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PDF LM4905 Data sheet ( Hoja de datos )
| Número de pieza | LM4905 | |
| Descripción | 1 Watt Audio Power Amplifier | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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July 2003
LM4905
1 Watt Audio Power Amplifier
General Description
The LM4905 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 LM4905 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 LM4905 features a low-power consumption shutdown
mode, which is achieved by driving the shutdown pin with
logic low. Additionally, the LM4905 features an internal ther-
mal shutdown protection mechanism.
The LM4905 contains advanced pop & click circuitry which
eliminates noise which would otherwise occur during turn-on
and turn-off transitions.
The LM4905 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, 8Ω
j Shutdown Current
62dB
1.07W (typ)
390mW (typ)
0.1µA (typ)
Features
n Available in space-saving LLP 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 2.2 - 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
200508D3
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation DS200508
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
20050830
THD+N vs Frequency
at VDD = 2.6V, 8Ω RL, and PWR = 150mW
20050831
THD+N vs Power Out
at VDD = 5V, 8Ω RL, 1kHz
20050832
5
20050834
www.national.com
5 Page 
Application Information (Continued)
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 LM4905 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4905 is unity-gain stable which gives the designer
maximum system flexibility. The LM4905 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete explanation of proper gain selection.
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.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
Ci. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 VDD). This
charge comes from the output via the feedback and is apt to
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, CB, is the most critical component to minimize
turn-on pops since it determines how fast the LM4905 turns
on. The slower the LM4905’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
Input Impedance
Bandwidth
1 Wrms
8Ω
1 Vrms
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 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 LM4905 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.
Rf/Ri = AVD/2
(2)
From Equation 2, the minimum AVD is 2.83; use AVD = 3.
Since the desired input impedance was 20 kΩ, and with a
AVD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an
allocation of Ri = 20 kΩ and Rf = 30 kΩ. The final design step
is to address the bandwidth requirements which must be
stated as a pair of −3 dB frequency points. Five times away
from a −3 dB point is 0.17 dB down from passband response
which is better than the required ±0.25 dB specified.
fL = 100 Hz/5 = 20 Hz
fH = 20 kHz * 5 = 100 kHz
As stated in the External Components section, Ri in con-
junction with Ci create a highpass filter.
Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
The high frequency pole is determined by the product of the
desired frequency pole, fH, and the differential gain, AVD.
With a AVD = 3 and fH = 100 kHz, the resulting GBWP =
300kHz which is much smaller than the LM4905 GBWP of
2.5MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4905 can still be used without running into bandwidth
limitations.
The LM4905 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-
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| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet LM4905.PDF ] | |
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