAX’ results in Equation (5). This equation gives the
maximum ambient temperature that still allows maximum
stereo power dissipation without violating the LM4866’s
maximum junction temperature.
T
A
=T
JMAX
−2xP
DMAX
θ
JA
(5)
For a typical application with a 5V power supply and an 4Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 99˚C for the LLP
package and 45˚C for the MTE package.
T
JMAX
=P
DMAX
θ
JA
+T
A
(6)
Equation (6) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4866’s 150˚C, reduce the
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
If the result of Equation (2) is greater than that of Equation
(3), then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce θ
JA
. The heat sink can be created using additional
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the θ
JA
is the sum of θ
JC
,θ
CS
, and θ
SA
.(θ
JC
is the
junction−to−case thermal impedance,
CS
is the case−to−sink
thermal impedance, and θ
SA
is the sink−to−ambient thermal
impedance.) Refer to the Typical Performance Characteris-
tics curves for power dissipation information at lower output
power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4866’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation in the output signal. Keep the length of leads and
traces that connect capacitors between the LM4866’s power
supply pin and ground as short as possible. Connecting a
1µF capacitor, C
B
, between the BYPASS pin and ground
improves the internal bias voltage’s stability and improves
the amplifier’s PSRR. The PSRR improvements increase as
the bypass pin capacitor value increases. Too large, how-
ever, increases turn-on time and can compromise amplifier’s
click and pop performance. The selection of bypass capaci-
tor values, especially C
B
, depends on desired PSRR require-
ments, click and pop performance (as explained in the sec-
tion, Proper Selection of External Components), system
cost, and size constraints.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4866’s shutdown function. Activate micro-power shut-
down by applying V
DD
to the SHUTDOWN pin. When active,
the LM4866’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
DD
/2. The low 0.7µA typical
shutdown current is achieved by applying a voltage that is as
near as V
DD
as possible to the SHUTDOWN pin. A voltage
thrat is less than V
DD
may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 10kΩpull-up resistor between the
SHUTDOWN pin and V
DD
. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
DD
through the pull-up resistor, activat-
ing micro-power shutdown. The switch and resistor guaran-
tee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
TABLE 1. LOGIC LEVEL TRUTH TABLE FOR SHUT-
DOWN OPERATION
SHUTDOWN OPERATIONAL MODE
Low Full power, stereo BTL
amplifiers
High Micro-power Shutdown
LM4866
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Application Information (Continued)
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4866’s performance requires properly se-
lecting external components. Though the LM4866 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4866 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
RMS
(2.83V
P-P
). Please refer to the
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
i
in Figure 1). A high value capaci-
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
i
has an affect on
the LM4866’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
DD
/2)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
f
. Thus, pops can be minimized by selecting an input
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (R
I
) and the input
capacitor, C
I
produce a −3dB high pass filter cutoff frequency
that is found using Equation (7).
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, C
I
, using Equation (4), is 0.063µF. The 1.0µF
C
I
shown in Figure 1 allows the LM4866 to drive high effi-
ciency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
B
, the capacitor con-
nected to the BYPASS pin. Since C
B
determines how fast
the LM4866 settles to quiescent operation, its value is critical
when minimizing turn−on pops. The slower the LM4866’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
DD
), the smaller the turn−on pop. Choosing C
B
equal to
1.0µF along with a small value of C
i
(in the range of 0.1µF to
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
i
no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION PERFOR-
MANCE
The LM4866 contains circuitry to minimize turn-on and shut-
down transients or "clicks and pop". For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4866’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the BYPASS pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches 1/2 V
DD
. As soon as the voltage on the
BYPASS pin is stable, the device becomes fully operational.
Although the bypass pin current cannot be modified, chang-
ing the size of C
B
alters the device’s turn-on time and the
magnitude of "clicks and pops". Increasing the value of C
B
reduces the magnitude of turn-on pops. However, this pre-
sents a tradeoff: as the size of C
B
increases, the turn-on time
increases. There is a linear relationship between the size of
C
B
and the turn-on time. Here are some typical turn-on times
for various values of C
B
:
C
B
T
ON
0.01µF 20 ms
0.1µF 200 ms
0.22µF 440 ms
0.47µF 940 ms
1.0µF 2 Sec
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
NO LOAD STABILITY
The LM4866 may exhibit low level oscillation when the load
resistance is greater than 10kΩ. This oscillation only occurs
as the output signal swings near the supply voltages. Pre-
vent this oscillation by connecting a 5kΩbetween the output
pins and ground.
LM4866
www.national.com13
Application Information (Continued)
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8ΩLoad
The following are the desired operational parameters:
Power Output: 1W
RMS
Load Impedance: 8Ω
Input Level: 1V
RMS
Input Impedance: 20kΩ
Bandwidth: 100Hz−20 kHz ±0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-
acteristics section. Another way, using Equation (4), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (9).
(8)
V
DD
≥(V
OUTPEAK
+(V
OD
TOP +V
OD
BOT)) (9)
The Output Power vs Supply Voltage graph for an 8Ωload
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4866 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4866’s power dissipation require-
ments, the minimum differential gain is found using Equation
(10).
(10)
Thus, a minimum gain of 2.83 allows the LM4866’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
VD
=3.
The amplifier’s overall gain is set using the input (R
i
) and
feedback (R
f
) resistors. With the desired input impedance
set at 20kΩ, the feedback resistor is found using Equation
(11).
R
f
/R
i
=A
VD
/2 (11)
The value of R
f
is 30kΩ.
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired ±0.25dB
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one−fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ±0.25dB
desired limit. The results are an
f
L
= 100Hz/5 = 20Hz (12)
and an
F
H
= 20kHzx5 = 100kHz (13)
As mentioned in the External Components section, R
i
and C
i
create a highpass filter that sets the amplifier’s lower
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (14).
(14)
the result is
1/(2π*20kΩ*20Hz) = 0.398µF (15)
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain, A
VD
, determines the
upper passband response limit. With A
VD
= 3 and f
H
=
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4866’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-lrestricting
bandwidth limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 2 through 6 show the recommended four-layer PC
board layout that is optimized for the 24-pin LQ-packaged
LM4866 and associated external components. Figures 7
through 11 show the recommended four-layer PC board
layout that is optimized for the 20-pin MTE-packaged
LM4866 and associated components. Figures 12 through 14
show the recommended two-layer PC board layout that is
optimized for the 20-pin MT-packaged LM4866 and associ-
ated components. These circuits are designed for use with
an external 5V supply and 3Ω(or greater) speakers for the
LQ- and MTE-packaged LM4866 and 4Ω(or greater) speak-
ers for the MT-packaged LM4866.
This circuit board is easy to use. Apply 5V and ground to the
board’s V
DD
and GND pads, respectively. Connect speakers
between the board’s -OUTA and +OUTA and OUTB and
+OUTB pads. Apply the stereo input signal to the input pins
labeled "-INA" and "-INB." The stereo input signal’s ground
references are connected to the respective input channel’s
"GND" pin, adjacent to the input pins.
LM4866
www.national.com 14
Application Information (Continued)
20018631
FIGURE 2. Recommended LQ PC board layout:
Component-side Silkscreen
20018632
FIGURE 3. Recommended LQ PC board layout:
Component-side layout
20018633
FIGURE 4. Recommended LQ PC board layout:
upper inner-layer layout
20018634
FIGURE 5. Recommended LQ PC board layout:
lower inner-layer layout
LM4866
www.national.com15
Application Information (Continued)
20018635
FIGURE 6. Recommended LQ PC board layout:
bottom-side layout
20018644
FIGURE 7. Recommended MTE board layout:
component-side silkscreen
20018645
FIGURE 8. Recommended MTE PC board layout:
component-side layout
20018646
FIGURE 9. Recommended MTE board layout:
upper inner-layer layout
LM4866
www.national.com 16
Application Information (Continued)
20018647
FIGURE 10. Recommended MTE PC board layout:
lower inner-layer layout
20018648
FIGURE 11. Recommended MTE board layout:
bottom-side layout
20018649
FIGURE 12. Recommended MT PC board layout:
component-side silkscreen
20018651
FIGURE 13. Recommended MT board layout:
component-side layout
LM4866
www.national.com17
Application Information (Continued)
20018650
FIGURE 14. Recommended MT PC board layout:
bottom-side layout
LM4866
www.national.com 18
Revision History
Rev Date Description
1.1 04/28/05 Changed (min) to (max) for I
SD
units
LM4866
www.national.com19
Physical Dimensions inches (millimeters) unless otherwise noted
20-Lead Molded PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH
Order Number LM4866MT
NS Package Number MTC20
20-Lead Molded TSSOP, Exposed Pad, 6.5x4.4x0.9mm
Order Number LM4866MTE
NS Package Number MXA20A
LM4866
www.national.com 20
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
24-Lead Molded pkg, Leadframe Package LLP
Order Number LM4866LQ
NS Package Number LQA24A
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.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products
Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain
no ‘‘Banned Substances’’ as defined in CSP-9-111S2.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
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Support Center
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Tel: 81-3-5639-7560
www.national.com
LM4866 2.2W Stereo Audio Amplifier
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