LM4900 265mW at 3.3V Supply Audio Power Amplifier - Mall

LM4900 265mW at 3.3V Supply Audio Power Amplifier - Mall
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LM4900
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LM4900 Boomer® Audio Power Amplifier Series 265mW at 3.3V Supply Audio Power
Amplifier with Shutdown Mode
Check for Samples: LM4900
FEATURES
DESCRIPTION
•
•
The LM4900 is a bridged audio power amplifier
capable of delivering 265mW of continuous average
power into an 8Ω load with 1% THD+N from a 3.3V
power supply.
1
23
•
•
•
•
MSOP, LLP, and SOP Packaging
No Output Coupling Capacitors, Bootstrap
Capacitors, or Snubber Circuits are Necessary
Thermal Shutdown Protection Circuitry
Unity-gain Stable
External Gain Configuration Capability
Latest Generation "Click and Pop"
Suppression Circuitry
APPLICATIONS
•
•
•
Cellular Phones
PDA's
Any Portable Audio Application
KEY SPECIFICATIONS
•
•
•
Boomer® audio power amplifiers were designed
specifically to provide high quality output power from
a low supply voltage while requiring a minimal
amount of external components. Since the LM4900
does not require output coupling capacitors, bootstrap
capacitors or snubber networks, it is optimally suited
for low-power portable applications.
The LM4900 features an externally controlled, low
power consumption shutdown mode, and thermal
shutdown protection.
The closed loop response of the unity-gain stable
LM4900 can be configured by external gain-setting
resistors.
THD+N at 1kHz for 265mW Continuous
Average Output Power into 8Ω,
– VDD = 3.3V: 1.0% (max)
THD+N at 1kHz for 675mW Continuous
Average Output Power into 8Ω,
– VDD = 5V: 1.0% (max)
Shutdown Current: 0.1µA (typ)
1
2
3
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
Boomer is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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TYPICAL APPLICATION
Figure 1. Typical Audio Amplifier Application Circuit
CONNECTION DIAGRAM
VSSOP and SOIC Package (Top View)
Package Number DGK, D
WSON Package (Top View)
Package Number NGL
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
2
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ABSOLUTE MAXIMUM RATINGS (1) (2)
Supply Voltage
6.0V
−65°C to +150°C
Storage Temperature
−0.3V to VDD + 0.3V
Input Voltage
Power Dissipation (3)
Internally limited
(4)
2000V
ESD Susceptibility
ESD Susceptibility (5)
200V
Junction Temperature
150°C
Soldering Information
Small Outline Package
Vapor Phase (60 sec.)
215°C
Infrared (15 sec.)
220°C
Thermal Resistance
θJC (D)
35°C/W
θJA (D)
170°C/W
θJC (DGK)
56°C/W
θJA (DGK)
190°C/W
θJA (NGL)
67°C/W
(1)
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
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 the Absolute Maximum Ratings,
whichever is lower. For the LM4900, TJMAX = 150°C. The typical junction-to-ambient thermal resistance, when board mounted, is
190°C/W for package number DGK.
Human body model, 100pF discharged through a 1.5kΩ resistor.
Machine Model, 220pF–240pF discharged through all pins.
(2)
(3)
(4)
(5)
OPERATING RATINGS
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ +85°C
2.0V ≤ VDD ≤ 5.5V
Supply Voltage
ELECTRICAL CHARACTERISTICS (1) (2)
The following specifications apply for VDD = 5V, for all available packages, unless otherwise specified. Limits apply for TA =
25°C
LM4900
Symbol
Parameter
Conditions
Typical
Limit
(4)
Units
(Limits)
4
6.0
mA (max)
(3)
IDD
Quiescent Power Supply
Current
VIN = 0V, IO = 0A (5)
ISD
Shutdown Current
VPIN1 = VDD
VOS
Output Offset Voltage
VIN = 0V
(1)
(2)
(3)
(4)
(5)
0.1
5
μA (max)
5
50
mV (max)
All voltages are measured with respect to the ground pin, unless otherwise specified.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric normal.
Datasheet min/max specification limits are specified by design, test, or statistical analysis.
The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
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ELECTRICAL CHARACTERISTICS(1)(2) (continued)
The following specifications apply for VDD = 5V, for all available packages, unless otherwise specified. Limits apply for TA =
25°C
LM4900
Symbol
Parameter
Conditions
Typical
Limit
(4)
Units
(Limits)
300
mW (min)
(3)
PO
Output Power
THD = 1% (max); f = 1kHz; RL = 8Ω;
675
THD+N
Total Harmonic
Distortion+Noise
PO = 400 mWrms; AVD = 2; RL = 8Ω;
20Hz ≤ f ≤ 20kHz, BW < 80kHz
0.4
PSRR
(6)
(7)
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
f = 217Hz (6)
70
f = 1KHz (6)
67
f = 217Hz (7)
55
f = 1KHz (7)
55
%
dB
Unterminated input.
10Ω terminated input.
ELECTRICAL CHARACTERISTICS (1) (2)
The following specifications apply for VDD = 3.3V, for all available packages, unless otherwise specified. Limits apply for TA =
25°C
LM4900
Symbol
Parameter
Conditions
Typical
Limit
(4)
Units
(Limits)
3
5
mA (max)
(3)
IDD
Quiescent Power Supply
Current
VIN = 0V, IO = 0A (5)
ISD
Shutdown Current
VPIN1 = VDD
VOS
Output Offset Voltage
VIN = 0V
PO
Output Power
THD = 1% (max); f = 1kHz; RL = 8Ω;
THD+N
Total Harmonic
Distortion+Noise
PO = 250 mWrms; AVD = 2; RL = 8Ω;
20Hz ≤ f ≤ 20kHz, BW < 80kHz
f = 217Hz
PSRR
(1)
(2)
(3)
(4)
(5)
(6)
(7)
4
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
f = 1KHz
(6)
(6)
0.1
3
μA (max)
5
50
mV (max)
265
mW (min)
0.4
%
73
70
f = 217Hz (7)
60
f = 1KHz (7)
68
dB
All voltages are measured with respect to the ground pin, unless otherwise specified.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric normal.
Datasheet min/max specification limits are specified by design, test, or statistical analysis.
The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Unterminated input.
10Ω terminated input.
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ELECTRICAL CHARACTERISTICS (1) (2)
The following specifications apply for VDD = 2.6V, for all available packages, unless otherwise specified. Limits apply for TA =
25°C
LM4900
Symbol
Parameter
Conditions
Typical
Limit
(4)
Units
(Limits)
(3)
IDD
Quiescent Power Supply
Current
VIN = 0V, IO = 0A (5)
2.6
4
mA (max)
ISD
Shutdown Current
VPIN1 = VDD
0.1
2.0
μA (max)
VOS
Output Offset Voltage
VIN = 0V
5
mV
PO
Output Power
THD = 1% (max); f = 1kHz; RL = 8Ω
130
mW
THD+N
Total Harmonic
Distortion+Noise
PO = 100 mWrms; AVD = 2; RL = 8Ω;
20Hz ≤ f ≤ 20kHz, BW < 80kHz
0.4
%
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
(1)
(2)
(3)
(4)
(5)
(6)
f = 217Hz (6)
58
f = 1KHz (6)
63
dB
All voltages are measured with respect to the ground pin, unless otherwise specified.
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
Typicals are measured at 25°C and represent the parametric normal.
Datasheet min/max specification limits are specified by design, test, or statistical analysis.
The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
10Ω terminated input.
EXTERNAL COMPONENTS DESCRIPTION
(Figure 1)
Components
Functional Description
1.
Ri
Inverting input resistance which sets the closed-loop gain in conjunction with RF. This resistor also forms a high pass filter
with Ci at fc = 1/(2π RiCI).
2.
Ci
Input coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a highpass filter with Ri
at fc = 1/(2π RiCi). Refer to the section, PROPER SELECTION OF EXTERNAL COMPONENTS for an explanation of how
to determine the value of Ci.
3.
RF
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
4.
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.
5.
CB
Bypass pin capacitor which provides half-supply filtering. Refer to the PROPER SELECTION OF EXTERNAL
COMPONENTS for information concerning proper placement and selection of CB.
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TYPICAL PERFORMANCE CHARACTERISTICS
6
THD+N vs Frequency
THD+N vs Frequency
Figure 2.
Figure 3.
THD+N
vs
Frequency
THD+N vs Frequency
Figure 4.
Figure 5.
THD+N vs Frequency
THD+N vs Frequency
Figure 6.
Figure 7.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD+N vs Frequency
THD+N vs Frequency
Figure 8.
Figure 9.
THD+N vs Frequency
THD+N vs Frequency
Figure 10.
Figure 11.
THD+N vs Frequency
THD+N vs Output Power
Figure 12.
Figure 13.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
8
THD+N vs
Output Power
THD+N vs
Output Power
Figure 14.
Figure 15.
THD+N vs
Output Power
THD+N vs
Output Power
Figure 16.
Figure 17.
THD+N vs
Output Power
THD+N vs
Output Power
Figure 18.
Figure 19.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD+N vs
Output Power
THD+N vs
Output Power
Figure 20.
Figure 21.
THD+N vs
Output Power
THD+N vs
Output Power
Figure 22.
Figure 23.
Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
Figure 24.
Figure 25.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
10
Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
Figure 26.
Figure 27.
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
Figure 28.
Figure 29.
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
Figure 30.
Figure 31.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Clipping Voltage vs
Supply Voltage
Noise Floor
Figure 32.
Figure 33.
Noise Floor
Frequency Response vs
Input Capacitor Size
Figure 34.
Figure 35.
Power Supply
Rejection Ratio
Power Supply
Rejection Ratio
Figure 36.
Figure 37.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
12
Power Supply
Rejection Ratio
Power Supply
Rejection Ratio
Figure 38.
Figure 39.
Power Supply Rejection Ratio
vs Supply Voltage
Power Supply Rejection Ratio
vs Supply Voltage
Figure 40.
Figure 41.
Supply Current vs
Shutdown Voltage
LM4900MM Power Derating Curve
Figure 42.
Figure 43.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Supply Current vs
Supply Voltage
Figure 44.
LM4900LD Power Derating Curve
This curve shows the LM4900LD's thermal dissipation ability at
different ambient temperatures given the exposed-DAP of the part is
soldered to a plane of 1oz. Cu with an area given in the label of each
curve.
Figure 45.
Open Loop Frequency Response
Figure 46.
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APPLICATION INFORMATION
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION
The LM4900's exposed-DAP (die-attach paddle) package (NGL) provides a low thermal resistance between the
die and the PCB to which the part is mounted and soldered. This allows rapid heat from the die to the
surrounding PCB copper traces, ground plane, and surrounding air. This allows the NGL to operate at higher
output power levels in higher ambient temperatures than the DGK, D package. Failing to optimize thermal design
may compromise the high power performance and activate unwanted, though necessary, thermal shutdown
protection.
The NGL 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, 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 2 vias. The via diameter should be 0.012in - 0.013in with a 1.27mm pitch. Ensure efficient
thermal conductivity by plating through the vias.
Best thermal performance is achieved with the largest practical heat sink area. The power derating curve in the
TYPICAL PERFORMANCE CHARACTERISTICS shows the maximum power dissipation versus temperature for
several different areas of heat sink area. Placing the majority of the heat sink area on another plane is preferred
as heat is best dissipated through the bottom of the chip. Further detailed and specific information concerning
PCB layout, fabrication, and mounting an NGL (WSON) package is available from Texas Instrument's Package
Engineering Group under application note AN1187.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4900 has two operational amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier's gain is externally configurable, while the second amplifier is internally fixed in
a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of RF to
Ri while the second amplifier's gain is fixed by the two internal 10 kΩ resistors. Figure 1 shows that the output of
amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in
magnitude, but out of phase 180°. Consequently, the differential gain for the IC is
AVD = 2*(RF/Ri)
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides
differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output
power is possible as compared to 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 closedloop gain without causing excessive clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN section.
A bridge configuration, such as the one used in LM4900, also creates a second advantage over single-ended
amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across
the load. This eliminates the need for an output coupling capacitor which is required in a single supply, singleended amplifier configuration. If an output coupling capacitor is not used in a single-ended configuration, the halfsupply bias across the load would result in both increased internal lC power dissipation as well as permanent
loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended. Equation 1 states the maximum power dissipation point for a bridge amplifier operating at a given
supply voltage and driving a specified output load.
PDMAX = (VDD)2/(2π2RL)
Single-Ended
(1)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase
in internal power dissipation point for a bridge amplifier operating at the same conditions.
PDMAX = 4(VDD)2/(2π2RL)
14
Bridge Mode
(2)
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Since the LM4900 has two operational amplifiers in one package, the maximum internal power dissipation is 4
times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4900 does
not require heatsinking. From Equation 1, assuming a 5V power supply and an 8Ω load, the maximum power
dissipation point is 625 mW. The maximum power dissipation point obtained from Equation 2 must not be greater
than the power dissipation that results from Equation 3:
PDMAX = (TJMAX − TA)/θJA
(3)
For package DGK, θJA = 190°C/W. TJMAX = 150°C for the LM4900. Depending on the ambient temperature, TA,
of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by
the IC packaging. If the result of Equation 2 is greater than that of Equation 3, 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 a 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. Internal power dissipation is a function of output power. If typical operation is not around
the maximum power dissipation point, the ambient temperature can be increased. Refer to the TYPICAL
PERFORMANCE CHARACTERISTICS curves for power dissipation information for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as
possible. The effect of a larger half supply bypass capacitor is improved PSRR due to increased half-supply
stability. Typical applications employ a 5V regulator with 10μF and a 0.1μF bypass capacitors which aid in supply
stability, but do not eliminate the need for bypassing the supply nodes of the LM4900. The selection of bypass
capacitors, especially CB, is thus dependent upon desired PSRR requirements, click and pop performance as
explained in the section, PROPER SELECTION OF EXTERNAL COMPONENTS, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4900 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the
shutdown pin. The trigger point between a logic low and logic high level is typically half supply. It is best to switch
between ground and supply to provide maximum device performance. By switching the shutdown pin to VDD, the
LM4900 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin
voltages less than VDD, 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
provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground
and enables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4900. This
scheme ensures 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 to optimize
device and system performance. While the LM4900 is tolerant to a variety of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4900 is unity-gain stable, giving a designer maximum system flexibility. The LM4900 should be used in
low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain
configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1
Vrms are available from sources such as audio codecs. Please refer to the section, AUDIO POWER AMPLIFIER
DESIGN, for a more complete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, Ci,
forms a first order high pass filter which limits low frequency response. This value should be chosen based on
needed frequency response for a few distinct reasons.
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SELECTION OF INPUT CAPACITOR SIZE
Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers
used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. In this
case using a large input capacitor may not increase system performance.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
½ VDD). This charge comes from the output via the feedback and is apt to create pops upon device enable. Thus,
by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value.
Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the
LM4900 turns on. The slower the LM4900's outputs ramp to their quiescent DC voltage (nominally ½ VDD), the
smaller the turn-on pop. Choosing CB equal to 1.0 μF along with a small value of Ci (in the range of 0.1μF to
0.39μF), should produce a clickless and popless shutdown function. While the device will function properly, (no
oscillations or motorboating), with CB equal to 0.1μF, the device will be much more susceptible to turn-on clicks
and pops. Thus, a value of CB equal to 1.0μF or larger is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
DESIGN A 300 mW/8Ω AUDIO AMPLIFIER
Given:
Power Output
300mWrms
Load Impedance
8Ω
Input Level
1Vrms
Input Impedance
20kΩ
Bandwidth
100Hz–20 kHz ± 0.25dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from the Output Power vs Supply Voltage graphs in the TYPICAL PERFORMANCE CHARACTERISTICS
section, the supply rail can be easily found. A second way to determine the minimum supply rail is to calculate
the required Vopeak using Equation 4 and add the dropout voltage. Using this method, the minimum supply
voltage would be (Vopeak + (2*VOD)), where VOD is extrapolated from the Dropout Voltage vs Supply Voltage curve
in the TYPICAL PERFORMANCE CHARACTERISTICS section.
(4)
Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 3.5V. But since 5V is
a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4900 to reproduce peaks in excess of 700 mW 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 5.
(5)
(6)
RF/Ri = AVD/2
From Equation 5, the minimum AVD is 1.55; use AVD = 2.
Since the desired input impedance was 20 kΩ, and with a AVD of 2, a ratio of 1:1 of RF to Ri results in an
allocation of Ri = RF = 20 kΩ. The final design step is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away from a pole gives 0.17 dB down from passband
response which is better than the required ±0.25 dB specified.
fL = 100Hz/5 = 20Hz
fH = 20kHz × 5 = 100kHz
16
(7)
(8)
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As stated in the EXTERNAL COMPONENTS DESCRIPTION section, Ri in conjunction with Ci create a highpass
filter.
(9)
(10)
Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397μF; use 0.39μF
The high frequency pole is determined by the product of the desired high frequency pole, fH, and the differential
gain, AVD. With a AVD = 2 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the
LM4900 GBWP of 25MHz. This figure displays that if a designer has a need to design an amplifier with a higher
differential gain, the LM4900 can still be used without running into bandwidth problems.
Figure 47. Differential Amplifier Configuration for LM4900
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REVISION HISTORY
Changes from Revision C (April 2013) to Revision D
•
18
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 17
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