LM4898
LM4898 1 Watt Fully Differential Audio Power Amplifier With Shutdown
Select
Literature Number: SNAS216D
LM4898
1 Watt Fully Differential Audio Power Amplifier With
Shutdown Select
General Description
The LM4898 is a fully differential audio power amplifier
primarily designed for demanding applications in mobile
phones and other portable communication device applica-
tions. It is capable of delivering 1 watt of continuous average
power to an 8ΩBTL load 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 LM4898 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 LM4898 features a low-power consumption shutdown
mode. To facilitate this, Shutdown may be enabled by either
logic high or low depending on mode selection. Driving the
shutdown mode pin either high or low enables the shutdown
select pin to be driven in a likewise manner to enable Shut-
down. Additionally, the LM4898 features an internal thermal
shutdown protection mechanism.
The LM4898 contains advanced pop & click circuitry which
virtually eliminates noises which would otherwise occur dur-
ing turn-on and turn-off transitions.
Key Specifications
jImproved PSRR at 217Hz 83dB(typ)
jPower Output at 5.0V & 1% THD 1.0W(typ)
jPower Output at 3.3V & 1% THD 400mW(typ)
jShutdown Current 0.1µA(typ)
Features
nFully differential amplification
nAvailable in space-saving packages micro SMD, MSOP,
and LLP
nUltra low current shutdown mode
nCan drive capacitive loads up to 500pF
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
nShutdown high or low selectivity
Applications
nMobile phones
nPDAs
nPortable electronic devices
Connection Diagrams
Mini Small Outline (MSOP) Package MSOP Marking
20073723
Top View
Order Number LM4898MM
See NS Package Number MUB10A
20073774
Z -Assembly Code
X - Date Code
TT - Die Run Traceability
G - Boomer Family
B3 - LM4898MM
Boomer®is a registered trademark of National Semiconductor Corporation.
July 2003
LM4898 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
© 2003 National Semiconductor Corporation DS200737 www.national.com
Connection Diagrams (Continued)
LLP Package LD Marking
20073735
Top View
Order Number LM4898LD
See NS Package Number LDA10B
20073756
Z- Assembly Code
XY - Date Code
TT - Die Run Traceability
L4898 - LM4898LD
9 Bump micro SMD Package 9 Bump micro SMD Marking
20073736
Top View
Order Number LM4898ITL, LM4898ITLX
See NS Package Number TLA09AAA
20073784
X - Date Code
T - Die Run Traceability
G - Boomer Family
C3 - LM4898ITLX
LM4898
www.national.com 2
Typical Application
20073701
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4898
www.national.com3
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
JC
(LLP) 12˚C/W
θ
JA
(LLP) 63˚C/W
θ
JA
(micro SMD) 220˚C/W
θ
JC
(MSOP) 56˚C/W
θ
JA
(MSOP) 190˚C/W
Soldering Information
See AN-1112 "microSMD Wafers Level Chip Scale
Package."
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, 8) The following specifications apply for V
DD
= 5V,
8Ωload, and A
V
= 1V/V, unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4898 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, no load 3 6 mA (max)
V
IN
= 0V, R
L
=8Ω510
I
SD
Shutdown Current V
SDMODE
=V
SHUTDOWN
= GND 0.1 1 µA (max)
P
o
Output Power THD = 1% (max);f=1kHz
LM4898LD, R
L
=4Ω(Note 11) 1.4 W (min)
LM4898, R
L
=8Ω1 0.9
THD+N Total Harmonic Distortion+Noise P
o
= 0.4 Wrms; f = 1kHz 0.05 %
PSRR Power Supply Rejection Ratio V
ripple
= 200mV sine p-p
f = 217Hz (Note 9) 83 dB (min)
f = 1kHz (Note 9) 90
f = 217Hz (Note 10) 83 71
f = 1kHz (Note 10) 83 71
CMRR Common_Mode Rejection Ratio f = 217Hz
V
CM
= 200mV
DD
50 dB
V
OS
Output Offset V
IN
=0V 2 mV
V
SDIH
Shutdown Voltage Input High SD Mode = GND 0.9 V
V
SDIL
Shutdown Voltage Input Low SD Mode = GND 0.7 V
V
SDIH
Shutdown Voltage Input High SD Mode = V
DD
0.9 V
V
SDIL
Shutdown Voltage Input Low SD Mode = V
DD
0.7 V
LM4898
www.national.com 4
Electrical Characteristics V
DD
=3V (Notes 1, 2, 8)
The following specifications apply for V
DD
= 3V, 8Ωload and A
V
= 1V/V, unless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions
LM4898 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, no load 2.5 5 .5 mA (max)
V
IN
= 0V, R
L
=8Ω49
I
SD
Shutdown Current V
SDMODE
=V
SHUTDOWN
= GND 0.1 1 µA (max)
P
o
Output Power THD = 1% (max); f = 1kHz
LM4898, RL = 8Ω
0.35 W
THD+N Total Harmonic Distortion+Noise P
o
= 0.25Wrms; f = 1kHz 0.03 %
PSRR Power Supply Rejection Ratio V
ripple
= 200mV sine p-p
f = 217Hz (Note 9) 83 dB
f = 1kHz (Note 9) 84
f = 217Hz (Note 10) 83
f = 1kHz (Note 10) 83
CMRR Common-Mode Rejection Ratio f = 217Hz
V
CM
= 200mV
PP
50 dB
V
OS
Output Offset V
IN
=0V 2 mV
V
SDIH
Shutdown Voltage Input High SD Mode = GND 0.8 V
V
SDIL
Shutdown Voltage Input Low SD Mode = GND 0.6 V
V
SDIH
Shutdown Voltage Input High SD Mode = V
DD
0.8 V
V
SDIL
Shutdown Voltage Input Low SD Mode = V
DD
0.6 V
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.
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: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 8: For micro SMD only, shutdown current is measured in a Normal Room Environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA.
Note 9: Unterminated input.
Note 10: 10Ωterminated input.
Note 11: When driving 4Ωloads from a 5V supply, the LM4898LD must be mounted to a circuit board with the exposed-DAP area soldered down to a 1sq. in plane
of 1oz. copper..
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 section, Proper Selection of External
Components, 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
Feedback resistance which sets the closed-loop gain in conjunction with R
i
.
LM4898
www.national.com5
Typical Performance Characteristics
LD Specific Characteristics
THD+N vs Output Power
V
DD
= 5V, R
L
=4Ω
THD+N vs Frequency
V
DD
= 5V, R
L
=4Ω,P
O
=1W
200737A1 200737A3
LM4898LD
Power Dissipation vs Output Power
LM4898LD
Power Derating Curve
200737A4 200737A5
LM4898
www.national.com 6
Typical Performance Characteristics
Non-LD Specific Characteristics
THD+N vs Frequency
V
DD
= 5V, R
L
=8Ω,P
O
= 400mW
THD+N vs Frequency
V
DD
= 3V, R
L
=8Ω,P
O
= 275mW
200737A7 200737A9
THD+N vs Frequency
V
DD
= 3V, R
L
=4Ω,P
O
= 225mW
THD+N vs Frequency
V
DD
= 2.6V, R
L
=8Ω,P
O
= 150mW
200737B1
200737B3
THD+N vs Frequency
V
DD
= 2.6V, R
L
=4Ω,P
O
= 150mW
THD+N vs Output Power
V
DD
= 5V, R
L
=8Ω
200737B5 200737B7
LM4898
www.national.com7
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
THD+N vs Output Power
V
DD
= 3V, R
L
=8Ω
THD+N vs Output Power
V
DD
= 3V, R
L
=4Ω
200737B9 200737C1
THD+N vs Output Power
V
DD
= 2.6V, R
L
=8Ω
THD+N vs Output Power
V
DD
= 2.6V, R
L
=4Ω
200737C3
200737C5
PSRR vs Frequency
V
DD
= 5V, R
L
=8Ω, Input 10ΩTerminated
PSRR vs Frequency
V
DD
= 3V, R
L
=8Ω, Input 10ΩTerminated
200737C7 200737D0
LM4898
www.national.com 8
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
Output Power vs Supply Voltage
R
L
=8Ω
Output Power vs Supply Voltage
R
L
=4Ω
200737D3 200737D5
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
200737D6 200737D6
Power Dissipation vs
Output Power
Output Power vs
Load Resistance
200737D8 200737D9
LM4898
www.national.com9
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
Supply Current vs Shutdown Voltage
Shutdown Low
Supply Current vs Shutdown Voltage
Shutdown High
200737E0 200737E1
Clipping (Dropout) Voltage vs
Supply Voltage Open Loop Frequency Response
200737E2
200737E3
Power Derating Curve
Noise Floor
200737E4
200737E6
LM4898
www.national.com 10
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
CMRR vs Frequency
V
DD
= 5V, R
L
=8Ω, 200mV
pp
CMRR vs Frequency
V
DD
= 3V, R
L
=8Ω, 200mV
pp
200737E8 200737F0
PSRR vs Common Mode Voltage
V
DD
=5V
PSRR vs Common Mode Voltage
V
DD
= 3V, R
L
=8Ω, 217Hz, 200mV
pp
200737F2 200737F4
Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4898 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 two "halves". Each half uses an input and feedback
resistor (R
i
1 and R
f
1) to set its respective closed-loop gain
(see Figure 1). With R
i
1=R
i
2 and R
f
1=R
f
2, the gain is set
at -R
f
/R
i
for each half. 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 External
Components section for more information. A differential am-
plifier 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 LM4898 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 LM4898 simply amplifies the differ-
ence between them.
All of these applications, either single-ended or fully differ-
ential, provide what is known as a "bridged mode" output
(bridge-tied-load, BTL). This results in output signals at Vo1
and Vo2 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 be-
tween the amplifier output and ground. A bridged amplifier
design has distinct advantages over the single-ended con-
figuration: it provides differential drive to the load, thus dou-
bling 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 assumes that the
LM4898
www.national.com11
Application Information (Continued)
amplifier is not current limited or clipped. In order to choose
an amplifier’s closed-loop gain without causing excess clip-
ping, please refer to the Audio Power Amplifier Design sec-
tion.
A bridged configuration, such as the one used in theLM4898,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2,are biased at
half-supply, no net DC voltage exists across the load. This
assumes that the input resistor pair and the feedback resis-
tor pair are properly matched (see Proper Selection of Ex-
ternal 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 configuration, 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 LM4898 include
increased power supply rejection ratio, common-mode noise
reduction, and click and pop reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4898’s exposed-DAP (die attach paddle) package
(LD) provides 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. The result is a low voltage audio power amplifier that
produces 1.4W 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 LM4898’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The LD 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 4 (2x2) vias.
The via diameter should be 0.012in - 0.013in with a 0.050in
pitch. 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. 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 LM4898 should be
5in
2
(min) for the same supply voltage and load resistance.
The last two area recommendations apply for 25˚C ambient
temperature. In all circumstances and conditions, the junc-
tion temperature must be held below 150˚C to prevent acti-
vating the LM4898’s thermal shutdown protection. The
LM4898’s power derating curve in the Typical Performance
Characteristics shows the maximum power dissipation ver-
sus temperature. Further detailed and specific information
concerning PCB layout, fabrication, and mounting an LLP
package is available from National Semiconductor’s pack-
age Engineering Group under application note AN-1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS 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 1.4W
to1.37W. This problem of decreased load dissipation is ex-
acerbated 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 amplifier, 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.
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 (3)
Since the LM4898 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 LM4898 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8. load,the
maximum power dissipation point is 625mW. The maximum
power dissipation point obtained from Equation 3 must not
be greater than the power dissipation results from Equa-
tion4:
P
DMAX
=(T
JMAX
-T
A
)/θ
JA
(4)
The LM4898’s θ
JA
in an MUA10A package is 190˚C/W.
Depending on the ambient temperature, T
A
, of the system
surroundings, Equation 4 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 3 is greater than that of Equation 4,
then either the supply voltage must be decreased, the load
impedance increased, the ambient temperature reduced, or
theθ
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, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 30˚C pro-
vided that device operation is around the maximum power
dissipation point. Recall that internal power dissipation is a
LM4898
www.national.com 12
Application Information (Continued)
function of output power. If typical operation is not around the
maximum power dissipation point, the LM4898 can operate
at higher ambient temperatures. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion.
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
and0.1µF bypass capacitors that increase supply stability.
This, however, does not eliminate the need for bypassing the
supply nodes of the LM4898. Although the LM4898 will
operate without the bypass capacitor C
B
, the PSRR may
decrease. 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 as explained in the section
Proper Selection of External Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4898 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4898 con-
tains a Shutdown Mode pin, allowing the designer to desig-
nate whether the part will be driven into shutdown with a high
level logic signal or a low level logic signal. This allows the
designer maximum flexibility in device use, as the Shutdown
Mode pin may simply be tied permanently to either V
DD
or
GND to set the LM4898 as either a "shutdown-high" device
or a "shutdown-low" device, respectively. The device may
then be placed into shutdown mode by toggling the Shut-
down Select pin to the same state as the Shutdown Mode
pin. For simplicity’s sake, this is called "shutdown same", as
the LM4898 enters shutdown mode whenever the two pins
are in the same logic state. The trigger point for either
shutdown high or shutdown low 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
maybe 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 (or pull-down, depending on shut-
down high or low application). 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 LM4898 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4898 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4898 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 LM4898
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 5:
V
CMi
<(V
DD
-1.2)*((R
f
+(R
i
)/(R
f
)-V
DD
*(R
i
/2R
f
) (5)
R
f
/R
i
=A
VD
(6)
Special care must be taken to match the values of the
feedback resistors (R
f
1 and R
f
2) to each other as well as
matching the input resistors (R
i
1 and R
i
2) to each other (see
Figure 1). 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 consumption, internal IC power dissipation, reduce
PSRR, and possibly damaging the loudspeaker. The chart
below demonstrates this problem by showing the effects of
differing values between the feedback resistors while as-
suming that the input resistors are perfectly matched. The
results 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
f
1R
f
2 Vo2-Vo1 I
LOAD
20% 0.8R 1.2R -0.5V 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 RRR 0
Similar results would occur if the input resistors were not
carefully matched. Adding input coupling capacitors 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.
LM4898
www.national.com13
Application Information (Continued)
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8ΩAudio Amplifier
Given:
•Power Output 1W
•Load Impedance 8Ω
•Input Level 1Vrms
•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 Characteristics
section. A second way to determine the minimum supply rail
is to calculate the required Vopeak using Equation 7 and add
the dropout voltages. Using this method, the minimum sup-
ply 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 5V. Extra supply
voltage creates headroom that allows the LM4898 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 Dissipation
section. Once the power dissipation equations have been
addressed, the required differential gain can be determined
from Equation 8.
(8)
R
f
/R
i
=A
VD
From Equation 8, the minimum A
VD
is 2.83. Since the de-
sired input impedance was 20kΩ, a ratio of 2.83:1 of R
f
to R
i
results in an allocation of R
i
= 20kΩfor both input resistors
and R
f
= 60kΩfor both feedback resistors. 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 =
150kHz which is much smaller than the LM4898 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4898 can still be used without running into bandwidth
limitations.
LM4898
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted
LLP
Order Number LM4898LD
NSPackage Number LDA10B
LM4898
www.national.com15
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Mini Small Outline (MSOP)
Order Number LM4898MM
NSPackage Number MUB10A
LM4898
www.national.com 16
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
9-Bump micro SMD
Order Number LM4898ITL, LM4898ITLX
NS Package Number TLA09AAA
X1 = 1.514±0.03 X2 = 1.514±0.03 X3 = 0.600±0.075
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.
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
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: ap.support@nsc.com
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM4898 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
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.
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,
and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should
obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are
sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard
warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where
mandated by government requirements, testing of all parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and
applications using TI components. To minimize the risks associated with customer products and applications, customers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right,
or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a
warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual
property of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied
by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive
business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional
restrictions.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all
express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not
responsible or liable for any such statements.
TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably
be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing
such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and
acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products
and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be
provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in
such safety-critical applications.
TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are
specifically designated by TI as military-grade or "enhanced plastic."Only products designated by TI as military-grade meet military
specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at
the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use.
TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are
designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated
products in automotive applications, TI will not be responsible for any failure to meet such requirements.
Following are URLs where you can obtain information on other Texas Instruments products and application solutions:
Products Applications
Audio www.ti.com/audio Communications and Telecom www.ti.com/communications
Amplifiers amplifier.ti.com Computers and Peripherals www.ti.com/computers
Data Converters dataconverter.ti.com Consumer Electronics www.ti.com/consumer-apps
DLP®Products www.dlp.com Energy and Lighting www.ti.com/energy
DSP dsp.ti.com Industrial www.ti.com/industrial
Clocks and Timers www.ti.com/clocks Medical www.ti.com/medical
Interface interface.ti.com Security www.ti.com/security
Logic logic.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense
Power Mgmt power.ti.com Transportation and Automotive www.ti.com/automotive
Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video
RFID www.ti-rfid.com
OMAP Mobile Processors www.ti.com/omap
Wireless Connectivity www.ti.com/wirelessconnectivity
TI E2E Community Home Page e2e.ti.com
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright ©2011, Texas Instruments Incorporated
