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PDF LM4929 Data sheet ( Hoja de datos )
| Número de pieza | LM4929 | |
| Descripción | Stereo 40mW Low Noise Headphone Amplifier with OCL Output | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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December 2004
LM4929
Stereo 40mW Low Noise Headphone Amplifier
with OCL Output
General Description
The LM4929 is an stereo audio power amplifier capable of
delivering 40mW per channel of continuous average power
into a 16Ω load or 25mW per channel into a 32Ω load at 1%
THD+N from a 3V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. Since the LM4929 does not require
bootstrap capacitors or snubber networks, it is optimally
suited for low-power portable systems. The LM4929 is con-
figured for OCL (Output Capacitor-Less) outputs, operating
with no DC blocking capacitors on the outputs.
The LM4929 features a low-power consumption shutdown
mode with a faster turn on time. Additionally, the LM4929
features an internal thermal shutdown protection mecha-
nism.
The LM4929 is unity gain stable and may be configured with
external gain-setting resistors.
Key Specifications
j PSRR at 217Hz and 1kHz
j Output Power at 1kHz with VDD = 2.4V,
1% THD+N into a 16Ω load
j Output Power at 1kHz with VDD = 3V,
1% THD+N into a 16Ω load
j Shutdown current
j Output Voltage change on release
from Shutdown VDD = 2.4V, RL = 16Ω
65dB (typ)
25mW (typ)
40mW (typ)
2.0µA (max)
1mV (max)
Features
n OCL outputs — No DC Blocking Capacitors
n External gain-setting capability
n Available in space-saving MSOP package
n Ultra low current shutdown mode
n 2V - 5.5V operation
n Ultra low noise
Applications
n Portable CD players
n PDAs
n Portable electronics devices
Block Diagram
FIGURE 1. Block Diagram
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation DS201324
20132441
www.national.com
1 page  
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
THD+N vs Frequency
20132482
THD+N vs Frequency
20132483
THD+N vs Frequency
20132406
THD+N vs Frequency
20132403
20132405
5
20132404
www.national.com
5 Page 
Application Information (Continued)
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a
system with a microprocessor or microcontroller, use a digi-
tal output to apply the control voltage to the SHUTDOWN
pin. Driving the SHUTDOWN pin with active circuitry elimi-
nates the pull-up resistor.
Shutdown enable/disable times are controlled by a combina-
tion of CB and VDD. Larger values of CB results in longer turn
on/off times from Shutdown. Smaller VDD values also in-
crease turn on/off time for a given value of CB. Longer
shutdown times also improve the LM4929’s resistance to
click and pop upon entering or returning from shutdown. For
a 2.4V supply and CB = 4.7µF, the LM4929 requires about 2
seconds to enter or return from shutdown. This longer shut-
down time enables the LM4929 to have virtually zero pop
and click transients upon entering or release from shutdown.
Smaller values of CB will decrease turn-on time, but at the
cost of increased pop and click and reduced PSRR. Since
shutdown enable/disable times increase dramatically as
supply voltage gets below 2.2V, this reduced turn-on time
may be desirable if extreme low supply voltage levels are
used as this would offset increases in turn-on time caused by
the lower supply voltage. This technique is not recom-
mended for OCL mode since shutdown enable/disable times
are very fast (0.5s) independent of supply voltage.
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 LM4929 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4929 is unity-gain stable which gives the designer
maximum system flexibility. The LM4929 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 1Vrms are available
from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
Ri and Rf should be less than 1MΩ. 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 2. 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 and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, Ci. A high value capacitor can
be expensive and may compromise space efficiency in por-
table designs. In many cases, however, the headphones
used in portable systems have little ability to reproduce
signals below 60Hz. Applications using headphones with this
limited frequency response reap little improvement by using
a high value input capacitor.
In addition to system cost and size, turn on time is affected
by the size of the input coupling capacitor Ci. A larger input
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of Ci (in the range of 0.1µF to
0.39µF), is recommended.
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32Ω AUDIO AMPLIFIER
Given:
Power Output
25mWrms
Load Impedance
32Ω
Input Level
1Vrms
Input Impedance
20kΩ
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.
3V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4929 to reproduce peak in excess of 25mW
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 gain can be determined from Equation 2.
(4)
From Equation 4, the minimum AV is 0.89; use AV = 1. Since
the desired input impedance is 20kΩ, and with a AV gain of
1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The
values are chosen with Ri = 20kΩ and Rf = 20kΩ. 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 speci-
fied.
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
As stated in the External Components section, Ri in con-
junction with Ci creates a
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, AV. With
an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz
which is much smaller than the LM4929 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4929 can still
be used without running into bandwidth limitations.
Figure 3 shows an optional resistor connected between the
amplifier output that drives the headphone jack sleeve and
ground. This resistor provides a ground path that supressed
power supply hum. Thishum may occur in applications such
as notebook computers in a shutdown condition and con-
nected to an external powered speaker. The resistor’s 100Ω
value is a suggested starting point. Its final value must be
11 www.national.com
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| PDF Descargar | [ Datasheet LM4929.PDF ] | |
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