NSC LM4894ITL

LM4894
1 Watt Fully Differential Audio Power Amplifier With
Shutdown Select
General Description
j Power Output at 5.0V & 1% THD
1.0W(typ)
The LM4894 is a fully differential audio power amplifier
primarily designed for demanding applications in mobile
phones and other portable communication device applications. It is capable of delivering 1 watt of continuous average
power to an 8Ω BTL load with less than 1% distortion
(THD+N) from a 5VDC power supply.
j Power Output at 3.3V & 1% THD
400mW(typ)
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4894 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other low voltage applications where minimal power consumption is a primary requirement.
The LM4894 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 Shutdown. Additionally, the LM4894 features an internal thermal
shutdown protection mechanism.
The LM4894 contains advanced pop & click circuitry which
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
The LM4894 is unity-gain stable and can be configured by
external gain-setting resistors.
j Shutdown Current
0.1µA(typ)
Features
n Fully differential amplification
n Available in space-saving packages micro SMD, MSOP,
and LLP
n Ultra low current shutdown mode
n Can drive capacitive loads up to 500pF
n Improved pop & click circuitry eliminates noises during
turn-on and turn-off transitions
n 2.2 - 5.5V operation
n No output coupling capacitors, snubber networks or
bootstrap capacitors required
n Unity-gain stable
n External gain configuration capability
n Shutdown high or low selectivity
n High CMRR
Applications
n Mobile phones
n PDAs
n Portable electronic devices
Key Specifications
j Improved PSRR at 217Hz
80dB(typ)
Connection Diagrams
Mini Small Outline (MSOP) Package
LLP Package
20013723
Top View
Order Number LM4894MM
See NS Package Number MUB10A
20013735
Top View
Order Number LM4894LD
See NS Package Number LDA10B
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation
DS200137
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LM4894 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
June 2003
LM4894
Connection Diagrams
(Continued)
9 Bump micro SMD Package
9 Bump micro SMD Package
20013794
Top View
Order Number LM4894ITL, LM4894ITLX
See NS Package Number TLA09AAA
20013736
Top View
Order Number LM4894IBP
See NS Package Number BPA09CDB
Typical Application
20013701
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
θJA (micro SMD)
(Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
−0.3V to VDD +0.3V
Power Dissipation (Note 3)
Internally Limited
ESD Susceptibility (Note 4)
2000V
ESD Susceptibility (Note 5)
200V
Junction Temperature
56˚C/W
θJA (MSOP)
190˚C/W
See AN-1112 "microSMD Wafers Level Chip Scale
Package."
−65˚C to +150˚C
Input Voltage
θJC (MSOP)
Soldering Information
6.0V
Storage Temperature
220˚C/W
See AN-1187 "Leadless
Leadframe Package (LLP)."
Operating Ratings
150˚C
Temperature Range
Thermal Resistance
θJC (LLP)
12˚C/W
θJA (LLP)
63˚C/W
TMIN ≤ TA ≤ TMAX
−40˚C ≤ TA ≤ 85˚C
2.2V ≤ VDD ≤ 5.5V
Supply Voltage
Electrical Characteristics VDD = 5V (Notes 1, 2, 8)
The following specifications apply for VDD = 5V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4894
Symbol
Parameter
Conditions
Typical
Limit
(Note 6)
(Note 7)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, Io = 0A
4
8
mA (max)
ISD
Shutdown Current
Vshutdown = GND
0.1
1
µA (max)
Po
Output Power
THD = 1% (max); f = 1 kHz
LM4894LD, RL= 4Ω (Note 11)
0.1
%
f = 217Hz (Note 9)
87
dB (min)
f = 1kHz (Note 9)
83
f = 217Hz (Note 10)
83
Total Harmonic Distortion+Noise
Po = 0.4 Wrms; f = 1kHz
PSRR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
f = 1kHz (Note 10)
Common_Mode Rejection Ratio
W (min)
1
THD+N
CMRR
1.4
LM4894, RL= 8Ω
f = 217Hz
0.850
63
80
50
dB
Electrical Characteristics VDD = 3V (Notes 1, 2, 8)
The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4894
Symbol
Parameter
IDD
Quiescent Power Supply Current
Conditions
VIN = 0V, Io = 0A
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
3.5
6
mA (max)
1
µA (max)
ISD
Shutdown Current
Vshutdown = GND
0.1
Po
Output Power
THD = 1% (max); f = 1kHz
0.35
W
THD+N
Total Harmonic Distortion+Noise
Po = 0.25Wrms; f = 1kHz
0.325
%
PSRR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
f = 217Hz (Note 9)
87
dB
f = 1kHz (Note 9)
83
f = 217Hz (Note 10)
80
CMRR
Common-Mode Rejection Ratio
f = 1kHz (Note 10)
78
f = 217Hz
49
3
dB
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LM4894
Absolute Maximum Ratings
LM4894
Electrical Characteristics VDD = 3V (Notes 1, 2, 8)
The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA =
25˚C. (Continued)
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 = (TJMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4894, see power derating
currents for additional information.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
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: 10Ω terminated input.
Note 11: When driving 4Ω loads from a 5V supply, the LM4894LD must be mounted to a circuit board.
External Components Description
(Figure 1)
Components
Functional Description
1.
Ri
Inverting input resistance which sets the closed-loop gain in conjunction with Rf.
2.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
3.
CS
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.
4.
CB
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 CB.
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LM4894
Typical Performance Characteristics
LD Specific Characteristics
LM4894LD
THD+N vs Output Power
VDD = 5V, 4Ω RL
VDD
LM4894LD
THD+N vs Frequency
= 5V, 4Ω RL, and Power = 1W
20013790
20013791
LM4894LD
Power Dissipation vs
Output Power
LM4894LD
Power Derating Curve
20013792
20013793
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LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics
THD+N vs Frequency
at VDD = 3V, 8Ω RL, and PWR = 250mW
THD+N vs Frequency
at VDD = 5V, 8Ω RL, and PWR = 400mW
20013737
20013738
THD+N vs Frequency
at VDD = 2.6V, 8Ω RL, and PWR = 150mW
THD+N vs Frequency
at VDD = 3V, 4Ω RL, and PWR = 225mW
20013739
20013740
THD+N vs Output Power
@ VDD = 5V, 8Ω RL
THD+N vs Frequency
@ VDD = 2.6V, 4Ω RL, and PWR = 150mW
20013741
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20013742
6
LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
THD+N vs Output Power
@ VDD = 3V, 8Ω RL
THD+N vs Output Power
@ VDD = 3V, 4Ω RL
20013743
20013744
THD+N vs Output Power
@ VDD = 2.6V, 8Ω RL
THD+N vs Output Power
@ VDD = 2.6V, 4Ω RL
20013745
20013773
Power Supply Rejection Ratio (PSRR) @ VDD = 5V
Input 10Ω Terminated
Power Supply Rejection Ratio (PSRR) @ VDD = 5V
Input Floating
20013747
20013746
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LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
Power Supply Rejection Ratio (PSRR) @ VDD = 3V
Input 10Ω Terminated
Power Supply Rejection Ratio (PSRR) @ VDD = 3V
Input Floating
20013749
20013748
Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
20013751
20013750
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
20013753
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20013752
8
LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
Power Dissipation vs
Output Power
Output Power vs
Load Resistance
20013755
20013754
Supply Current vs Shutdown Voltage
Shutdown Low
Supply Current vs Shutdown Voltage
Shutdown High
20013769
20013756
Clipping (Dropout) Voltage vs
Supply Voltage
Open Loop Frequency Response
20013784
20013774
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LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
Power Derating Curve
Noise Floor
20013776
20013777
Input CMRR vs
Frequency
Input CMRR vs
Frequency
20013778
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20013779
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LM4894
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
PSRR vs
DC Common-Mode Voltage
PSRR vs
DC Common-Mode Voltage
20013780
20013781
THD vs
Common-Mode Voltage
THD vs
Common-Mode Voltage
20013782
20013783
amplifier performance. See the Proper Selection of External 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 LM4894 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 LM4894 simply amplifies the difference between them. Figures 2 and 3 show single-ended
applications of the LM4894 that still take advantage of the
differential nature of the amplifier and the benefits to PSRR,
common-mode noise reduction, and "click and pop" reduction.
Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4894 is a fully differential audio amplifier that features 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 VDD/2. When
setting the differential gain, the amplifier can be considered
to have "halves". Each half uses an input and feedback
resistor (Ri1and RF1) to set its respective closed-loop gain
(see Figure 1). With Ri1 = Ri2 and RF1 = RF2, the gain is set
at -RF/Ri for each half. This results in a differential gain of
AVD = -RF/Ri
(1)
All of these applications, either single-ended or fully differential, 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
It is extremely important to match the input resistors to each
other, as well as the feedback resistors to each other for best
11
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LM4894
Application Information
temperature. In all circumstances and conditions, the junction temperature must be held below 150˚C to prevent activating the LM4894’s thermal shutdown protection. The
LM4894’s power de-rating curve in the Typical Performance
Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts for the exposedDAP 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 available from National Semiconductor’s package Engineering Group under application note
AN1187.
(Continued)
other. Bridged mode operation is different from the singleended 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 assumes that the
amplifier is not current limited or clipped. In order to choose
an amplifier’s closed-loop gain without causing excess clipping, please refer to the Audio Power Amplifier Design
section.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 1.4W to
1.37W. 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.
A bridged configuration, such as the one used in the
LM4894, also creates a second advantage over singleended amplifiers. 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 resistor pair are properly matched (see Proper
Selection of External Components). BTL configuration
eliminates the output coupling capacitor required in singlesupply, 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 LM4894 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
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 supplies, trace resistance creates the same effects as poor
sup-ply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4894’s exposed-DAP (die attach paddle) package
(LD) 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. The result is a low voltage audio power amplifier that
produces 1.4W at ≤ 1% THD with a 4Ω load. This high power
is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4894’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 platingthrough and solder-filling the vias.
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 dissipation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
PDMAX=(VDD)2/(2π2RL) Single-Ended
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in
internal power dissipation versus a single-ended amplifier
operating at the same conditions.
PDMAX = 4*(VDD)2/(2π2RL) Bridge Mode
(3)
Since the LM4894 has bridged outputs, the maximum internal power dissipation is 4 times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4894 does not require additional heatsinking
under most operating conditions and output loading. From
Equation 3, assuming a 5V power supply and an 8Ω load,
the maximum power dissipation point is 625mW. The maxi-
Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in2 (min) area is
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4894 should be
5in2 (min) for the same supply voltage and load resistance.
The last two area recommendations apply for 25˚C ambient
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(2)
12
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 performance. 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.
(Continued)
mum power dissipation point obtained from Equation 3 must
not be greater than the power dissipation results from Equation 4:
(4)
PDMAX = (TJMAX - TA)/θJA
The LM4894’s θJA in an MUA10A package is 190˚C/W.
Depending on the ambient temperature, TA, 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
the θJA reduced with heatsinking. In many cases, larger
traces near the output, VDD, and GND pins can be used to
lower the θJA. 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 8Ω load, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 30˚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 LM4894 can operate
at higher ambient temperatures. Refer to the Typical Performance Characteristics curves for power dissipation information.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which provides a quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shutdown 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 using integrated power amplifiers is critical when optimizing
device and system performance. Although the LM4894 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4894 is unity-gain stable, giving the designer maximum system flexibility. The LM4894 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 LM4894
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than VDD. Exact
allowable input common-mode voltage levels are actually a
function of VDD, Ri, and Rf and may be determined by
Equation 5:
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 LM4894. Although the LM4894 will
operate without the bypass capacitor CB, although the PSRR
may decrease. A 1µF capacitor is recommended for CB. This
value maximizes PSRR performance. Lesser values may be
used, but PSRR decreases at frequencies below 1kHz. The
issue of CB 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
LM4894 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4894 contains a Shutdown Mode pin, allowing the designer to designate 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 VDD or
GND to set the LM4894 as either a "shutdown-high" device
or a "shutdown-low" device, respectively. The device may
then be placed into shutdown mode by toggling the Shutdown Select pin to the same state as the Shutdown Mode
pin. For simplicity’s sake, this is called "shutdown same", as
the LM4894 enters shutdown mode whenever the two pins
VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri/2Rf)
(5)
VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri/2Rf)
(6)
Special care must be taken to match the values of the
feedback resistors (RF1 and RF2) to each other as well as
matching the input resistors (Ri1 and Ri2) to each other (see
Figure 1) more infront. 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
assuming that the input resistors are perfectly matched. The
results below apply to the application circuit shown in Figure
1, and assumes that VDD = 5V, RL = 8Ω, and the system has
DC coupled inputs tied to ground.
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LM4894
Application Information
LM4894
Application Information
(Continued)
(7)
Tolerance RF1
RF2
V02 - V01
ILOAD
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
0
R
0
Using the Output Power vs Supply Voltage graph for an 8W
load, the minimum supply rail just about 5V. Extra supply
voltage creates headroom that allows the LM4894 to reproduce peaks in excess of 1W without producing audible distortion. 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 7.
Similar results would occur if the input resistors were not
carefully matched. Adding input coupling capacitors in between 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% tolerance or better.
(8)
Rf / Ri = AVD
AUDIO POWER AMPLIFIER DESIGN
From Equation 7, the minimum AVD is 2.83. Since the desired input impedance was 20kΩ, a ratio of 2.83:1 of Rf to Ri
results in an allocation of Ri = 20kΩ for both input resistors
and Rf= 60kΩ for 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.
Design a 1W/8Ω Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
1Wrms
8Ω
1Vrms
20kΩ
–20kHz ± 0.25dB
fH = 20kHz * 5 =100kHz
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 supply voltage is (Vopeak +(VDO TOP+(VDO BOT )),
where VDO BOT and VDO TOP are extrapolated from the
Dropout Voltage vs Supply Voltage curve in the Typical
Performance Characteristics section.
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The high frequency pole is determined by the product of the
desired frequency pole, fH , and the differential gain, AVD .
With a AVD = 2.83 and fH = 100kHz, the resulting GBWP =
150kHz which is much smaller than the LM4894 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4894 can still be used without running into bandwidth
limitations.
14
LM4894
Application Information
(Continued)
20013787
FIGURE 2. Single-Ended Input, "Shutdown-Low" Configuration
20013785
FIGURE 3. Single-Ended Input, "Shutdown-High" Configuration
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LM4894
Physical Dimensions
inches (millimeters) unless otherwise noted
9-Bump micro SMD
Order Number LM4894IBP
NS Package Number BPA09CDB
X1 = 1.336 ± 0.03 X2 = 1.361 ± 0.03 X3 = 0.850 ± 0.10
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16
LM4894
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
LLP
Order Number LM4894LD
NSPackage Number LDA10B
17
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LM4894
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
Mini Small Outline (MSOP)
Order Number LM4894MM
NSPackage Number MUB10A
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18
inches (millimeters) unless otherwise noted (Continued)
9-Bump micro SMD
Order Number LM4894ITL, LM4894ITLX
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.
National Semiconductor
Americas Customer
Support Center
Email: [email protected]
Tel: 1-800-272-9959
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National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: [email protected]
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
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
Asia Pacific Customer
Support Center
Email: [email protected]
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedbac[email protected]
Tel: 81-3-5639-7560
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.
LM4894 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
Physical Dimensions