NSC LM4923 1.1 watt fully differential audio power amplifier with shutdown select Datasheet

LM4923
1.1 Watt Fully Differential Audio Power Amplifier With
Shutdown Select
General Description
Key Specifications
The LM4923 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 1.1 watt of continuous average power to an 8Ω BTL load with less than 1% distortion
(THD+N) from a 5VDC power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4923 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 LM4923 features a low-power consumption shutdown
mode. To facilitate this, Shutdown may be enabled by logic
low. Additionally, the LM4923 features an internal thermal
shutdown protection mechanism.
The LM4923 contains advanced pop & click circuitry which
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
j Improved PSRR at 217Hz
85dB(typ)
j Power Output at 5.0V @ 1% THD+N
1.1W(typ)
j Power Output at 3.3V @ 1% THD+N
400mW(typ)
j Shutdown Current
0.1µA(typ)
Features
Fully differential amplification
Available in space-saving LLP package
Ultra low current shutdown mode
Can drive capacitive loads up to 100pF
Improved pop & click circuitry eliminates noises during
turn-on and turn-off transitions
n 2.4 - 5.5V operation
n No output coupling capacitors, snubber networks or
bootstrap capacitors required
n
n
n
n
n
Applications
n Mobile phones
n PDAs
n Portable electronic devices
Connection Diagrams
LQ Package
8 Pin LQ Marking
20071302
20071330
Top View
Order Number LM4923LQ
See NS Package Number LQB08A
X − Date Code
TT − Die Traceability
G − Boomer
B2 − LM4923LQ
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation
DS200713
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LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
July 2004
LM4923
Typical Application
20071313
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
Thermal Resistance
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Soldering Information
Supply Voltage
θJA (LLP)
140˚C/W
See AN-1187
6.0V
Storage Temperature
−65˚C to +150˚C
Operating Ratings
−0.3V to VDD +0.3V
Input Voltage
Power Dissipation (Note 3)
Internally Limited
ESD Susceptibility (Note 4)
2000V
ESD Susceptibility (Note 5)
Junction Temperature
Temperature Range
TMIN ≤ TA ≤ TMAX
−40˚C ≤ TA ≤ 85˚C
2.4V ≤ VDD ≤ 5.5V
Supply Voltage
200V
150˚C
Electrical Characteristics VDD = 5V (Notes 1, 2)
The following specifications apply for VDD = 5V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4923
Symbol
Parameter
Conditions
Typical
Limit
Units
(Limits)
(Note 6)
(Note 7)
IDD
Quiescent Power Supply Current
VIN = 0V, no load
VIN = 0V, RL = 8Ω
4
4
9
9
mA (max)
0.1
1
µA (max)
1.1
1
ISD
Shutdown Current
VSHUTDOWN = GND
Po
Output Power
THD = 1% (max); f = 1 kHz
THD+N
Total Harmonic Distortion+Noise
Po = 0.4 Wrms; f = 1kHz
PSRR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
LM4923, RL = 8Ω
0.02
%
f = 217Hz (Note 8)
85
73
f = 1kHz (Note 8)
85
73
CMRR
Common_Mode Rejection Ratio
f = 217Hz,
VCM = 200mVpp
VOS
Output Offset
VIN = 0V
VSDIH
Shutdown Voltage Input High
VSDIL
Shutdown Voltage Input Low
0.7
V
50
dB
4
mV
0.9
V
Electrical Characteristics VDD = 3V (Notes 1, 2)
The following specifications apply for VDD = 3V, AV = 1, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4923
Symbol
Parameter
IDD
Quiescent Power Supply Current
Conditions
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
VIN = 0V, no load
VIN = 0V, RL = 8Ω
3
3
5.5
5.5
mA (max)
0.1
1
µA (max)
ISD
Shutdown Current
VSHUTDOWN = GND
Po
Output Power
THD = 1% (max); f = 1kHz
LM4923, RL = 8Ω
0.375
W
THD+N
Total Harmonic Distortion+Noise
Po = 0.25Wrms; f = 1kHz
0.02
%
PSRR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
f = 217Hz (Note 8)
85
f = 1kHz (Note 8)
85
CMRR
Common-Mode Rejection Ratio
f = 217Hz
VCM = 200mVpp
VIN = 0V
73
50
dB
VOS
Output Offset
4
mV
VSDIH
Shutdown Voltage Input High
0.8
V
VSDIL
Shutdown Voltage Input Low
0.6
V
3
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LM4923
Absolute Maximum Ratings (Note 2)
LM4923
Electrical Characteristics VDD = 3V (Notes 1, 2)
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 LM4923, see power
derating curve 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: 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.
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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4
LM4923
Typical Performance Characteristics
THD+N vs Frequency
VDD = 2.6V, RL = 4Ω, PO = 150mW
THD+N vs Frequency
VDD = 2.6V, RL = 8Ω, PO = 150mW
20071306
20071305
THD+N vs Frequency
VDD = 3V, RL = 8Ω, PO = 275mW
THD+N vs Frequency
VDD = 5V, RL = 8Ω, PO = 400mW
20071309
20071308
THD+N vs Output Power
VDD = 2.6V, RL = 8Ω
THD+N vs Frequency
VDD = 3V, RL = 4Ω, PO = 225mW
20071307
20071311
5
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LM4923
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
VDD = 2.6V, RL = 4Ω
THD+N vs Output Power
VDD = 5V, RL = 8Ω
20071310
20071315
THD+N vs Output Power
VDD = 3V, RL = 4Ω
THD+N vs Output Power
VDD = 3V, RL = 8Ω
20071314
20071312
PSRR vs Frequency
VDD = 3V, RL = 8Ω, Input terminated
PSRR vs Frequency
VDD = 5V, RL = 8Ω, Input terminated
20071304
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20071303
6
LM4923
Typical Performance Characteristics
(Continued)
Output Power vs Supply Voltage
RL = 8Ω
CMRR vs Frequency
VDD = 5V, RL = 8Ω
20071301
20071317
PSRR vs Common Mode Voltage
VDD = 3V, RL = 8Ω, f = 217Hz
CMRR vs Frequency
VDD = 3V, RL = 8Ω
20071326
20071322
Power Dissipation vs Output Power
VDD = 2.6V, RL = 8Ω and 4Ω
PSRR vs Common Mode Voltage
VDD = 5V, RL = 8Ω, f = 217Hz
20071316
20071321
7
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LM4923
Typical Performance Characteristics
(Continued)
Power Dissipation vs Output Power
VDD = 5V, RL = 8Ω
Power Dissipation vs Output Power
VDD = 3V, RL = 8Ω
20071319
20071325
Power Derating Curve
Noise Floor
VDD = 5V
20071318
20071324
Clipping Voltage vs Supply Voltage
Noise Floor
VDD = 3V
20071320
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20071323
8
(Continued)
Output Power vs Load Resistance
Supply Current Shutdown Voltage
20071328
20071327
sumes 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.
Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4923 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 (Ri1 and 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
LM4923
Typical Performance Characteristics
A bridged configuration, such as the one used in the
LM4923, 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 LM4923 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
(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
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 LM4923 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 LM4923 simply amplifies the difference between them.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4923’s exposed-DAP (die attach paddle) package
(LLP) provide a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. Failing to optimize thermal design may compromise the
LM4923’s high power performance and activate unwanted,
though necessary, thermal shutdown protection. The LLP
package must have its DAP soldered to a copper pad on the
PCB. The DAP’s PCB copper pad is connected to a large
plane of continuous unbroken copper. This plane forms a
thermal mass and heat sink and radiation area. Place the
heat sink area on either outside plane in the case of a
two-sided PCB, or on an inner layer of a board with more
than two layers. Connect the DAP copper pad to the inner
layer or backside copper heat sink area with a thermal via.
All of these applications 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 between the amplifier output and ground. A bridged
amplifier design has distinct advantages over the singleended 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 as9
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LM4923
Application Information
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 maximum power dissipation point obtained from Equation 3 must
not be greater than the power dissipation results from Equation 4:
(4)
PDMAX = (TJMAX - TA) / θJA
(Continued)
The via diameter should be 0.012in - 0.013in. Ensure efficient thermal conductivity by plating-through and solderfilling the vias.
Best thermal performance is achieved with the largest practical copper heat sink area. In all circumstances and conditions, the junction temperature must be held below 150˚C to
prevent activating the LM4923’s thermal shutdown protection. The LM4923’s power de-rating curve in the Typical
Performance Characteristics shows the maximum power
dissipation versus temperature. Example PCB layouts are
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout,
fabrication, and mounting an LLP package is available from
National Semiconductor’s package Engineering Group under application note AN1187.
The LM4923’s θJA in an LQB08A package is 140˚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 62˚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 LM4923 can operate
at higher ambient temperatures. Refer to the Typical Performance Characteristics curves for power dissipation information.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 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. 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.
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 LM4923. The LM4923 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.
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
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifer, whether the amplifier is bridged or
single-ended. Equation 2 states the maximum power 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
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4923 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. The device may then be placed
into shutdown mode by toggling the Shutdown Select pin to
logic low. The trigger point for shutdown is shown as a typical
value in the Supply Current vs Shutdown Voltage graphs in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for maximum 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.
(2)
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 LM4923 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 LM4923 does not require additional heatsinking
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In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro10
minimum component count it is highly recommended that
both the feedback and input resistors matched to 1% tolerance or better.
(Continued)
vides a quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction with an
external pull-up resistor. This scheme guarantees that the
shutdown pin will not float, thus preventing unwanted state
changes.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Given:
PROPER SELECTION OF EXTERNAL COMPONENTS
Power Output
Proper selection of external components in applications using integrated power amplifiers is critical when optimizing
device and system performance. Although the LM4923 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
Load Impedance
Input Level
Input Impedance
Bandwidth
The LM4923 is unity-gain stable, giving the designer maximum system flexibility. The LM4923 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 LM4923
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:
VCMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri / 2Rf)
(5)
-RF / RI = AVD
(6)
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
8Ω
1Vrms
20kΩ
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
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.
(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 LM4923 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 8.
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 in front. 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.
Tolerance RF1
1Wrms
(8)
Rf / Ri = AVD
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.
fH = 20kHz * 5 = 100kHz
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 LM4923 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4923 can still be used without running into bandwidth
limitations.
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
11
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LM4923
Application Information
LM4923 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
Physical Dimensions
inches (millimeters) unless otherwise noted
LQ Package
Order Number LM4923LQ
NS Package Number LQB08A
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