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LM4891 Boomer® Audio Power Amplifier Series 1 Watt Audio Power Amplifier
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FEATURES
DESCRIPTION
•
The LM4891 is an 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.
1
23
•
•
•
•
•
•
•
•
Available in Space-Saving Packages: Micro
SMD, VSSOP, SOIC, and WSON
Ultra Low Current Shutdown Mode
BTL Output Can Drive Capacitive Loads
Improved Pop and Click Circuitry Eliminates
Noises During Turn-On and Turn-Off
Transitions
2.2 to 5.5V Operation
No Output Coupling Capacitors, Snubber
Networks or Bootstrap Capacitors Required
Thermal Shutdown Protection
Unity-Gain Stable
External Gain Configuration Capability
APPLICATIONS
•
•
•
Mobile Phones
PDAs
Portable Electronic Devices
KEY SPECIFICATIONS
•
•
•
•
PSRR at 217Hz, VDD = 5V, 8Ω Load: 62dB (typ)
Power Output at 5.0V & 1% THD: 1.0W (typ)
Power Output at 3.3V & 1% THD: 400mW (typ)
Shutdown Current: 0.1μA (typ)
Boomer® audio power amplifiers were designed
specifically to provide high quality output power with a
minimal amount of external components. The
LM4891 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 LM4891 features a low-power consumption
shutdown mode, which is achieved by driving the
shutdown pin with logic high. Additionally, the
LM4891 features an internal thermal shutdown
protection mechanism.
The LM4891 contains advanced pop & click circuitry
which eliminates noises which would otherwise occur
during turn-on and turn-off transitions.
The LM4891 is unity-gain stable and can be
configured by external gain-setting resistors.
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
Figure 2. 8 Bump micro SMD (Top View)
Package Number BPA08DDB
2
Figure 3. Small Outline (SOIC) Package (Top View)
Package Number D
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Figure 4. Mini Small Outline (VSSOP) Package
(Top View)
Package Number NGZ
Figure 5. WSON Package (Top View)
Package Number NGZ
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.
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ABSOLUTE MAXIMUM RATINGS (1) (2)
Supply Voltage (3)
6.0V
−65°C to +150°C
Storage Temperature
−0.3V to VDD +0.3V
Input Voltage
Power Dissipation (4) (5)
ESD Susceptibility
Internally Limited
(6)
2000V
ESD Susceptibility (7)
250V
Junction Temperature
150°C
Thermal Resistance
θJC (SOIC)
35°C/W
θJA (SOIC)
150°C/W
θJA (micro SMD)
220°C/W
θJC (VSSOP)
56°C/W
θJA (VSSOP)
190°C/W
θJA (WSON)
220°C/W
Soldering Information
See AN-1112 "DSBGA Wafer Level Chip Scale Package ".
See AN-1187 "Leadlesss Leadframe Package (LLP)".
(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.
If the product is in shutdown mode and VDD exceeds 6V (to a max of 8V VDD), then most of the excess current will flow through the ESD
protection circuits. If the source impedance limits the current to a max of 10 mA, then the part will be protected. If the part is enabled
when VDD is greater than 5.5V and less than 6.5V, no damage will occur, although operational life will be reduced. Operation above
6.5V with no current limit will result in permanent damage.
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 LM4891, see TYPICAL PERFORMANCE CHARACTERISTICS curves for additional information.
Maximum power dissipation (PDMAX) in the device occurs at an output power level significantly below full output power. PDMAX can be
calculated using Equation 1 shown in the Application section. It may also be obtained from the power dissipation graphs.
Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Machine Model, 220 pF–240 pF discharged through all pins.
(2)
(3)
(4)
(5)
(6)
(7)
OPERATING RATINGS
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ 85°C
2.2V ≤ VDD ≤ 5.5V
Supply Voltage
ELECTRICAL CHARACTERISTICS VDD = 5V (1) (2) (3)
The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.
LM4891
Symbol
Parameter
Conditions
Typical
(4)
IDD
(1)
(2)
(3)
(4)
(5)
4
Quiescent Power Supply Current
VIN = 0V, Io = 0A
4
Limit
(5)
10
Units
(Limits)
mA (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.
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.
Typicals are measured at 25°C and represent the parametric normal.
Limits are specified to AOQL (Average Outgoing Quality Level).
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ELECTRICAL CHARACTERISTICS VDD = 5V(1)(2)(3) (continued)
The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.
LM4891
Symbol
Parameter
Conditions
Typical
Limit
(4)
(5)
Units
(Limits)
ISD
Shutdown Current
Vshutdown = VDD
PO
Output Power
THD = 2% (max); f = 1 kHz
THD+N
Total Harmonic Distortion+Noise
Po = 0.4 Wrms; f = 1kHz
0.1
%
PSRR (6)
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
62 (f = 217Hz)
66 (f = 1kHz)
dB
(6)
0.1
µA (max)
1
W
PSRR is a function of system gain. Specifications apply to the circuit in Figure 1 where AV = 2. Higher system gains will reduce PSRR
value by the amount of gain increase. A system gain of 10 represents a gain increase of 14dB. PSRR will be reduced by 14dB and
applies to all operating voltages.
ELECTRICAL CHARACTERISTICS VDD = 3.3V (1) (2) (3)
The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25°C.
LM4891
Symbol
Parameter
Conditions
Typical
Limit
(4)
(5)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, Io = 0A
3.5
mA (max)
ISD
Shutdown Current
Vshutdown = VDD
0.1
µA (max)
PO
Output Power
THD = 1% (max); f = 1kHz
0.4
W
THD+N
Total Harmonic Distortion+Noise
Po = 0.15Wrms; f = 1kHz
0.1
%
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
60 (f = 217Hz)
62 (f = 1kHz)
dB
PSRR
(1)
(2)
(3)
(4)
(5)
(6)
(6)
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.
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.
Typicals are measured at 25°C and represent the parametric normal.
Limits are specified to AOQL (Average Outgoing Quality Level).
PSRR is a function of system gain. Specifications apply to the circuit in Figure 1 where AV = 2. Higher system gains will reduce PSRR
value by the amount of gain increase. A system gain of 10 represents a gain increase of 14dB. PSRR will be reduced by 14dB and
applies to all operating voltages.
ELECTRICAL CHARACTERISTICS VDD = 2.6V (1) (2) (3)
The following specifications apply for VDD = 2.6V, AV = 2, and 8Ω Load unless otherwise specified. Limits apply for TA = 25°C.
LM4891
Symbol
Parameter
Conditions
Typical
(4)
Limit
(5)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, Io = 0A
2.6
mA (max)
ISD
Shutdown Current
Vshutdown = VDD
0.1
µA (max)
(1)
(2)
(3)
(4)
(5)
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.
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.
Typicals are measured at 25°C and represent the parametric normal.
Limits are specified to AOQL (Average Outgoing Quality Level).
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ELECTRICAL CHARACTERISTICS VDD = 2.6V(1)(2)(3) (continued)
The following specifications apply for VDD = 2.6V, AV = 2, and 8Ω Load unless otherwise specified. Limits apply for TA = 25°C.
LM4891
Symbol
Parameter
Conditions
Typical
(4)
(5)
Units
(Limits)
PO
Output Power ( 8Ω )
Output Power ( 4Ω )
THD = 1% (max); f = 1 kHz THD =
1% (max); f = 1 kHz
THD+N
Total Harmonic Distortion+Noise
Po = 0.1Wrms; f = 1kHz
0.08
%
PSRR (6)
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
44 (f = 217Hz)
44 (f = 1kHz)
dB
(6)
0.25
0.28
Limit
W
W
PSRR is a function of system gain. Specifications apply to the circuit in Figure 1 where AV = 2. Higher system gains will reduce PSRR
value by the amount of gain increase. A system gain of 10 represents a gain increase of 14dB. PSRR will be reduced by 14dB and
applies to all operating voltages.
EXTERNAL COMPONENTS DESCRIPTION
Components (1)
(1)
6
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 amplifiers 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 section, PROPER SELECTION OF EXTERNAL
COMPONENTS, for information concerning proper placement and selection of CB.
ROUT is measured from each of the output pins to ground. This value represents the parallel combination of the 10k ohm output
resistors and the two 20k ohm resistors.
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TYPICAL PERFORMANCE CHARACTERISTICS
THD+N vs Frequency
at VDD = 5V, 8Ω RL, and PWR = 250mW
THD+N vs Frequency
at VDD = 3.3V, 8Ω RL, and PWR = 150mW
Figure 6.
Figure 7.
THD+N vs Frequency
at VDD = 2.6V, 8Ω RL, and PWR = 100mW
THD+N vs Frequency
at VDD = 2.6V, 4Ω RL, and PWR = 100mW
Figure 8.
Figure 9.
THD+N vs Power Out
@ VDD = 5V, 8Ω RL, 1kHz
THD+N vs Power Out
@ VDD = 3.3V, 8Ω RL, 1kHz
Figure 10.
Figure 11.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD+N vs Power Out
@ VDD = 2.6V, 8Ω RL, 1kHz
THD+N vs Power Out
@ VDD = 2.6V, 4Ω RL, 1kHz
Figure 12.
Figure 13.
Power Supply Rejection Ratio (PSRR) @ VDD = 5V
Power Supply Rejection Ratio (PSRR) @ VDD = 5V
Input terminated with 10Ω R
Input Floating
Figure 14.
Figure 15.
Power Supply Rejection Ratio (PSRR) @ VDD = 2.6V
Power Supply Rejection Ratio (PSRR) @ VDD = 3.3V
Input terminated with 10Ω R
Input terminated with 10Ω R
Figure 16.
8
Figure 17.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Power Dissipation vs
Output Power
VDD = 3.3V
Power Dissipation vs
Output Power
@ VDD = 5V, 1kHz, 8Ω, THD ≤ 1.0%
Figure 18.
Figure 19.
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
VDD = 2.6V
Figure 20.
Figure 21.
Supply Current vs
Shutdown Voltage
Clipping (Dropout) Voltage vs
Supply Voltage
Figure 22.
Figure 23.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
10
Open Loop Frequency Response
VDD = 5V No Load
Open Loop Frequency Response
VDD = 3V No Load
Figure 24.
Figure 25.
Power Derating Curves
(PDMAX = 670mW)
Power Derating Curves
for 8 Bump microSMD (PDMAX = 670mW)
Figure 26.
Figure 27.
Power Derating - 10 Pin LD Pkg
PDMAX = 670mW, 5V, 8Ω
Frequency Response vs
Input Capacitor Size
Figure 28.
Figure 29.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Noise Floor
Figure 30.
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APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4891 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 20 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 by 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 the 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 LM4891, 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. Without an output coupling capacitor, the half-supply bias across the load would
result in both increased internal IC power dissipation and also possible loudspeaker damage.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS FOR THE LM4891LD
The LM4891LD's exposed-DAP (die attach paddle) package (NGZ) provides a low thermal resistance between
the die and the PCB to which the part is mounted and soldered. The LM4891LD package should have its DAP
soldered to the grounded copper pad (heatsink) under the NGZ (the NC pins, no connect, and ground pins
should also be directly connected to this copper pad-heatsink area). The area of the copper pad (heatsink) can
be determined from the NGZ Power Derating graph. If the multiple layer copper heatsink areas are used, then
these inner layer or backside copper heatsink areas should be connected to each other with 4 (2 x 2) vias. The
diameter for these vias should be between 0.013 inches and 0.02 inches with a 0.050 inch pitch-spacing. Ensure
efficient thermal conductivity by plating through and solder-filling the vias. Further detailed information concerning
PCB layout, fabrication, and mounting an WSON package is available from Texas Instrument's Package
Engineering Group under application note AN1187.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Since the LM4891 has two operational amplifiers in one package, the
maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation
for a given application can be derived from the power dissipation graphs or from Equation 1.
PDMAX = 4*(VDD)2/(2π2RL)
(1)
It is critical that the maximum junction temperature (TJMAX) of 150°C is not exceeded. TJMAX can be determined
from the power derating curves by using PDMAX and the PC board foil area. By adding additional copper foil, the
thermal resistance of the application can be reduced from a free air value of 150°C/W, resulting in higher PDMAX.
Additional copper foil can be added to any of the leads connected to the LM4891. It is especially effective when
connected to VDD, GND, and the output pins. Refer to the application information on the LM4891 reference
design board for an example of good heat sinking. If TJMAX still exceeds 150°C, then additional changes must be
made. These changes can include reduced supply voltage, higher load impedance, or reduced ambient
temperature. Internal power dissipation is a function of output power. Refer to the TYPICAL PERFORMANCE
CHARACTERISTICS curves for power dissipation information for different output powers and output loading.
12
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POWER SUPPLY BYPASSING
As with any 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. Typical applications employ a 5V regulator with 10 µF tantalum or electrolytic capacitor and a ceramic
bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of
the LM4891. The selection of a bypass capacitor, especially CB, is dependent upon 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 LM4891 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. By switching the shutdown pin to VDD, the LM4891 supply current draw will be minimized in idle
mode. While the device will be disabled with shutdown pin voltages more than 1.0VDC, the idle current may be
greater than the typical value of 0.1µA. (Idle current is measured with the shutdown pin tied to VDD).
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to
provide 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
which enables the amplifier. If the switch is open, then the external pull-up resistor to VDD will disable the
LM4891. 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 to optimize
device and system performance. While the LM4891 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4891 is unity-gain stable which gives the designer maximum system flexibility. The LM4891 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.
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 100 Hz to
150 Hz. Thus, using a large input capacitor may not increase actual 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
1/2 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
LM4891 turns on. The slower the LM4891's outputs ramp to their quiescent DC voltage (nominally 1/2 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 0.1 µF to
0.39 µF), should produce a virtually clickless and popless shutdown function. While the device will function
properly, (no oscillations or motorboating), with CB equal to 0.1 µF, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of CB equal to 1.0 µF is recommended in all but the most cost sensitive
designs.
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AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω AUDIO AMPLIFIER
Given:
Power Output
1 Wrms
Load Impedance
8Ω
Input Level
1 Vrms
Input Impedance
20 kΩ
Bandwidth
100 Hz–20 kHz ± 0.25 dB
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 2 and add the output voltage. Using this method, the minimum supply voltage
would be (Vopeak + (VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs
Supply Voltage curve in the TYPICAL PERFORMANCE CHARACTERISTICS section.
(2)
5V is a standard voltage, in most applications, chosen for the supply rail. Extra supply voltage creates headroom
that allows the LM4891 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 3.
(3)
AVD = (Rf/Ri) 2
From Equation 3, the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20 kΩ, and with a AVD of 3, a ratio of 1.5:1 of Rf to Ri results in an
allocation of Ri = 20 kΩ and Rf = 30 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 −3 dB point is 0.17 dB down from
passband response which is better than the required ±0.25 dB specified.
fL = 100 Hz/5 = 20 Hz
fH = 20 kHz * 5 = 100 kHz
As stated in the EXTERNAL COMPONENTS DESCRIPTION section, Ri in conjunction with Ci create a highpass
filter.
Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain,
AVD. With a AVD = 3 and fH = 100 kHz, the resulting GBWP = 300 kHz which is much smaller than the LM4891
GBWP of 2.5 MHz. This figure displays that if a designer has a need to design an amplifier with a higher
differential gain, the LM4891 can still be used without running into bandwidth limitations.
14
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Figure 31. Higher Gain Audio Amplifier
The LM4891 is unity-gain stable and requires no external components besides gain-setting resistors, an input
coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential
gain of greater than 10 is required, a feedback capacitor (C4) may be needed as shown in Figure 31 to
bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high
frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect
combination of R3 and C4 will cause rolloff before 20kHz. A typical combination of feedback resistor and
capacitor that will not produce audio band high frequency rolloff is R3 = 20kΩ and C4 = 25pf. These components
result in a -3dB point of approximately 320 kHz.
Figure 32. Differential Amplifier Configuration for LM4891
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Figure 33. Reference Design Board and Layout - micro SMD
LM4891 MICRO SMD BOARD ARTWORK
16
Figure 34. Silk Screen
Figure 35. Top Layer
Figure 36. Bottom Layer
Figure 37. Inner Layer Ground
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Figure 38. Inner Layer VDD
Figure 39. Reference Design Board and PCB Layout Guidelines - VSSOP & SOIC Boards
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LM4891 SOIC DEMO BOARD ARTWORK
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LM4891 VSSOP DEMO BOARD ARTWORK
Figure 40. Silk Screen
Figure 43. Silk Screen
Figure 41. Top Layer
Figure 44. Top Layer
Figure 42. Bottom Layer
18
Figure 45. Bottom Layer
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Table 1. Mono LM4891 Reference Design Boards Bill of Material for all 3 Demo Boards
Item
Part Number
Part Description
Qty
1
551011208-001
LM4891 Mono Reference Design Board
1
Ref Designator
10
482911183-001
LM4891 Audio AMP
1
U1
20
151911207-001
Tant Cap 1uF 16V 10
1
C1
21
151911207-002
Cer Cap 0.39uF 50V Z5U 20% 1210
1
C2
25
152911207-001
Tant Cap 1uF 16V 10
1
C3
30
472911207-001
Res 20K Ohm 1/10W 5
3
R1, R2, R3
35
210007039-002
Jumper Header Vertical Mount 2X1 0.100
2
J1, J2
PCB LAYOUT GUIDELINES
This section provides practical guidelines for mixed signal PCB layout that involves various digital/analog power
and ground traces. Designers should note that these are only "rule-of-thumb" recommendations and the actual
results will depend heavily on the final layout.
GENERAL MIXED SIGNAL LAYOUT RECOMMENDATION
POWER AND GROUND CIRCUITS
For 2 layer mixed signal design, it is important to isolate the digital power and ground trace paths from the
analog power and ground trace paths. Star trace routing techniques (bringing individual traces back to a central
point rather than daisy chaining traces together in a serial manner) can have a major impact on low level signal
performance. Star trace routing refers to using individual traces to feed power and ground to each circuit or even
device. This technique will take require a greater amount of design time but will not increase the final price of the
board. The only extra parts required may be some jumpers.
SINGLE-POINT POWER / GROUND CONNECTIONS
The analog power traces should be connected to the digital traces through a single point (link). A "Pi-filter" can
be helpful in minimizing high frequency noise coupling between the analog and digital sections. It is further
recommended to put digital and analog power traces over the corresponding digital and analog ground traces to
minimize noise coupling.
PLACEMENT OF DIGITAL AND ANALOG COMPONENTS
All digital components and high-speed digital signals traces should be located as far away as possible from
analog components and circuit traces.
AVOIDING TYPICAL DESIGN / LAYOUT PROBLEMS
Avoid ground loops or running digital and analog traces parallel to each other (side-by-side) on the same PCB
layer. When traces must cross over each other do it at 90 degrees. Running digital and analog traces at 90
degrees to each other from the top to the bottom side as much as possible will minimize capacitive noise
coupling and cross talk.
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REVISION HISTORY
Changes from Revision D (April 2013) to Revision E
•
20
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 19
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