LM4892IBP [TI]
1W, 1 CHANNEL, AUDIO AMPLIFIER, PBGA8, MICRO, SMD-8;
| 型号： | LM4892IBP |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 1W, 1 CHANNEL, AUDIO AMPLIFIER, PBGA8, MICRO, SMD-8 放大器 商用集成电路 |
| 文件： | 总22页 (文件大小：1106K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
OBSOLETE
LM4892
www.ti.com
SNAS130E –MAY 2001–REVISED APRIL 2013
LM4892 Boomer® Audio Power Amplifier Series 1 Watt Audio Power Amplifier with
Headphone Sense
Check for Samples: LM4892
1
FEATURES
DESCRIPTION
The LM4892 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. Switching between bridged speaker mode
and headphone (single-ended) mode is accomplished
using the headphone sense pin.
2
•
Available in Space-Saving Packages: WSON,
DSBGA, VSSOP, and SOIC
•
•
Ultra Low Current Shutdown Mode
BTL Output Can Drive Capacitive Loads up to
500pF
•
Improved Pop & Click Circuitry Eliminates
Noise During Turn-On and Turn-Off
Transitions
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 applications
where minimal power consumption is a primary
requirement.
•
•
2.2 - 5.5V Operation
No Output Coupling Capacitors, Snubber
Networks or Bootstrap Capacitors Required
•
•
•
•
Thermal Shutdown Protection
Unity-Gain Stable
External Gain Configuration Capability
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 thermal shutdown protection
mechanism.
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)
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
•
•
•
Mobile Phones
PDAs
Portable Electronic Devices
TYPICAL APPLICATION
Figure 1. Typical Audio Amplifier Application Circuit (Pin #'s apply to M & MM packages)
1
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Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2
All 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.
Copyright © 2001–2013, Texas Instruments Incorporated
OBSOLETE
LM4892
SNAS130E –MAY 2001–REVISED APRIL 2013
www.ti.com
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.
CONNECTION DIAGRAM
8 Bump micro SMD
Small Outline (SOIC) Package
Figure 2. (Top View)
See Package Number YPB0008
Figure 3. Top View
See Package Number D0008A
X - Date Code
T - Die Traceability
G - Boomer Family
H - LM4892IBP
Mini Small Outline (VSSOP) Package
Figure 4. Top View
Figure 5. micro SMD Marking (Top View)
See Package Number DGK0008A
XY - Date Code
TT - Die Traceability
Bottom 2 lines - Part Number
G - Boomer Family
92 - LM4892MM
Figure 6. SOIC Marking (Top View)
Figure 7. VSSOP Marking (Top View)
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Figure 8. WSON Package (Top View)
Package Number NGZ0010B
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ABSOLUTE MAXIMUM RATINGS(1)(2)
Supply Voltage
6.0V
−65°C to +150°C
−0.3V to VDD +0.3V
Internally Limited
2500V
Storage Temperature
Input Voltage
Power Dissipation(3)
ESD Susceptibility(4)
ESD Susceptibility(5)
250V
Junction Temperature
Thermal Resistance
150°C
θJC (VSSOP)
35°C/W
150°C/W
220°C/W
56°C/W
θJA (VSSOP)
θJA (micro SMD)
θJC (VSSOP)
θJA (VSSOP)
190°C/W
220°C/W(6)
θJA (WSON)
Soldering Information
See AN-1112 "microSMD Wafers Level Chip Scale Package".
See AN-1187 "Leadless Leadframe Package (WSON)".
(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.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(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 LM4892, see Figure 28 for additional information.
(4) Human body model, 100pF discharged through a 1.5kΩ resistor.
(5) Machine Model, 220pF–240pF discharged through all pins.
(6) 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.
OPERATING RATINGS
Temperature Range
T
MIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ 85°C
2.2V ≤ VDD ≤ 5.5V
Supply Voltage
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ELECTRICAL CHARACTERISTICS VDD = 5V(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
Limit
(Limits)
(3)
(4)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 5V
4
10
mA (max)
mA (max)
µA (max)
W
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
2.5
(5)
Vshutdown = GND
0.1
THD = 2% (max), f = 1kHz,
RL = 8Ω, HP Sense < 0.8V
1
Po
Output Power
THD = 1% (max), f = 1kHz,
RL = 32Ω, HP Sense > 4V
mW
90
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
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
(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 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.
(3) Typicals are measured at 25°C and represent the parametric norm.
(4) Datasheet min/max specification limits are specified by design, test, or statistical analysis.
(5) 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.
ELECTRICAL CHARACTERISTICS VDD = 3.3V(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
Limit
(Limits)
(3)
(4)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 3.3V
Vshutdown = GND(5)
3.5
2.0
0.1
mA (max)
mA (max)
µA (max)
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
THD = 1% (max), f = 1kHz,
RL = 8Ω, HP Sense < 0.8V
0.4
35
W
Po
Output Power
THD = 1% (max), f = 1kHz,
RL = 32Ω, HP Sense > 3V
mW
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
%
PSSR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
60(f = 217Hz)
62 (f = 1kHz)
dB
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
(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 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.
(3) Typicals are measured at 25°C and represent the parametric norm.
(4) Datasheet min/max specification limits are specified by design, test, or statistical analysis.
(5) 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.
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ELECTRICAL CHARACTERISTICS VDD = 2.6V(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
Limit
(Limits)
(3)
(4)
VIN = 0V, Io = 0A, HP sense = 0V
VIN = 0V, Io = 0A, HP sense = 2.6V
Vshutdown = GND(5)
2.6
1.5
0.1
mA (max)
mA (max)
µA (max)
IDD
ISD
Quiescent Power Supply Current
Shutdown Current
THD = 1% (max), f = 1kHz,
RL = 8Ω, HP Sense < 0.8V
0.25
0.28
20
W
W
THD = 1% (max), f = 1kHz,
RL = 4Ω, HP Sense < 0.8V
Po
Output Power
THD = 1% (max), f = 1kHz, RL
32Ω, HP Sense > 2.5V
=
mW
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 ≤
0.1
80kHz
PSSR
Power Supply Rejection Ratio
Vripple = 200mV sine p-p
44(f = 217Hz)
44 (f = 1kHz)
dB
(1) All voltages are measured with respect to the ground pin, unless otherwise specified.
(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 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.
(3) Typicals are measured at 25°C and represent the parametric norm.
(4) Datasheet min/max specification limits are specified by design, test, or statistical analysis.
(5) 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.
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.
4.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
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.
6.
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.
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.
8.
9.
RPU
RS
This is the pull up resistor to activate headphone operation when a headphone plug is plugged into the headphone jack.
This is the current limiting resistor for the headphone input pin.
RPD
This is the pull down resistor to de-activate headphone operation when no headphone is plugged into the headphone
jack.
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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 9.
Figure 10.
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 11.
Figure 12.
THD+N vs Power Out
at VDD = 5V, 8Ω RL, 1kHz
THD+N vs Power Out
at VDD = 3.3V, 8Ω RL, 1kHz
Figure 13.
Figure 14.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD+N vs Power Out
at VDD = 2.6V, 8Ω RL, 1kHz
THD+N vs Power Out
at VDD = 2.6V, 4Ω RL, 1kHz
Figure 15.
Figure 16.
Power Supply Rejection Ratio (PSRR)
Power Supply Rejection Ratio (PSRR)
vs
vs
Frequency
at VDD = 5V, 8Ω RL
Frequency
at VDD = 5V, 8Ω RL
Figure 17. Input terminated with 10Ω R
Figure 18. Input Floating
Power Supply Rejection Ratio (PSRR)
Power Supply Rejection Ratio (PSRR)
vs
vs
Frequency
at VDD = 2.6V, 8Ω RL
Frequency
at VDD = 3.3V, 8Ω RL
Figure 19. Input terminated with 10Ω R
Figure 20. Input terminated with 10Ω R
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Power Dissipation vs
Output Power
VDD = 5V
Power Dissipation vs Output Power
VDD = 3.3V
Figure 21.
Figure .
Output Power vs
Load Resistance
Power Dissipation vs Output Power
VDD = 2.6V
Figure 22.
Figure 23.
Supply Current vs
Shutdown Voltage
Clipping (Dropout) Voltage vs
Supply Voltage
Figure 24.
Figure 25.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Open Loop Frequency Response
VDD = 5V No Load
Open Loop Frequency Response
VDD = 3V No Load
Figure 26.
Figure 27.
Power Derating Curves vs
for 8 Bump microSMD
Power Derating Curves
Figure 28.
Figure 29.
Frequency Response vs
Input Capacitor Size
Noise Floor
Figure 30.
Figure 31.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD+N vs Frequency
at VDD = 5V, RL = 32Ω, PWR = 70mW, Headphone mode
THD+N vs Power Out
at VDD = 5V, RL = 32Ω, 1kHz, Headphone mode
Figure 32.
Figure 33.
Output Power vs Supply Voltage
Output Power vs Supply Voltage
RL = 8Ω
RL = 16Ω
Figure 34.
Figure 35.
Output Power vs Supply Voltage
Output Power vs Supply Voltage
Headphone Output, RL = 32Ω
RL = 32Ω
Figure 36.
Figure 37.
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APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
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 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 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
AVD= 2 *(Rf/Ri)
(1)
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 closed-
loop gain without causing excessive 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
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, single-
ended 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.
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 LM4892 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 2.
PDMAX = 4*(VDD)2/(2π2RL)
(2)
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 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.
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 10µF 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 LM4892. 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.
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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 amplifier. If the switch is open, then the external pull-up resistor will enable the LM4892. This
scheme ensures that the shutdown pin will not float thus preventing unwanted state changes.
Table 1. Table 1. Logic Level Truth Table for Shutdown and HP Sense Operation
Shutdown
Logic High
Logic High
Logic Low
Logic Low
HP Sense Pin
Logic Low
Operational Mode
Bridged Amplifier
Logic High
Logic Low
Single-Ended Amplifier
Micro-Power Shutdown
Micro-Power Shutdown
Logic High
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.
Figure 38 shows the implementation of the LM4892's headphone control function. With no headphones
connected to the headphone jack, the R4-R6 voltage divider sets the voltage 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 potential 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. Connecting 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 headphones is 32Ω.
The output coupling capacitor blocks the amplifier's half supply DC voltage, protecting the headphones.
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 headphones.
Figure 38. Headphone Circuit (Pin #'s apply to M & MM packages)
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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 LM4892 is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
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 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 100Hz to
150Hz. 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
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.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω AUDIO AMPLIFIER
Given:
Power Output
Load Impedance
Input Level
1 Wrms
8Ω
1 Vrms
Input Impedance
Bandwidth
20 kΩ
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 3 and add the output voltage. Using this method, the minimum supply voltage
would be (Vopeak + (VOD + VODBOT)), where V
and VOD are extrapolated from the Dropout Voltage vs
TOP
BOT
OD
TOP
Supply Voltage curve in the TYPICAL PERFORMANCE CHARACTERISTICS section.
(3)
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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 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 4.
(4)
Rf/Ri = AVD/2
From Equation 4, 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.
fL = 100Hz/5 = 20Hz
(5)
(6)
fH = 20kHz * 5 = 100kHz
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
(7)
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. The following 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.
Figure 39. Higher Gain Audio Amplifier
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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 application. 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 eliminates possible high frequency
oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect 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.
Figure 40. Reference Design Schematic For Demo Boards
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LM4892 micro SMD BOARD ARTWORK
Figure 41. Silk Screen
Figure 42. Top Layer
Figure 43. Bottom Layer
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LM4892 SO DEMO BOARD ARTWORK
LM4892 MSOP DEMO BOARD ARTWORK
Figure 44. Silk Screen
Figure 45. Top Layer
Figure 46. Bottom Layer
Figure 47. Silk Screen
Figure 48. Top Layer
Figure 49. Bottom Layer
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LM4892 WSON DEMO BOARD ARTWORK
Figure 50. Composite View
Figure 51. Silk Screen
Figure 52. Top Layer
Figure 53. Bottom Layer
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Table 2. Mono LM4892 Reference Design Boards Bill of Material for all Demo Boards
Part Description
Qty
1
Ref Designator
LM4892 Audio Amplifier
Tantalum Capacitor, 1µF
Ceramic Capacitor, 0.39µF
Capacitor, 100µF
U1
2
Cs, Cb
Ci
1
1
Cout
Resistor, 1kΩ, 1/10W
1
Rpd
Resistor, 20kΩ, 1/10W
3
Ri, Rf, Rpu2
Rpu1, Rs
J1
Resistor, 100kΩ, 1/10W
Jumper Header Vertical Mount 2X1, 0.100" spacing
2
1
3.5mm Audio Jack (PC mount, w/o nut), PN# SJS-0357-B Shogyo
International Corp. (www.shogyo.com)
1
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 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.
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 signal 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
Page
•
Changed layout of National Data Sheet to TI format .......................................................................................................... 20
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