LM4928
1.2 Watt Stereo Fully Differential Audio Amplifier with RF
Suppression and Shutdown Low
General Description
The LM4928 is an stereo fully differential stereo audio power
amplifier primarily designed for demanding applications in
mobile phones and other portable communication devices. It
is capable of delivering 1.2 watts of continuous average
power to a 8Ωload with less than 1% distortion (THD+N)
from a 5V
DC
power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4928 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other low voltage appli-
cations where minimal power consumption is a primary re-
quirement.
The LM4928 features a low-power consumption shutdown
mode. To facilitate this, shutdown may be enabled by logic
low. Additionally, the LM4928 features an internal thermal
shutdown protection mechanism.
The LM4928 contains advanced pop & click circuitry which
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
Key Specifications
jImproved PSRR at 217Hz 90dB (typ)
jOutput Power at 5.0V @1% THD+N (8Ω) 1.2W (typ)
jOutput Power at 3.0V @1% THD+N (8Ω)400mW (typ)
jShutdown Current 0.1µA (typ)
Features
nRF Suppression Circuitry
nFully differential amplification
nAvailable in space-saving micro SMD and LLP packages
nUltra low current shutdown mode
nCan drive capacitive loads up to 100pF
nImproved pop & click circuitry eliminates noises during
turn-on and turn-off transitions
n2.4 - 5.5V operation
nNo output coupling capacitors, snubber networks or
bootstrap capacitors required
Applications
nMobile phones
nPDAs
nPortable electronic devices and accessories
Boomer®is a registered trademark of National Semiconductor Corporation.
February 2006
LM4928 1.2 Watt Stereo Fully Differential Audio Power Amplifier with RF Suppression and
Shutdown Low
© 2006 National Semiconductor Corporation DS201600 www.national.com
Typical Application
201600B7
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4928
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Connection Diagrams
LLP Package LLP Package Marking
20160006
Top View
Order Number LM4928SD
See NS Package Number SDA14A
20160004
Top View
Z — Assembly Plant Code
XY — 2 Digit Date Code
TT — Die Traceability
L4928 — LM4928
micro SMD Package micro SMD Package Marking
20160003
Top View
Order Number LM4928TL
See NS Package Number TLA16
20160005
Top View
XY — 2 Digit Date COde
TT — Die Traceability
G — Boomer Family
F9 — LM4928T
LM4928TL Pin Descriptions
A1 IN1+
B1 IN1–
C1 IN2–
D1 IN2+
A2 VDD
B2 BYPASS
C2 SHUTDOWN
D2 VDD
A3 OUT1–
B3 OUT1+
C3 OUT2+
D3 OUT2–
A4 GND
B4 NC
C4 NC
D4 GND
LM4928
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Absolute Maximum Ratings (Note 2)
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 3) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JA
(SD) 50˚C/W
θ
JA
(micro SMD) 74˚C/W
Soldering Information
See AN-1187
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2.4V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
=5V(Notes 1, 2)
The following specifications apply for V
DD
= 5V, A
V
= 1, and 8Ωload unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4928 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, no load
V
IN
= 0V, R
L
=8Ω
(Both amplifiers)
4
4 7.5 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND
(Both amplifiers)
0.1 1.0 µA (max)
P
o
Output Power THD = 1% (max);f=1kHz
LM4928SD, R
L
=4Ω(Note 9)
R
L
=8Ω
1.8
1.2 1.0 W
THD = 10% (max);f=1kHz
LM4928SD, R
L
=4Ω(Note 9)
R
L
=8Ω
2.2
1.5 W
THD+N Total Harmonic Distortion + Noise P
o
= 1 Wrms; f = 1kHz 0.04 %
PSRR Power Supply Rejection Ratio
V
ripple
= 200mV sine p-p
f = 217Hz (Note 8) 90 dB
f = 1kHz (Note 8) 90
CMRR Common-Mode Rejection Ratio f = 217Hz, V
CM
= 200mV
pp
70 50 dB (min)
V
OS
Output Offset V
IN
= 0V 4 18 mV (max)
V
SDIH
Shutdown Voltage Input High 1.4 V
V
SDIL
Shutdown Voltage Input Low 0.4 V
SNR Signal-to-Noise Ratio P
O
= 1W, f = 1kHz 105 dB
T
WU
Wake-up time from Shutdown Cbypass = 1µF 13 ms
Electrical Characteristics V
DD
=3V (Notes 1, 2)
The following specifications apply for V
DD
= 3V, A
V
= 1, and 8Ωload unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4928 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, no load
V
IN
= 0V, R
L
=8Ω
(Both amplifiers)
3.5
3.5 mA
I
SD
Shutdown Current V
SHUTDOWN
= GND
(Both amplifiers)
0.1 1 µA (max)
LM4928
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Electrical Characteristics V
DD
=3V(Notes 1, 2)
The following specifications apply for V
DD
= 3V, A
V
= 1, and 8Ωload unless otherwise specified. Limits apply for T
A
=
25˚C. (Continued)
Symbol Parameter Conditions
LM4928 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
P
o
Output Power
THD = 1% (max);f=1kHz
R
L
=4Ω
R
L
=8Ω
0.55
0.40 W
THD = 10% (max);f=1kHz
R
L
=4Ω
R
L
=8Ω
0.68
0.50 W
THD+N Total Harmonic Distortion + Noise P
o
= 0.25Wrms; f = 1kHz 0.05 %
PSRR Power Supply Rejection Ratio
V
ripple
= 200mV sine p-p
f = 217Hz (Note 8) 90 dB
f = 1kHz (Note 8) 90
CMRR Common-Mode Rejection Ratio f = 217Hz, V
CM
= 200mV
pp
70 50 dB (min)
V
OS
Output Offset V
IN
= 0V 4 18 mV (max)
V
SDIH
Shutdown Voltage Input High 1.4 V
V
SDIL
Shutdown Voltage Input Low 0.4 V
SNR Signal-to-Noise Ratio P
O
= 0.4W, f = 1kHz 105 dB
T
WU
Wake-up time from Shutdown Cbypass = 1µF 9 ms
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 –T
A)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4928, see power
derating curve for additional information.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model, 220pF – 240pF 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: Inputs are AC terminated to GND.
Note 9: When driving 4Ωloads from a 5V power supply, the LM4928SD must be mounted to a circuit board with the exposed-DAP area soldered down to at least
4in2plane of 1oz, copper.
Note 10: Data taken with BW = 80kHz and AV= 1 except where specified.
Note 11: Maximum Power Dissipation (PDMAX) in the device occurs at an output power level significantly below full output power. PDMAX can be calculated using
Equation 4 shown in the Application section. It may also be obtained from the Power Dissipation graphs.
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
B
Bypass pin capacitor which provides half-supply filtering. Refer to the Power Supply Bypassing section for
information concerning proper placement and selection of C
B
.
3. R
i
Inverting input resistance which sets the closed-loop gain in conjunction with R
f
.
4. R
f
External feedback resistance which sets the closed-loop gain in conjunction with R
i
.
LM4928
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Typical Performance Characteristics (Note 10)
THD+N vs Frequency
V
DD
= 2.6V, R
L
=4Ω,P
O
= 150mW
THD+N vs Frequency
V
DD
= 2.6V, R
L
=8Ω,P
O
= 150mW
20160016 20160017
THD+N vs Frequency
V
DD
= 3V, R
L
=4Ω,P
O
= 250mW
THD+N vs Frequency
V
DD
= 3V, R
L
=8Ω,P
O
= 250mW
20160018 20160019
THD+N vs Frequency
V
DD
= 5V, R
L
=4Ω,P
O
=1W
THD+N vs Frequency
V
DD
= 5V, R
L
=8Ω,P
O
=1W
20160020 20160021
LM4928
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Typical Performance Characteristics (Note 10) (Continued)
THD+N vs Output Power
V
DD
= 2.6V, R
L
=4Ω
THD+N vs Output Power
V
DD
= 2.6V, R
L
=8Ω
20160022 20160023
THD+N vs Output Power
V
DD
= 3V, R
L
=4Ω
THD+N vs Output Power
V
DD
= 3V, R
L
=8Ω
20160024 20160025
THD+N vs Output Power
V
DD
= 5V, R
L
=4Ω
THD+N vs Output Power
V
DD
= 5V, R
L
=8Ω
20160026 20160027
LM4928
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Typical Performance Characteristics (Note 10) (Continued)
PSRR vs Common Mode Voltage
V
DD
= 3V, R
L
=8Ω, f = 217Hz
PSRR vs Common Mode Voltage
V
DD
= 5V, R
L
=8Ω, f = 217Hz
201600C8 201600C9
PSRR vs Frequency
V
DD
= 3V, R
L
=8Ω
Input Terminated to GND, BW = 500kHz
PSRR vs Frequency
V
DD
= 5V, R
L
=8Ω
Input Terminated to GND, BW = 500kHz
201600D0 201600D1
Output Power vs Supply Voltage
R
L
=4Ω
Output Power vs Supply Voltage
R
L
=8Ω
201600D2 201600C5
LM4928
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Typical Performance Characteristics (Note 10) (Continued)
CMRR vs Frequency
V
DD
= 3V, R
L
=8Ω
CMRR vs Frequency
V
DD
= 5V, R
L
=8Ω
201600C0 201600C1
Crosstalk vs Frequency
V
DD
= 5V, R
L
=8Ω,P
O
=1W
Top = Vin Left driven, Vout Right measured
Bot = Vin Right driven, Vout Left measured
Crosstalk vs Frequency
V
DD
= 5V, R
L
=4Ω,P
O
=1W
Top = Vin Left driven, Vout Right measured
Bot = Vin Right driven, Vout Left measured
20160069 20160072
Crosstalk vs Frequency
V
DD
= 3V, R
L
=4Ω,P
O
= 500mW
Top = Vin Left driven, Vout Right measured
Bot = Vin Right driven, Vout Left measured
Crosstalk vs Frequency
V
DD
= 3V, R
L
=8Ω,P
O
= 250mW
Top = Vin Left driven, Vout Right measured
Bot = Vin Right driven, Vout Left measured
20160070 20160071
LM4928
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Typical Performance Characteristics (Note 10) (Continued)
Power Dissipation vs Output Power
V
DD
=3V
Power Dissipation vs Output Power
V
DD
=5V
201600C6 201600C7
Noise Floor
V
DD
=3V
Noise Floor
V
DD
=5V
201600C2 201600C3
Output Power vs Load Resistance Clipping Voltage vs Supply Voltage
201600C4 20160030
LM4928
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Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4928 is a fully differential audio amplifier that fea-
tures differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a
common mode feedback amplifier that adjusts the output
voltages so that the average value remains V
DD
/ 2. When
setting the differential gain, the amplifier can be considered
to have "halves". Each half uses an input and feedback
resistor (R
i1
and R
F1
) to set its respective closed-loop gain
(see Figure 1). With R
i1
=R
i2
and R
F1
=R
F2
, the gain is set
at -R
F
/R
i
for each half per channel. This results in a
differential gain of
A
VD
=-R
F
/R
i
(1)
It is extremely important to match the input resistors to each
other, as well as the feedback resistors to each other for best
amplifier performance. See the Proper Selection of Exter-
nal Components section for more information. A differential
amplifier works in a manner where the difference between
the two input signals is amplified. In most applications, this
would require input signals that are 180˚ out of phase with
each other. The LM4928 can be used, however, as a single
ended input amplifier while still retaining its fully differential
benefits. In fact, completely unrelated signals may be placed
on the input pins. The LM4928 simply amplifies the differ-
ence between them.
All of these applications provide what is known as a "bridged
mode" output (bridge-tied-load, BTL). This results in output
signals at V
o1
and V
o2
that are 180˚ out of phase with
respect to each other. Bridged mode operation is different
from the single-ended amplifier configuration that connects
the load between the amplifier output and ground. A bridged
amplifier design has distinct advantages over the single-
ended configuration: it provides differential drive to the load,
thus doubling maximum possible output swing for a specific
supply voltage. Four times the output power is possible
compared with a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or clipped. In
order to choose an amplifier’s closed-loop gain without caus-
ing excess clipping, please refer to the Audio Power Am-
plifier Design section.
A bridged configuration, such as the one used in the
LM4928, also creates a second advantage over single-
ended amplifiers. Since the differential outputs, V
o1
and V
o2
,
are biased at half-supply, no net DC voltage exists across
the load. This assumes that the input resistor pair and the
feedback resistor pair are properly matched (see Proper
Selection of External Components). BTL configuration
eliminates the output coupling capacitor required in single-
supply, single-ended amplifier configurations. If an output
coupling capacitor is not used in a single-ended output con-
figuration, the half-supply bias across the load would result
in both increased internal IC power dissipation as well as
permanent loudspeaker damage. Further advantages of
bridged mode operation specific to fully differential amplifiers
like the LM4928 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4928’s exposed-DAP (die attach paddle) package
(LLP) 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 surrounding
PCB copper traces, ground plane and, finally, surrounding
air. Failing to optimize thermal design may compromise the
LM4928’s high power performance and activate unwanted,
though necessary, thermal shutdown protection. The LLP
package must have its DAP 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 at least 4 vias
thermal via. The via diameter should be 0.012in - 0.013in.
Ensure efficient thermal conductivity by plating-through and
solder-filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. In all circumstances and condi-
tions, the junction temperature must be held below 150˚C to
prevent activating the LM4928’s thermal shutdown protec-
tion. The LM4928’s power de-rating curve in the Typical
Performance Characteristics shows the maximum power
dissipation versus temperature. Example PCB layouts are
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout,
fabrication, and mounting an LLP package is available from
National Semiconductor’s package Engineering Group un-
der application note AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 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. This problem of decreased load dissipation
is exacerbated as load impedance decreases. Therefore, to
maintain the highest load dissipation and widest output volt-
age 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.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 states the maximum power dissi-
pation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
LM4928
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Application Information (Continued)
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 an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.
P
DMAX
= 4(V
DD
)
2
/(2π
2
R
L
) Bridge Mode per channel (3)
P
DMAX
= 8(V
DD
)
2
/(2π
2
R
L
) Bridge Mode both channel (4)
Since the LM4928 has bridged outputs, the maximum inter-
nal power dissipation is 4 times that of a single-ended am-
plifier. Even with this substantial increase in power dissipa-
tion, the LM4928 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8Ωload,
the maximum power dissipation point is 625mW per chan-
nel. Then multiply by two or use equation 4 to get 1.25W total
power dissipation for both channels. The maximum power
dissipation point obtained from Equation 4 must not be
greater than the power dissipation results from Equation 5:
P
DMAX
=(T
JMAX
-T
A
)/θ
JA
(5)
Depending on the ambient temperature, T
A
, of the system
surroundings, Equation 5 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 4 is greater than that of Equation 5,
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
the θ
JA
reduced with heatsinking. In many cases, larger
traces near the output, V
DD
, and GND pins can be used to
lower the θ
JA
. The larger areas of copper provide a form of
heatsinking allowing higher power dissipation. For the typical
application of a 5V power supply, with an 8Ωload in the LLP
package, the maximum ambient temperature possible with-
out violating the maximum junction temperature is approxi-
mately 85˚C provided that device operation is around the
maximum power dissipation point. Recall that internal power
dissipation is a function of output power. If typical operation
is not around the maximum power dissipation point, the
LM4928 can operate at higher ambient temperatures. Refer
to the Typical Performance Characteristics curves for
power dissipation information.
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 location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.
Typical applications employ a 5V regulator with 10µF and
0.1µF bypass capacitors that increase supply stability. This,
however, does not eliminate the need for bypassing the
supply nodes of the LM4928. The LM4928 will operate with-
out the bypass capacitor C
B
, although the PSRR may de-
crease. A 1µF capacitor is recommended for C
B
. This value
maximizes PSRR performance. Lesser values may be used,
but PSRR decreases at frequencies below 1kHz. The issue
of C
B
selection is thus dependant upon desired PSRR and
click and pop performance.
OPTIMIZING RF IMMUNITY
The internal circuitry of the LM4928 suppresses the amount
of RF signal that is coupled into the chip. However, certain
external factors, such as output trace length, output trace
orientation, distance between the chip and the antenna,
antenna strength, speaker type, and type of RF signal, may
affect the RF immunity of the LM4928. In general, the RF
immunity of the LM4928 is application specific. Neverthe-
less, optimal RF immunity can be achieved by using short
output traces and increasing the distance between the
LM4928 and the antenna.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4928 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. The device may then be placed
into shutdown mode by toggling the Shutdown Select pin to
logic low. The trigger point for shutdown is shown as a typical
value in the Supply Current vs Shutdown Voltage graphs in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for maximum perfor-
mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current may
be greater than the typical value of 0.1µA. In either case, the
shutdown pin should be tied to a definite voltage to avoid
unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor. This scheme guarantees that the
shutdown pin will not float, thus preventing unwanted state
changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical when optimizing
device and system performance. Although the LM4928 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4928 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4928 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1Vrms are available
from sources such as audio codecs. Please refer to the
Audio Power Amplifier Design section for a more complete
explanation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4928
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
DD
. Exact
allowable input common-mode voltage levels are actually a
function of V
DD
,R
i
, and R
f
and may be determined by
Equation 6:
V
CMi
<(V
DD
-1.2)(R
i
+R
f
)/R
f
-V
DD
/2(R
i
/R
f
) (6)
LM4928
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Application Information (Continued)
Special care must be taken to match the values of the input
resistors (R
i1
and R
i2
) and (R
f1
and R
f2
) to each other.
Because of the balanced nature of differential amplifiers,
resistor matching differences can result in net DC currents
across the load. This DC current can increase power con-
sumption, internal IC power dissipation, reduce PSRR,
CMRR, and possibly damaging the loudspeaker. The chart
below demonstrates this problem by showing the effects of
differing values between the input resistors while assuming
that the feedback resistors are perfectly matched. The re-
sults below apply to the application circuit shown in Figure 1,
and assumes that V
DD
=5V,R
L
=8Ω, and the system has
DC coupled inputs tied to ground.
Tolerance R
i1
R
i2
V
02
-V
01
I
LOAD
20% 0.8R 1.2R -0.500V 62.5mA
10% 0.9R 1.1R -0.250V 31.25mA
5% 0.95R 1.05R -0.125V 15.63mA
1% 0.99R 1.01R -0.025V 3.125mA
0% R R 0 0
Similar results would occur if the feedback resistors were not
carefully matched. Adding input coupling resistors in be-
tween the signal source and the input resistors will eliminate
this problem, however. To achieve best performance with
minimum component count, it is highly recommended that
both the feedback and input resistors matched to 1% toler-
ance or better for best performance.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8ΩAudio Amplifier
Given:
Power Output 1Wrms
Load Impedance 8Ω
Maximum Input Level 1Vrms
Maximum Input
Impedance 20kΩ
Bandwidth 100Hz–20kHz ±0.25dB
A designer must first determine the minimum supply rail to
obtain the specified output power. The supply rail can easily
be found by extrapolating from the Output Power vs Supply
Voltage graphs in the Typical Performance Characteris-
tics section. A second way to determine the minimum supply
rail is to calculate the required V
OPEAK
using Equation 7 and
add the dropout voltages. Using this method, the minimum
supply voltage is (Vopeak + (V
DO TOP
+V
DO BOT
), where V
DO
BOT
and V
DO TOP
are extrapolated from the Dropout Voltage
vs Supply Voltage curve in the Typical Performance Char-
acteristics section.
(7)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail just about 4.5V. Extra supply
voltage creates headroom that allows the LM4928 to repro-
duce peaks in excess of 1W without producing audible dis-
tortion. 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 Dissipa-
tion section. Once the power dissipation equations have
been addressed, the required differential gain can be deter-
mined from Equation 8.
(8)
R
f
/R
i
=A
VD
From Equation 8, the minimum A
VD
is 2.83. With R
f
= 40kΩ,
a ratio of R
f
to R
i
of 2.83 gives R
i
= 14kΩ. The final design
step is to address the bandwidth requirement which must be
stated as a single -3dB frequency point. Five times away
from a -3dB point is 0.17dB down from passband response
which is better than the required ±0.25dB specified.
f
H
= 20kHz*5=100kHz
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
VD
.
With a A
VD
= 2.83 and f
H
= 100kHz, the resulting GBWP =
283kHz which is much smaller than the LM4928 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4928 can still be used without running into bandwidth
limitations.
LM4928
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LM4928 Demo Board Schematic
20160073
LM4928
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LM4928 LLP Demo Board Artwork
Top Silkscreen Top Layer
20160009 20160008
Bottom Layer and Ground Plane
20160007
LM4928
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LM4928 microSMD Board Artwork
Top Silkscreen Top Layer
20160013 20160012
Middle Layer Bottom Layer and Ground Plane
20160010 20160011
LM4928
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Revision History
Rev Date Description
1.0 7/13/05 Input first set of edits.
1.1 10/3/05 More edits input.
1.2 10/10/05 Input few text edits.
1.3 10/25/04 Added the Typ Perf section.
1.4 11/02/05 Added the X1, X2, and X3 values on the
TLA1611A mktg outline.
1.5 11/15/05 Added 3 more curves (66, 67, and 68) and
some texts edits.
1.6 11/16/05 Texts edits.
1.7 12/13/05 Added 4 more curves (69, 70, 71, and 72)
and did some texts edits.
1.8 12/14/05 First WEB released (per Kashif).
1.9 12/16/05 Coded the LM4928TL ( Future Product )
for it will be released soon ( early January,
2006) per Kashif.
Re-released D/S to the WEB.
2.0 01/04/06 Released the TL package to the WEB.
2.1 01/09/06 Edited B7 and B8 (now 73), then
re-released D/S to the WEB (per Kashif).
2.2 02/01/06 Text edits, then re-released D/S to the
WEB.
LM4928
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Physical Dimensions inches (millimeters) unless otherwise noted
LLP Package
Order Number LM4928SD
NS Package Number SDA14A
16–bump micro SMD
Order Number LM4928TL
NS Package Number TLA1611A
X1 = 1.970±0.03, X2 = 1.970±0.03, X3 = 0.600±0.075
LM4928
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Notes
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.
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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.
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LM4928 1.2 Watt Stereo Fully Differential Audio Power Amplifier with RF Suppression and
Shutdown Low
