LM4990LDX [NSC]
IC 2 W, 1 CHANNEL, AUDIO AMPLIFIER, DSO10, LLP-10, Audio/Video Amplifier;
| 型号： | LM4990LDX |
| 厂家： | 
National Semiconductor |
| 描述： | IC 2 W, 1 CHANNEL, AUDIO AMPLIFIER, DSO10, LLP-10, Audio/Video Amplifier 放大器 |
| 文件： | 总20页 (文件大小：1155K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
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October 2004
LM4990
2 Watt Audio Power Amplifier with Selectable Shutdown
Logic Level
General Description
Key Specifications
The LM4990 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 1.25 watts of continuous average power to an 8Ω
BTL load and 2 watts of continuous average power (LD and
MH only) to a 4Ω BTL load with less than 1% distortion
(THD+N+N) from a 5VDC power supply.
j
Improved PSRR at 217Hz & 1KHz
62dB
j
Power Output at 5.0V, 1%
THD+N,
4Ω (LD and MH only)
Power Output at 5.0V, 1% THD+N, 8Ω
2W (typ)
j
j
j
j
1.25W (typ)
Power Output at 3.0V, 1% THD+N, 4Ω 600mW (typ)
Power Output at 3.0V, 1% THD+N, 8Ω 425mW (typ)
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4990 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.
Shutdown Current
0.1µA (typ)
Features
n Available in space-saving packages: LLP, Exposed-DAP
TSSOP, MSOP, and ITL
The LM4990 features a low-power consumption shutdown
mode. To facilitate this, Shutdown may be enabled by either
logic high or low depending on mode selection. Driving the
shutdown mode pin either high or low enables the shutdown
pin to be driven in a likewise manner to enable shutdown.
n Ultra low current shutdown mode
n Improved pop & click circuitry eliminates noise during
turn-on and turn-off transitions
n 2.2 - 5.5V operation
The LM4990 contains advanced pop & click circuitry which
eliminates noise which would otherwise occur during turn-on
and turn-off transitions.
n No output coupling capacitors, snubber networks or
bootstrap capacitors required
n Unity-gain stable
The LM4990 is unity-gain stable and can be configured by
external gain-setting resistors.
n External gain configuration capability
n User selectable shutdown High or Low logic Level
Applications
n Mobile Phones
n PDAs
n Portable electronic devices
Connection Diagrams
Mini Small Outline (MSOP) Package
MSOP Marking
20051071
Top View
Z - Plant Code
200510B9
Top View
Order Number LM4990MM
See NS Package Number MUA08A
X - Date Code
TT - Die Traceability
G - Boomer Family
A5 - LM4990MM
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation
DS200510
www.national.com
Connection Diagrams (Continued)
LLP Package
LLP Marking
200510B4
Top View
Z - Plant Code
XY - Date Code
TT - Die Traceability
Bottom Line - Part Number
200510B3
Top View
Order Number LM4990LD
See NS Package Number LDA10B
Exposed-DAP TSSOP Package
20051096
Top View
Order Number LM4990MH
See NS Package Number MXF10A
9 Bump micro SMD
9 Bump micro SMD Marking
200510C1
Top View
X — Date Code
T — Die Traceability
G — Boomer Family
D2 — LM4990ITL
200510C0
Top View
Order Number LM4990ITL, LM4990ITLX
See NS Package Number TLA09ZZA
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2
Package
LD
MH
Selectable
2W
MM
Low
ITL
Low
Shutdown Mode
Typical Power Output at 5V,
1% THD+N
Selectable
2W
1.25W
(RL = 8Ω)
1.25W
(RL = 8Ω)
(RL = 4Ω)
(RL = 4Ω)
. A SD_MODE select pin determines the Shutdown Mode for the LD and MH packages, whether it is an Asserted High or an Asserted Low device, to activate
shutdown.
. The SD_MODE select pin is not available with the MM and ITL packaged devices. Shutdown occurs only with a low assertion.
Typical Application
20051001
Note: MM and ITL packaged devices are active low only; Shutdown Mode pin is internally tied to GND.
FIGURE 1. Typical Audio Amplifier Application Circuit (LD and MH)
200510C4
FIGURE 2. Typical Audio Amplifier Application Circuit (ITL and MM)
3
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Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
θJA (MSOP)
190˚C/W
180˚C/W
θJA (9 Bump micro SMD) (Note 15)
θJA (LLP)
63˚C/W (Note 13)
12˚C/W (Note 13)
θJC (LLP)
Supply Voltage (Note 11)
Storage Temperature
Input Voltage
6.0V
−65˚C to +150˚C
−0.3V to VDD +0.3V
Internally Limited
2000V
Soldering Information
See AN-1187 "Leadless
Leadframe Package (LLP)."
Power Dissipation (Notes 3, 12)
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Thermal Resistance
Operating Ratings
Temperature Range
TMIN ≤ TA ≤ TMAX
Supply Voltage
200V
150˚C
−40˚C ≤ TA ≤ 85˚C
2.2V ≤ VDD ≤ 5.5V
θJC (MSOP)
56˚C/W
Electrical Characteristics VDD = 5V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4990
Units
(Limits)
Symbol
Parameter
Conditions
Typical
Limit
(Note 6)
(Notes 7, 9)
VIN = 0V, Io = 0A, No Load
VIN = 0V, Io = 0A, 8Ω Load
VSD = VSD Mode (Note 8)
VSD MODE = VDD
3
7
mA (max)
IDD
Quiescent Power Supply Current
4
10
2.0
mA (max)
ISD
Shutdown Current
0.1
1.5
1.3
1.5
1.3
7
µA (max)
VSDIH
VSDIL
VSDIH
VSDIL
VOS
Shutdown Voltage Input High
Shutdown Voltage Input Low
Shutdown Voltage Input High
Shutdown Voltage Input Low
Output Offset Voltage
V
VSD MODE = VDD
V
V
VSD MODE = GND
VSD MODE = GND
V
50
9.7
7.0
0.9
mV (max)
kΩ (max)
kΩ (min)
W (min)
W
ROUT
Resistor Output to GND (Note 10)
8.5
Output Power (8Ω)
(4Ω) (Notes 13, 14)
Wake-up time
THD+N = 1% (max); f = 1kHz
THD+N = 1% (max); f = 1kHz
1.25
2
Po
TWU
100
0.2
ms
THD+N+N Total Harmonic Distortion+Noise
Po = 0.5Wrms; f = 1kHz
Vripple = 200mV sine p-p
Input terminated with 10Ω
%
60 (f =
PSRR Power Supply Rejection Ratio
217Hz)
55
dB (min)
64 (f = 1kHz)
Electrical Characteristics VDD = 3V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4990
Units
(Limits)
Symbol
Parameter
Conditions
Typical
Limit
(Note 6)
(Notes 7, 9)
VIN = 0V, Io = 0A, No Load
VIN = 0V, Io = 0A, 8Ω Load
VSD = VSD Mode (Note 8)
VSD MODE = VDD
2
7
9
mA (max)
mA (max)
µA (max)
V
IDD
Quiescent Power Supply Current
3
ISD
Shutdown Current
0.1
1.1
0.9
1.3
1.0
7
2.0
VSDIH
VSDIL
VSDIH
VSDIL
VOS
Shutdown Voltage Input High
Shutdown Voltage Input Low
Shutdown Voltage Input High
Shutdown Voltage Input Low
Output Offset Voltage
VSD MODE = VDD
V
VSD MODE = GND
V
VSD MODE = GND
V
50
9.7
7.0
mV (max)
kΩ (max)
kΩ (min)
ROUT
Resistor Output to GND (Note 10)
8.5
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4
Electrical Characteristics VDD = 3V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA
25˚C. (Continued)
=
LM4990
Units
(Limits)
Symbol
Parameter
Conditions
Typical
(Note 6)
425
Limit
(Notes 7, 9)
Output Power (8Ω)
(4Ω)
THD+N = 1% (max); f = 1kHz
THD+N = 1% (max); f = 1kHz
mW
mW
ms
%
Po
600
TWU
Wake-up time
75
THD+N+N Total Harmonic Distortion+Noise
Po = 0.25Wrms; f = 1kHz
Vripple = 200mV sine p-p
Input terminated with 10Ω
0.1
62 (f =
PSRR
Power Supply Rejection Ratio
217Hz)
55
dB (min)
68 (f = 1kHz)
Electrical Characteristics VDD = 2.6V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4990
Units
(Limits)
Symbol
Parameter
Conditions
Typical
(Note 6)
2.0
Limit
(Notes 7, 9)
VIN = 0V, Io = 0A, No Load
VIN = 0V, Io = 0A, 8Ω Load
VSD = VSD Mode (Note 8)
VSD MODE = VDD
mA
IDD
Quiescent Power Supply Current
3.0
mA
ISD
Shutdown Current
0.1
µA
VSDIH
VSDIL
VSDIH
VSDIL
VOS
Shutdown Voltage Input High
Shutdown Voltage Input Low
Shutdown Voltage Input High
Shutdown Voltage Input Low
Output Offset Voltage
1.0
V
VSD MODE = VDD
0.9
V
V
VSD MODE = GND
1.2
VSD MODE = GND
1.0
V
5
50
9.7
7.0
mV (max)
kΩ (max)
kΩ (min)
ROUT
Resistor Output to GND (Note 10)
8.5
Po
Output Power ( 8Ω )
( 4Ω )
THD+N = 1% (max); f = 1kHz
THD+N = 1% (max); f = 1kHz
300
400
70
mW
TWU
Wake-up time
ms
%
THD+N+N Total Harmonic Distortion+Noise
Po = 0.15Wrms; f = 1kHz
Vripple = 200mV sine p-p
Input terminated with 10Ω
0.1
51 (f =
PSRR Power Supply Rejection Ratio
217Hz)
dB
51 (f = 1kHz)
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 LM4990, see power derating
DMAX
JMAX A JA
curves 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: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
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: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 10: R
is measured from the output pin to ground. This value represents the parallel combination of the 10kΩ output resistors and the two 20kΩ resistors.
ROUT
Note 11: If the product is in Shutdown mode and V exceeds 6V (to a max of 8V V ), then most of the excess current will flow through the ESD protection circuits.
DD
DD
If the source impedance limits the current to a max of 10mA, then the device will be protected. If the device is enabled when V is greater than 5.5V and less than
DD
6.5V, no damage will occur, although operation life will be reduced. Operation above 6.5V with no current limit will result in permanent damage.
Note 12: Maximum power dissipation in the device (P
) occurs at an output power level significantly below full output power. P
can be calculated using
DMAX
DMAX
Equation 1 shown in the Application Information section. It may also be obtained from the power dissipation graphs.
5
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Electrical Characteristics VDD = 2.6V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA
25˚C. (Continued)
=
Note 13: The Exposed-DAP of the LDA10B package should be electrically connected to GND or an electrically isolated copper area. the LM4990LD demo board
has the Exposed-DAP connected to GND with a PCB area of 86.7mils x 585mils (2.02mm x 14.86mm) on the copper top layer and 550mils x 710mils (13.97mm
x 18.03mm) on the copper bottom layer.
Note 14: The thermal performance of the LLP and exposed-DAP TSSOP packages when used with the exposed-DAP connected to a thermal plane is sufficient for
driving 4Ω loads. The MSOP and ITL packages do not have the thermal performance necessary for driving 4Ω loads with a 5V supply and is not recommended for
this application.
Note 15: All bumps have the same thermal resistance and contribute equally when used to lower thermal resistance. All bumps must be connected to achieve
specified thermal resistance.
External Components Description
See (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.
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.
5.
CB
Typical Performance Characteristics
LD and MH Specific Characteristics
THD+N+N vs Frequency
THD+N+N vs Output Power
VDD = 5V, RL = 4Ω, and PO = 1W
VDD = 5V, RL = 4Ω, and f = 1 kHz
20051030
20051031
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Typical Performance Characteristics
THD+N+N vs Frequency
THD+N+N vs Frequency
VDD = 5V, RL = 8Ω, and PO = 500mW
VDD = 3V, RL = 4Ω, and PO = 500mW
20051032
20051033
THD+N+N vs Frequency
THD+N+N vs Frequency
VDD = 3V, RL = 8Ω, and PO = 250mW
VDD = 2.6V, RL = 4Ω, and PO = 150mW
20051034
20051083
THD+N+N vs Output Power
THD+N+N vs Output Power
VDD = 2.6V, RL = 8Ω, and PO = 150mW
VDD = 5V, RL = 8Ω, and f = 1kHz
20051084
20051085
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Typical Performance Characteristics (Continued)
THD+N+N vs Output Power
THD+N+N vs Output Power
VDD = 3V, RL = 4Ω, and f = 1kHz
VDD = 3V, RL = 8Ω, and f = 1kHz
20051002
20051003
THD+N+N vs Output Power
THD+N+N vs Output Power
VDD = 2.6V, RL = 4Ω, and f = 1kHz
VDD = 2.6V, RL = 8Ω, and f = 1kHz
20051004
20051005
Power Supply Rejection Ratio (PSRR) vs Frequency
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 5V, RL = 8Ω, input 10Ω terminated
VDD = 5V, RL = 8Ω, input floating
20051006
20051007
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio (PSRR) vs Frequency
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 3V, RL = 8Ω, input floating
VDD = 3V, RL = 8Ω, input 10Ω terminated
20051086
20051087
Power Supply Rejection Ratio (PSRR) vs Frequency
Power Supply Rejection Ratio (PSRR) vs Frequency
VDD = 2.6V, RL = 8Ω, input 10Ω terminated
VDD = 2.6V, RL = 8Ω, Input Floating
20051088
20051089
Noise Floor, 5V, 8Ω
80kHz Bandwidth, Input to GND
Open Loop Frequency Response, 5V
20051092
20051095
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Typical Performance Characteristics (Continued)
Power Dissipation vs
Power Dissipation vs
Output Power, VDD = 5V
Output Power, VDD = 3V
200510B5
200510B6
Power Dissipation vs
Output Power, VDD = 2.6V
Shutdown Hysteresis Voltage
VDD = 5V, SD Mode = VDD
200510B7
200510A0
Shutdown Hysteresis Voltage
VDD = 5V, SD Mode = GND
Shutdown Hysteresis Voltage
VDD = 3V, SD Mode = VDD
200510A1
200510A2
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Typical Performance Characteristics (Continued)
Shutdown Hysteresis Voltage
VDD = 3V, SD Mode = GND
Shutdown Hysteresis Voltage
VDD = 2.6V, SD Mode = VDD
200510A3
200510A4
200510B8
200510A7
Shutdown Hysteresis Voltage
VDD = 2.6V, SD Mode = GND
Output Power vs
Supply Voltage, RL = 4Ω
200510A5
Output Power vs
Supply Voltage, RL = 8Ω
Output Power vs
Supply Voltage, RL = 16Ω
200510A6
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Typical Performance Characteristics (Continued)
Output Power vs
Supply Voltage, RL = 32Ω
Frequency Response vs
Input Capacitor Size
200510A8
20051054
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12
especially effective when connected to VDD, GND, and the
output pins. Refer to the application information on the
LM4990 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.
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4990 has two internal opera-
tional amplifiers. The first amplifier’s gain is externally con-
figurable, 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 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 LM4990. 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.
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.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4990 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4990 con-
tains a Shutdown Mode pin (LD and MH packages only),
allowing the designer to designate whether the part will be
driven into shutdown with a high level logic signal or a low
level logic signal. This allows the designer maximum flexibil-
ity in device use, as the Shutdown Mode pin may simply be
tied permanently to either VDD or GND to set the LM4990 as
either a "shutdown-high" device or a "shutdown-low" device,
respectively. The device may then be placed into shutdown
mode by toggling the Shutdown pin to the same state as the
Shutdown Mode pin. For simplicity’s sake, this is called
"shutdown same", as the LM4990 enters shutdown mode
whenever the two pins are in the same logic state. The MM
package lacks this Shutdown Mode feature, and is perma-
nently fixed as a ‘shutdown-low’ device. The trigger point for
either shutdown high or shutdown low is shown as a typical
value in the Supply Current vs Shutdown Voltage graphs in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for maximum perfor-
mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current may
be greater than the typical value of 0.1µA. In either case, the
shutdown pin should be tied to a definite voltage to avoid
unwanted state changes.
A bridge configuration, such as the one used in LM4990,
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 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 LM4990 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.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
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 copper foil, the thermal resistance of the
PROPER SELECTION OF EXTERNAL COMPONENTS
application can be reduced from the free air value of θJA
,
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4990 is tolerant of
resulting in higher PDMAX values without thermal shutdown
protection circuitry being activated. Additional copper foil can
be added to any of the leads connected to the LM4990. It is
13
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AUDIO POWER AMPLIFIER DESIGN
Application Information (Continued)
A 1W/8Ω Audio Amplifier
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
Given:
Power Output
Load Impedance
Input Level
1Wrms
8Ω
The LM4990 is unity-gain stable which gives the designer
maximum system flexibility. The LM4990 should be used in
low gain configurations to minimize THD+N+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 1Vrms 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.
1Vrms
20kΩ
Input Impedance
Bandwidth
100Hz–20kHz 0.25dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found.
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.
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4990 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.
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.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 2.
(2)
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
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Rf/Ri = AVD/2
From Equation 2, the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20kΩ, and with a
AVD impedance of 2, 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.
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 LM4990 turns
on. The slower the LM4990’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.
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
As stated in the External Components section, Ri in con-
junction with Ci create a highpass filter.
Ci ≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF
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 =
300kHz which is much smaller than the LM4990 GBWP of
2.5MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4990 can still be used without running into bandwidth
limitations.
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14
Application Information (Continued)
20051024
FIGURE 3. HIGHER GAIN AUDIO AMPLIFIER
The LM4990 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 (C4) may be
needed as shown in Figure 2 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 R3 and C4 will cause rolloff before
20kHz. A typical combination of feedback resistor and ca-
pacitor that will not produce audio band high frequency rolloff
is R3 = 20kΩ and C4 = 25pf. These components result in a
-3dB point of approximately 320kHz.
15
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Application Information (Continued)
20051029
FIGURE 4. DIFFERENTIAL AMPLIFIER CONFIGURATION FOR LM4990
20051025
FIGURE 5. REFERENCE DESIGN BOARD SCHEMATIC
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16
Physical Dimensions inches (millimeters) unless otherwise noted
MSOP
Order Number LM4990MM
NS Package Number MUA08A
Exposed-DAP TSSOP
Order Number LM4990MH
NS Package Number MXF10A
17
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
LLP
Order Number LM4990LD
NS Package Number LDA10B
9–Bump micro SMD
Order Number LM4990ITL, LM4990ITLX
NS Package Number TLA09ZZA
X1 = 1.463 0.03 X2 = 1.463 0.03 X3 = 0.600 0.075
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18
Notes
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.
For the most current product information visit us at www.national.com.
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