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PDF LM4903 Data sheet ( Hoja de datos )
| Número de pieza | LM4903 | |
| Descripción | 1 Watt Audio Power Amplifier | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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February 2003
LM4903
1 Watt Audio Power Amplifier
General Description
The LM4903 is an audio power amplifier primarily designed
for demanding applications in mobile phones and other por-
table communication device applications. It is capable of
delivering 1 watt of continuous average power to an 8Ω BTL
load with less than 1% distortion (THD+N) from a 5VDC
power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4903 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 LM4903 features a low-power consumption shutdown
mode, which is achieved by driving the shutdown pin with
logic low. Additionally, the LM4903 features an internal ther-
mal shutdown protection mechanism.
The LM4903 contains advanced pop & click circuitry which
eliminates noise which would otherwise occur during turn-on
and turn-off transitions.
The LM4903 is unity-gain stable and can be configured by
external gain-setting resistors.
Key Specifications
j Improved PSRR at 217Hz & 1KHz
j Power Output at 5.0V, 1% THD, 8Ω
j Power Output at 3.0V, 1% THD, 4Ω
j Power Output at 3.0V, 1% THD, 8Ω
j Shutdown Current
62dB
1.07W (typ)
525mW (typ)
390mW (typ)
0.1µA (typ)
Features
n Available in space-saving MSOP package
n Ultra low current shutdown mode
n BTL output can drive capacitive loads
n Improved pop & click circuitry eliminates noise during
turn-on and turn-off transitions
n 2.0 - 5.5V operation
n No output coupling capacitors, snubber networks or
bootstrap capacitors required
n Unity-gain stable
n External gain configuration capability
Applications
n Mobile Phones
n PDAs
n Portable electronic devices
Typical Application
200467D3
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation DS200467
www.national.com
1 page  
External Components Description
(Figure 1)
Components
1. Ri
2. Ci
3. Rf
4. CS
5. CB
Functional Description
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).
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.
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
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.
Typical Performance Characteristics
THD+N vs Frequency
at VDD = 5V, 8Ω RL, and PWR = 500mW
THD+N vs Frequency
at VDD = 3V, 8Ω RL, and PWR = 250mW
20046730
THD+N vs Frequency
at VDD = 2.6V, 8Ω RL, and PWR = 150mW
20046731
THD+N vs Frequency
at VDD = 2.6V, 4Ω RL, and PWR = 150mW
20046732
5
20046733
www.national.com
5 Page 
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4903 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
AVD= 2 *(Rf/Ri)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4903,
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 LM4903 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
PDMAX = 4*(VDD)2/(2π2RL)
(1)
It is critical that the maximum junction temperature TJMAX of
150˚C is not exceeded. TJMAX can be determined from the
power derating curves by using PDMAX and the PC board foil
area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of θJA,
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 LM4903. It is
especially effective when connected to VDD, GND, and the
output pins. Refer to the application information on the
LM4903 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.
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 LM4903. The selection of
a bypass capacitor, especially CB, is dependent upon PSRR
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4903 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when logic low is placed on the shutdown pin.
By switching the shutdown pin to GND, the LM4903 supply
current draw will be minimized in idle mode. Idle current is
measured with the shutdown pin connected to GND. The
trigger point for shutdown is shown as a typical value in the
Shutdown Hysteresis Voltage graphs in the Typical Perfor-
mance Characteristics section. It is best to switch between
ground and supply for maximum performance. 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. 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 LM4903 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4903 is unity-gain stable which gives the designer
maximum system flexibility. The LM4903 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
11 www.national.com
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| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet LM4903.PDF ] | |
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