LM4892MM [NSC]
1 Watt Audio Power Amplifier with Headphone Sense; 1瓦音频功率放大器耳机检测
| 型号： | LM4892MM |
| 厂家： | 
National Semiconductor |
| 描述： | 1 Watt Audio Power Amplifier with Headphone Sense |
| 文件： | 总22页 (文件大小：1033K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
October 2002
LM4892
1 Watt Audio Power Amplifier with Headphone Sense
General Description
Key Specifications
The LM4892 is an audio power amplifier primarily designed
for demanding applications in mobile phones and other por-
table 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. Switching between bridged speaker mode and
headphone (single-ended) mode is accomplished using the
headphone sense pin.
j
PSRR at 217Hz, VDD = 5V, 8Ω Load
62dB (typ)
1.0W (typ)
j
j
j
Power Output at 5.0V & 1% THD
Power Output at 3.3V & 1% THD
Shutdown Current
400mW (typ)
0.1µA (typ)
Features
Boomer audio power amplifiers are designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4892 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other low voltage appli-
cations where minimal power consumption is a primary re-
quirement.
n Available in space-saving packages: LLP, micro SMD,
MSOP, and SOIC
n Ultra low current shutdown mode
n BTL output can drive capacitive loads up to 500pF
n Improved pop & click circuitry eliminates noise 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 Thermal shutdown protection
n Unity-gain stable
n External gain configuration capability
n Headphone amplifier mode
The LM4892 features a low-power consumption shutdown
mode, which is achieved by driving the shutdown pin with
logic low. Additionally, the LM4892 features an internal ther-
mal shutdown protection mechanism.
The LM4892 contains advanced pop & click circuitry which
eliminates noise which would otherwise occur during turn-on
and turn-off transitions.
The LM4892 is unity-gain stable and can be configured by
external gain-setting resistors.
Applications
n Mobile Phones
n PDAs
n Portable electronic devices
Typical Application
20012701
FIGURE 1. Typical Audio Amplifier Application Circuit (Pin #’s apply to M & MM packages)
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2002 National Semiconductor Corporation
DS200127
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Connection Diagrams
8 Bump micro SMD
Small Outline (SO) Package
20012735
Top View
Order Number LM4892M
See NS Package Number M08A
20012723
Top View
Order Number LM4892IBP, LM4892IBPX
See NS Package Number BPA08DDB
Mini Small Outline (MSOP) Package
micro SMD Marking
20012770
Top View
X - Date Code
20012736
Top View
Order Number LM4892MM
See NS Package Number MUA08A
T - Die Traceability
G - Boomer Family
H - LM4892IBP
SO Marking
MSOP Marking
20012772
Top View
20012771
Top View
XY - Date Code
TT - Die Traceability
Bottom 2 lines - Part Number
G - Boomer Family
92 - LM4892MM
LLP Package
20012789
Top View
Order Number LM4892LD
See NS Package Number LDA10B
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2
Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
θJC (MSOP)
θJA (MSOP)
56˚C/W
190˚C/W
θJA (LLP)
220˚C/W (Note 9)
Soldering Information
Supply Voltage
6.0V
−65˚C to +150˚C
−0.3V to VDD +0.3V
Internally Limited
2500V
See AN-1112 ’microSMD Wafers Level Chip Scale
Package’.
Storage Temperature
Input Voltage
See AN-1187 ’Leadless Leadframe Package (LLP)’.
Power Dissipation (Note 3)
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Thermal Resistance
θJC (SOP)
Operating Ratings
Temperature Range
250V
150˚C
TMIN ≤ TA ≤ TMAX
−40˚C ≤ TA ≤ 85˚C
2.2V ≤ VDD ≤ 5.5V
Supply Voltage
35˚C/W
150˚C/W
220˚C/W
θJA (SOP)
θJA (micro SMD)
Electrical Characteristics VDD = 5V (Notes 1, 2)
The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4892
Units
Symbol
Parameter
Conditions
Typical
(Note 6)
4
Limit
(Note 7)
10
(Limits)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 5V
Vshutdown = GND (Note 8)
mA (max)
mA (max)
µA (max)
W
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
2.5
0.1
THD = 2% (max), f = 1kHz,
1
<
RL = 8Ω, HP Sense 0.8V
Po
Output Power
THD = 1% (max), f = 1kHz,
mW
90
>
RL = 32Ω, HP Sense 4V
VIH
HP Sense high input voltage
HP Sense low input voltage
Total Harmonic Distortion+Noise
4
V (min)
V (max)
VIL
0.8
THD+N
Po = 0.4 Wrms; f = 1kHz 10Hz ≤
BW ≤ 80kHz
0.1
%
PSSR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
62 (f =
217Hz) 66 (f
= 1kHz)
dB
Electrical Characteristics VDD = 3.3V (Notes 1, 2)
The following specifications apply for VDD = 3.3V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4892
Units
Symbol
Parameter
Conditions
Typical
(Note 6)
3.5
Limit
(Limits)
(Note 7)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 3.3V
Vshutdown = GND (Note 8)
mA (max)
mA (max)
µA (max)
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
2.0
0.1
THD = 1% (max), f = 1kHz,
0.4
35
W
<
RL = 8Ω, HP Sense 0.8V
Po
Output Power
THD = 1% (max), f = 1kHz,
mW
>
RL = 32Ω, HP Sense 3V
VIH
HP Sense high input voltage
HP Sense low input voltage
Total Harmonic Distortion+Noise
2.6
0.8
V (min)
V (max)
VIL
THD+N
Po = 0.15 Wrms; f = 1kHz 10Hz ≤
BW ≤ 80kHz
0.1
%
3
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Electrical Characteristics VDD = 3.3V (Notes 1, 2)
The following specifications apply for VDD = 3.3V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA
=
25˚C. (Continued)
LM4892
Units
(Limits)
Symbol
Parameter
Conditions
Typical
(Note 6)
Limit
(Note 7)
PSSR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
60(f = 217Hz)
62 (f = 1kHz)
dB
Electrical Characteristics VDD = 2.6V (Notes 1, 2)
The following specifications apply for VDD = 2.6V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C.
LM4892
Units
Symbol
Parameter
Conditions
Typical
(Note 6)
2.6
Limit
(Limits)
(Note 7)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 2.6V
Vshutdown = GND (Note 8)
mA (max)
mA (max)
µA (max)
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
1.5
0.1
THD = 1% (max), f = 1kHz,
0.25
0.28
20
W
W
<
RL = 8Ω, HP Sense 0.8V
THD = 1% (max), f = 1kHz,
Po
Output Power
<
RL = 4Ω, HP Sense 0.8V
THD = 1% (max), f = 1kHz, RL
=
mW
>
32Ω, HP Sense 2.5V
VIH
HP Sense high input voltage
HP Sense low input voltage
Total Harmonic Distortion+Noise
2.0
0.8
V (min)
V (max)
%
VIL
THD+N
Po = 0.1 Wrms; f = 1kHz 10Hz ≤
BW ≤ 80kHz
0.1
PSSR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
44(f = 217Hz)
44 (f = 1kHz)
dB
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 T
, θ , and the ambient temperature T . The maximum
A
JMAX JA
allowable power dissipation is P
= (T
–T )/θ or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4892, see power derating
DMAX
JMAX A JA
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 I by a maximum of 2µA.
SD
Note 9: The Exposed-DAP of the LDA10B package should be electrically connected to GND or an electrically isolated copper area. The LM4892LD demo board
(views featured in the Application Information section) has the Exposed-DAP connected to GND with a PCB area of 353mils x 86.7mils (8.97mm x 2.20mm) on
the copper top layer and 714.7mils x 368mils (18.15mm x 9.35mm) on the copper bottom layer.
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 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.
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4
External Components Description (Figure 1) (Continued)
Components
Functional Description
4.
5.
6.
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.
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.
CB
COUT This output coupling capacitor blocks DC voltage while coupling the AC audio signal to the headphone
speaker. Combined with RL, the headphone impedance, it creates a high pass filter at fc = 1/(2πRLCOUT).
Refer to the section, Proper Selection of External Components for an explanation of how to determine the
value of COUT
.
7.
RPU
This is the pull up resistor to activate headphone operation when a headphone plug is plugged into the
headphone jack.
8.
9.
RS
This is the current limiting resistor for the headphone input pin.
This is the pull down resistor to de-activate headphone operation when no headphone is plugged into the
headphone jack.
RPD
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
at VDD = 5V, 8Ω RL, and PWR = 250mW
at VDD = 3.3V, 8Ω RL, and PWR = 150mW
20012737
20012738
THD+N vs Frequency
THD+N vs Frequency
at VDD = 2.6V, 8Ω RL, and PWR = 100mW
at VDD = 2.6V, 4Ω RL, and PWR = 100mW
20012739
20012740
5
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Typical Performance Characteristics (Continued)
THD+N vs Power Out
THD+N vs Power Out
at VDD = 5V, 8Ω RL, 1kHz
at VDD = 3.3V, 8Ω RL, 1kHz
20012784
20012742
THD+N vs Power Out
THD+N vs Power Out
at VDD = 2.6V, 8Ω RL, 1kHz
at VDD = 2.6V, 4Ω RL, 1kHz
20012785
20012786
Power Supply Rejection Ratio (PSRR) vs Frequency
Power Supply Rejection Ratio (PSRR) vs Frequency
at VDD = 5V, 8Ω RL
at VDD = 5V, 8Ω RL
20012745
20012773
Input terminated with 10Ω R
Input Floating
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio (PSRR) vs Frequency
Power Supply Rejection Ratio (PSRR) vs Frequency
at VDD = 2.6V, 8Ω RL
at VDD = 3.3V, 8Ω RL
20012747
20012746
Input terminated with 10Ω R
Input terminated with 10Ω R
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
VDD = 5V
VDD = 3.3V
20012749
20012748
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
VDD = 2.6V
20012751
20012750
7
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Typical Performance Characteristics (Continued)
Supply Current vs
Shutdown Voltage
Clipping (Dropout) Voltage vs
Supply Voltage
20012753
20012752
Open Loop Frequency Response
VDD = 5V No Load
Open Loop Frequency Response
VDD = 3V No Load
20012787
20012782
Power Derating Curves
Power Derating Curves vs
for 8 Bump microSMD
20012788
20012783
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Typical Performance Characteristics (Continued)
Frequency Response vs
Input Capacitor Size
Noise Floor
20012754
20012756
THD+N vs Frequency
THD+N vs Power Out
at VDD = 5V, RL = 32Ω, PWR = 70mW, Headphone mode
at VDD = 5V, RL = 32Ω, 1kHz, Headphone mode
20012777
20012776
Output Power vs Supply Voltage
Output Power vs Supply Voltage
RL = 8Ω
RL = 16Ω
20012778
20012779
9
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Typical Performance Characteristics (Continued)
Output Power vs Supply Voltage
Output Power vs Supply Voltage
RL = 32Ω
Headphone Output, RL = 32Ω
20012781
20012780
Application Information
BRIDGE CONFIGURATION EXPLANATION
POWER DISSIPATION
As shown in Figure 1, the LM4892 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable, 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 20kΩ
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
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 LM4892 has two opera-
tional 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 Equa-
tion 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 LM4892.
It is especially effective when connected to VDD, GND, and
the output pins. Refer to the application information on the
LM4892 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 re-
duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the Typical Performance Charac-
teristics curves for power dissipation information for differ-
ent output powers and output loading.
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 configura-
tion 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 con-
ditions. 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 ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4892,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. 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.
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 appli-
cations employ a 5V regulator with 10µF tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid
in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4892. The selection of
a bypass capacitor, especially CB, is dependent upon PSRR
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A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridged-
connected speaker is muted and Amp1 drives the head-
phones.
Application Information (Continued)
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
LM4892 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic low is placed on the shutdown pin.
By switching the shutdown pin to ground, the LM4892 supply
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than
0.5VDC, the idle current may be greater than the typical
value of 0.1µA. (Idle current is measured with the shutdown
pin grounded).
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 and disables the am-
plifier. If the switch is open, then the external pull-up resistor
will enable the LM4892. This scheme guarantees that the
shutdown pin will not float thus preventing unwanted state
changes.
20012774
FIGURE 2. Headphone Circuit (Pin #’s apply to M & MM
packages)
PROPER SELECTION OF EXTERNAL COMPONENTS
Table 1. Logic Level Truth Table for Shutdown and HP
Sense Operation
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4892 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
Shutdown
HP Sense
Pin
Operational Mode
Logic High
Logic High
Logic Low
Logic Low
Logic Low
Logic High
Logic Low
Bridged Amplifier
Single-Ended Amplifier
Micro-Power Shutdown
The LM4892 is unity-gain stable which gives the designer
maximum system flexibility. The LM4892 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 com-
plete explanation of proper gain selection.
Logic High Micro-Power Shutdown
HP SENSE FUNCTION
Applying a voltage between 4V and VCC to the LM4892’s
HP-Sense headphone control pin turns off Amp2 and mutes
a bridged-connected load. Quiescent current consumption is
reduced when the IC is in the single-ended mode.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width 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 re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Figure 2 shows the implementation of the LM4892’s head-
phone control function. With no headphones connected to
the headphone jack, the R4-R6 voltage divider sets the volt-
age applied to the HP-Sense pin (pin3) at approximately
50mV. This 50mV enables the LM4892 and places it in
bridged mode operation.
While the LM4892 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing can not cause a false single-ended trigger. Connect-
ing headphones to the headphone jack disconnects the
headphone jack contact pin from V01 and allows R4 to pull
the HP Sense pin up to VCC. This enables the headphone
function, turns off Amp2, and mutes the bridged speaker.
The amplifier then drives the headphone whose impedance
is in parallel with R6. Resistor R6 has negligible effect on
output drive capability since the typical impedance of head-
phones is 32Ω. The output coupling capacitor blocks the
amplifier’s half supply DC voltage, protecting the head-
phones.
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 attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance 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
11
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Application Information (Continued)
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
(2)
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4892 to reproduce peaks in excess of 1W
without producing audible distortion. At this time, the de-
signer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the Power Dissipation section.
Besides minimizing the input capacitor size, careful consid-
eration 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 LM4892 turns
on. The slower the LM4892’s outputs ramp to their quiescent
DC voltage (nominally 1/2 VDD), the smaller the turn-on pop.
Choosing CB equal to 1.0µF along with a small value of Ci (in
the range of 0.1µF to 0.39µF), should produce a virtually
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 is recommended in all but the most cost sensitive
designs.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 3.
(3)
Rf/Ri = AVD/2
From Equation 3, the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20kΩ, and with a
AVD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of
Ri = 20kΩ and Rf = 30kΩ. The final design step is to address
the bandwidth requirements which must be stated as a pair
of −3dB frequency points. Five times away from a −3dB point
is 0.17dB down from passband response which is better
than the required 0.25dB specified.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω AUDIO AMPLIFIER
Given:
Power Output
Load Impedance
Input Level
1 Wrms
8Ω
1 Vrms
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
Input Impedance
Bandwidth
20 kΩ
As stated in the External Components section, Ri in con-
junction with Ci create a highpass filter.
100 Hz–20 kHz 0.25 dB
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 = 100kHz, the resulting GBWP =
150kHz which is much smaller than the LM4892 GBWP of
4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4892 can still be used without running into bandwidth
limitations.
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 Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply 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 + (VOD
+ VODBOT)), where
VOD
and VOD
are extrapolated frToOmP the Dropout Volt-
TOP
age BvOsT Supply Voltage curve in the Typical Performance
Characteristics section.
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Application Information (Continued)
20012724
FIGURE 3. Higher Gain Audio Amplifier
The LM4892 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However, if a closed-loop differential gain of greater
than 10 is required, a feedback capacitor (Cf) may be
needed as shown in Figure 3 to bandwidth limit the amplifier.
This feedback capacitor creates a low pass filter that elimi-
nates possible high frequency oscillations. Care should be
taken when calculating the -3dB frequency in that an incor-
rect combination of Rf and Cf will cause rolloff before 20kHz.
A typical combination of feedback resistor and capacitor that
will not produce audio band high frequency rolloff is Rf =
20kΩ and Cf = 25pF. These components result in a -3dB
point of approximately 320 kHz.
13
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Application Information (Continued)
20012775
FIGURE 4. Reference Design Schematic For Demo Boards
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14
LM4892 micro SMD BOARD ARTWORK
Silk Screen
Application Information (Continued)
20012757
Top Layer
20012758
Bottom Layer
20012759
15
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LM4892 MSOP DEMO BOARD ARTWORK
Silk Screen
Application Information (Continued)
LM4892 SO DEMO BOARD ARTWORK
Silk Screen
20012765
20012762
Top Layer
Top Layer
20012766
20012763
Bottom Layer
Bottom Layer
20012767
20012764
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LM4892 LLP DEMO BOARD ARTWORK
Silk Screen
Application Information (Continued)
Composite View
20012790
20012791
Top Layer
Bottom Layer
20012792
20012793
17
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Application Information (Continued)
Mono LM4892 Reference Design Boards
Bill of Material for all Demo Boards
Part Description
Qty
Ref Designator
LM4892 Audio Amplifier
1
U1
Tantalum Capacitor, 1µF
Ceramic Capacitor, 0.39µF
Capacitor, 100µF
2
1
1
1
3
2
1
Cs, Cb
Ci
Cout
Resistor, 1kΩ, 1/10W
Rpd
Resistor, 20kΩ, 1/10W
Ri, Rf, Rpu2
Rpu1, Rs
J1
Resistor, 100kΩ, 1/10W
Jumper Header Vertical Mount 2X1,
0.100’ spacing
3.5mm Audio Jack (PC mount, w/o nut),
1
J2
#
PN SJS-0357-B Shogyo International
Corp. (www.shogyo.com)
PCB LAYOUT GUIDELINES
Single-Point Power / Ground Connections
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.
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 digi-
tal and analog ground traces to minimize noise coupling.
General Mixed Signal Layout Recommendation
Power and Ground Circuits
Placement of Digital and Analog Components
All digital components and high-speed digital signal traces
should be located as far away as possible from analog
components and circuit traces.
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 (bring-
ing 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
require a greater amount of design time but will not increase
the final price of the board. The only extra parts required will
be some jumpers.
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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Physical Dimensions inches (millimeters) unless otherwise noted
Note: Unless otherwise specified.
1. Epoxy coating.
2. 63Sn/37Pb eutectic bump.
3. Recommend non-solder mask defined landing pad.
4. Pin 1 is established by lower left corner with respect to text orientation pins are numbered counterclockwise.
5. Reference JEDEC registration MO-211, variation BC.
8-Bump micro SMD
Order Number LM4892IBP, LM4892IBPX
NS Package Number BPA08DDB
X1 = 1.361 0.03 X2 = 1.361 0.03 X3 = 0.850 0.10
19
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
MSOP
Order Number LM4892MM
NS Package Number MUA08A
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20
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
SO
Order Number LM4892M
NS Package Number M08A
21
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
LLP
Order Number LM4892LD
NS Package Number LDA10B
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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Support Center
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Fax: +49 (0) 180-530 85 86
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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.
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