LM4990
LM4990 2 Watt Audio Power Amplifier with Selectable Shutdown Logic Level
Literature Number: SNAS184D
LM4990
2 Watt Audio Power Amplifier with Selectable Shutdown
Logic Level
General Description
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 4ΩBTL load with less than 1% distortion
(THD+N+N) from a 5V
DC
power supply.
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.
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.
The LM4990 contains advanced pop & click circuitry which
eliminates noise which would otherwise occur during turn-on
and turn-off transitions.
The LM4990 is unity-gain stable and can be configured by
external gain-setting resistors.
Key Specifications
jImproved PSRR at 217Hz & 1KHz 62dB
jPower Output at 5.0V, 1%
THD+N,
4Ω(LD and MH only) 2W (typ)
jPower Output at 5.0V, 1% THD+N, 8Ω1.25W (typ)
jPower Output at 3.0V, 1% THD+N, 4Ω600mW (typ)
jPower Output at 3.0V, 1% THD+N, 8Ω425mW (typ)
jShutdown Current 0.1µA (typ)
Features
nAvailable in space-saving packages: LLP, Exposed-DAP
TSSOP, MSOP, and ITL
nUltra low current shutdown mode
nImproved pop & click circuitry eliminates noise during
turn-on and turn-off transitions
n2.2 - 5.5V operation
nNo output coupling capacitors, snubber networks or
bootstrap capacitors required
nUnity-gain stable
nExternal gain configuration capability
nUser selectable shutdown High or Low logic Level
Applications
nMobile Phones
nPDAs
nPortable electronic devices
Connection Diagrams
Mini Small Outline (MSOP) Package MSOP Marking
200510B9
Top View
Order Number LM4990MM
See NS Package Number MUA08A
20051071
Top View
Z - Plant Code
X - Date Code
TT - Die Traceability
G - Boomer Family
A5 - LM4990MM
Boomer®is a registered trademark of National Semiconductor Corporation.
October 2004
LM4990 2 Watt Audio Power Amplifier with Selectable Shutdown Logic Level
© 2004 National Semiconductor Corporation DS200510 www.national.com
Connection Diagrams (Continued)
LLP Package LLP Marking
200510B3
Top View
Order Number LM4990LD
See NS Package Number LDA10B
200510B4
Top View
Z - Plant Code
XY - Date Code
TT - Die Traceability
Bottom Line - Part Number
Exposed-DAP TSSOP Package
20051096
Top View
Order Number LM4990MH
See NS Package Number MXF10A
9 Bump micro SMD 9 Bump micro SMD Marking
200510C0
Top View
Order Number LM4990ITL, LM4990ITLX
See NS Package Number TLA09ZZA
200510C1
Top View
X — Date Code
T — Die Traceability
G — Boomer Family
D2 — LM4990ITL
LM4990
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Package LD MH MM ITL
Shutdown Mode Selectable Selectable Low Low
Typical Power Output at 5V,
1% THD+N
2W
(R
L
=4Ω)
2W
(R
L
=4Ω)
1.25W
(R
L
=8Ω)
1.25W
(R
L
=8Ω)
.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)
LM4990
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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.
Supply Voltage (Note 11) 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
+0.3V
Power Dissipation (Notes 3, 12) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JC
(MSOP) 56˚C/W
θ
JA
(MSOP) 190˚C/W
θ
JA
(9 Bump micro SMD) (Note 15) 180˚C/W
θ
JA
(LLP) 63˚C/W (Note 13)
θ
JC
(LLP) 12˚C/W (Note 13)
Soldering Information
See AN-1187 "Leadless
Leadframe Package (LLP)."
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2.2V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
=5V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4990 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 9)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
o
= 0A, No Load 3 7 mA (max)
V
IN
= 0V, I
o
= 0A, 8ΩLoad 4 10 mA (max)
I
SD
Shutdown Current V
SD
=V
SD Mode
(Note 8) 0.1 2.0 µA (max)
V
SDIH
Shutdown Voltage Input High V
SD MODE
=V
DD
1.5 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
=V
DD
1.3 V
V
SDIH
Shutdown Voltage Input High V
SD MODE
= GND 1.5 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
= GND 1.3 V
V
OS
Output Offset Voltage 7 50 mV (max)
R
OUT
Resistor Output to GND (Note 10) 8.5 9.7 kΩ(max)
7.0 kΩ(min)
P
o
Output Power (8Ω) THD+N = 1% (max); f = 1kHz 1.25 0.9 W (min)
(4Ω) (Notes 13, 14) THD+N = 1% (max); f = 1kHz 2 W
T
WU
Wake-up time 100 ms
THD+N+N Total Harmonic Distortion+Noise P
o
= 0.5Wrms; f = 1kHz 0.2 %
PSRR Power Supply Rejection Ratio
V
ripple
= 200mV sine p-p
Input terminated with 10Ω
60 (f =
217Hz)
64 (f = 1kHz)
55 dB (min)
Electrical Characteristics V
DD
=3V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4990 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 9)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
o
= 0A, No Load 2 7 mA (max)
V
IN
= 0V, I
o
= 0A, 8ΩLoad 3 9 mA (max)
I
SD
Shutdown Current V
SD
=V
SD Mode
(Note 8) 0.1 2.0 µA (max)
V
SDIH
Shutdown Voltage Input High V
SD MODE
=V
DD
1.1 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
=V
DD
0.9 V
V
SDIH
Shutdown Voltage Input High V
SD MODE
= GND 1.3 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
= GND 1.0 V
V
OS
Output Offset Voltage 7 50 mV (max)
R
OUT
Resistor Output to GND (Note 10) 8.5 9.7 kΩ(max)
7.0 kΩ(min)
LM4990
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Electrical Characteristics V
DD
=3V(Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T
A
=
25˚C. (Continued)
Symbol Parameter Conditions
LM4990 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 9)
P
o
Output Power (8Ω) THD+N = 1% (max); f = 1kHz 425 mW
(4Ω) THD+N = 1% (max); f = 1kHz 600 mW
T
WU
Wake-up time 75 ms
THD+N+N Total Harmonic Distortion+Noise P
o
= 0.25Wrms; f = 1kHz 0.1 %
PSRR Power Supply Rejection Ratio
V
ripple
= 200mV sine p-p
Input terminated with 10Ω
62 (f =
217Hz)
68 (f = 1kHz)
55 dB (min)
Electrical Characteristics V
DD
= 2.6V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4990 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 9)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
o
= 0A, No Load 2.0 mA
V
IN
= 0V, I
o
= 0A, 8ΩLoad 3.0 mA
I
SD
Shutdown Current V
SD
=V
SD Mode
(Note 8) 0.1 µA
V
SDIH
Shutdown Voltage Input High V
SD MODE
=V
DD
1.0 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
=V
DD
0.9 V
V
SDIH
Shutdown Voltage Input High V
SD MODE
= GND 1.2 V
V
SDIL
Shutdown Voltage Input Low V
SD MODE
= GND 1.0 V
V
OS
Output Offset Voltage 5 50 mV (max)
R
OUT
Resistor Output to GND (Note 10) 8.5 9.7 kΩ(max)
7.0 kΩ(min)
P
o
Output Power ( 8Ω) THD+N = 1% (max); f = 1kHz 300 mW
(4Ω) THD+N = 1% (max); f = 1kHz 400
T
WU
Wake-up time 70 ms
THD+N+N Total Harmonic Distortion+Noise P
o
= 0.15Wrms; f = 1kHz 0.1 %
PSRR Power Supply Rejection Ratio
V
ripple
= 200mV sine p-p
Input terminated with 10Ω
51 (f =
217Hz)
51 (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 TJMAX,θJA, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX =(T
JMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4990, see power derating
curves for additional information.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
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 ISD by a maximum of 2µA.
Note 9: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 10: RROUT is measured from the output pin to ground. This value represents the parallel combination of the 10kΩoutput resistors and the two 20kΩresistors.
Note 11: If the product is in Shutdown mode and VDD exceeds 6V (to a max of 8V VDD), then most of the excess current will flow through the ESD protection circuits.
If the source impedance limits the current to a max of 10mA, then the device will be protected. If the device is enabled when VDD is greater than 5.5V and less than
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 (PDMAX) occurs at an output power level significantly below full output power. PDMAX can be calculated using
Equation 1 shown in the Application Information section. It may also be obtained from the power dissipation graphs.
LM4990
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Electrical Characteristics V
DD
= 2.6V (Notes 1, 2)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for T
A
=
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 4Ωloads. The MSOP and ITL packages do not have the thermal performance necessary for driving 4Ωloads 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. R
i
Inverting input resistance which sets the closed-loop gain in conjunction with R
f
. This resistor also forms a
high pass filter with C
i
at f
C
= 1/(2πR
i
C
i
).
2. C
i
Input coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a
highpass filter with R
i
at f
c
= 1/(2πR
i
C
i
). Refer to the section, Proper Selection of External Components,
for an explanation of how to determine the value of C
i
.
3. R
f
Feedback resistance which sets the closed-loop gain in conjunction with R
i
.
4. C
S
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. C
B
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 C
B
.
Typical Performance Characteristics
LD and MH Specific Characteristics
THD+N+N vs Frequency
V
DD
= 5V, R
L
=4Ω, and P
O
=1W
THD+N+N vs Output Power
V
DD
= 5V, R
L
=4Ω,andf=1kHz
20051030 20051031
LM4990
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Typical Performance Characteristics
THD+N+N vs Frequency
V
DD
= 5V, R
L
=8Ω, and P
O
= 500mW
THD+N+N vs Frequency
V
DD
= 3V, R
L
=4Ω, and P
O
= 500mW
20051032 20051033
THD+N+N vs Frequency
V
DD
= 3V, R
L
=8Ω, and P
O
= 250mW
THD+N+N vs Frequency
V
DD
= 2.6V, R
L
=4Ω, and P
O
= 150mW
20051034 20051083
THD+N+N vs Output Power
V
DD
= 2.6V, R
L
=8Ω, and P
O
= 150mW
THD+N+N vs Output Power
V
DD
= 5V, R
L
=8Ω, and f = 1kHz
20051084 20051085
LM4990
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Typical Performance Characteristics (Continued)
THD+N+N vs Output Power
V
DD
= 3V, R
L
=4Ω, and f = 1kHz
THD+N+N vs Output Power
V
DD
= 3V, R
L
=8Ω, and f = 1kHz
20051002 20051003
THD+N+N vs Output Power
V
DD
= 2.6V, R
L
=4Ω, and f = 1kHz
THD+N+N vs Output Power
V
DD
= 2.6V, R
L
=8Ω, and f = 1kHz
20051004 20051005
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 5V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 5V, R
L
=8Ω, input floating
20051006 20051007
LM4990
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 3V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 3V, R
L
=8Ω, input floating
20051086 20051087
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 2.6V, R
L
=8Ω, input 10Ωterminated
Power Supply Rejection Ratio (PSRR) vs Frequency
V
DD
= 2.6V, R
L
=8Ω, Input Floating
20051088 20051089
Open Loop Frequency Response, 5V
Noise Floor, 5V, 8Ω
80kHz Bandwidth, Input to GND
20051092
20051095
LM4990
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Typical Performance Characteristics (Continued)
Power Dissipation vs
Output Power, V
DD
=5V
Power Dissipation vs
Output Power, V
DD
=3V
200510B5 200510B6
Power Dissipation vs
Output Power, V
DD
= 2.6V
Shutdown Hysteresis Voltage
V
DD
= 5V, SD Mode = V
DD
200510B7
200510A0
Shutdown Hysteresis Voltage
V
DD
= 5V, SD Mode = GND
Shutdown Hysteresis Voltage
V
DD
= 3V, SD Mode = V
DD
200510A1 200510A2
LM4990
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Typical Performance Characteristics (Continued)
Shutdown Hysteresis Voltage
V
DD
= 3V, SD Mode = GND
Shutdown Hysteresis Voltage
V
DD
= 2.6V, SD Mode = V
DD
200510A3 200510A4
Shutdown Hysteresis Voltage
V
DD
= 2.6V, SD Mode = GND
Output Power vs
Supply Voltage, R
L
=4Ω
200510A5 200510B8
Output Power vs
Supply Voltage, R
L
=8Ω
Output Power vs
Supply Voltage, R
L
=16Ω
200510A6 200510A7
LM4990
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Typical Performance Characteristics (Continued)
Output Power vs
Supply Voltage, R
L
=32Ω
Frequency Response vs
Input Capacitor Size
200510A8
20051054
LM4990
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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 R
f
to R
i
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
A
VD
= 2 *(R
f
/R
i
)
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 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.
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
) (1)
It is critical that the maximum junction temperature T
JMAX
of
150˚C is not exceeded. T
JMAX
can be determined from the
power derating curves by using P
DMAX
and the PC board foil
area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of θ
JA
,
resulting in higher P
DMAX
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
especially effective when connected to V
DD
, GND, and the
output pins. Refer to the application information on the
LM4990 reference design board for an example of good heat
sinking. If T
JMAX
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.
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 C
B
, is dependent upon PSRR
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
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 V
DD
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.
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.
PROPER SELECTION OF EXTERNAL COMPONENTS
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
LM4990
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Application Information (Continued)
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
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.
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, C
i
, 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.
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,
C
i.
A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD
). 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 consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B
, 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 V
DD
), the smaller the turn-on pop.
Choosing C
B
equal to 1.0µF along with a small value of C
i
(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
C
B
equal to 0.1µF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
B
equal to
1.0µF is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8ΩAudio Amplifier
Given:
Power Output 1Wrms
Load Impedance 8Ω
Input Level 1Vrms
Input Impedance 20kΩ
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.
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.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 2.
(2)
R
f
/R
i
=A
VD
/2
From Equation 2, the minimum A
VD
is 2.83; use A
VD
=3.
Since the desired input impedance was 20kΩ, and with a
A
VD
impedance of 2, a ratio of 1.5:1 of R
f
to R
i
results in an
allocation of R
i
= 20kΩand R
f
= 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.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz*5=100kHz
As stated in the External Components section, R
i
in con-
junction with C
i
create a highpass filter.
C
i
≥1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
VD
.
With a A
VD
= 3 and f
H
= 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.
LM4990
www.national.com 14
Application Information (Continued)
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 R
3
and C
4
will cause rolloff before
20kHz. A typical combination of feedback resistor and ca-
pacitor that will not produce audio band high frequency rolloff
is R
3
= 20kΩand C
4
= 25pf. These components result in a
-3dB point of approximately 320kHz.
20051024
FIGURE 3. HIGHER GAIN AUDIO AMPLIFIER
LM4990
www.national.com15
Application Information (Continued)
20051029
FIGURE 4. DIFFERENTIAL AMPLIFIER CONFIGURATION FOR LM4990
20051025
FIGURE 5. REFERENCE DESIGN BOARD SCHEMATIC
LM4990
www.national.com 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
LM4990
www.national.com17
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
LM4990
www.national.com 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.
LIFE SUPPORT POLICY
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.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship
Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: ap.support@nsc.com
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM4990 2 Watt Audio Power Amplifier with Selectable Shutdown Logic Level
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