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PDF LM4924 Data sheet ( Hoja de datos )
| Número de pieza | LM4924 | |
| Descripción | 2 Cell Battery / 40mW Per Channel Output Capacitor-Less(OCL) Stereo Headphone Audio Amplifier | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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October 2004
LM4924
2 Cell Battery, 40mW Per Channel Output Capacitor-Less
(OCL) Stereo Headphone Audio Amplifier
General Description
The LM4924 is a Output Capacitor-Less (OCL) stereo head-
phone amplifier, which when connected to a 3.0V supply,
delivers 40mW per channel to a 16Ω load with less than 1%
THD+N.
With the LM4924 packaged in the MM and SD packages, the
customer benefits include low profile and small size. These
packages minimizes PCB area and maximizes output power.
The LM4924 features circuitry that reduces output transients
(“clicks” and “pops”) during device turn-on and turn-off, and
Mute On and Off. An externally controlled, low-power con-
sumption, active-low shutdown mode is also included in the
LM4924. Boomer audio power amplifiers are designed spe-
cifically to use few external components and provide high
quality output power in a surface mount packages.
Key Specifications
n OCL output power
n (RL = 16Ω, VDD = 3.0V, THD+N = 1%) 40mW (typ)
n Micropower shutdown current
0.1µA (typ)
n Supply voltage operating range
1.5V < VDD < 3.6V
n PSRR 100Hz, VDD = 3.0V, AV = 2.5
66dB (typ)
Features
n 2-cell 1.5V to 3.6V battery operation
n OCL mode for stereo headphone operation
n Unity-gain stable
n “Click and pop” suppression circuitry for shutdown On
and Off transients
n Active low micropower shutdown
n Thermal shutdown protection circuitry
Applications
n Portable two-cell audio products
n Portable two-cell electronic devices
Typical Application
FIGURE 1. Block Diagram
20121057
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation DS201210
www.national.com
1 page  
Electrical Characteristics VDD = 1.8V (Notes 1, 5) (Continued)
The following specifications apply for the circuit shown in Figure 2, unless otherwise specified. AV = 2.5, RL = 16Ω.
Limits apply for TA = 25˚C.
Symbol
Parameter
Conditions
LM4924
Units
Typical
Limit
(Limits)
(Note 6) (Note 7)
THD
Crosstalk
PO = 5mW
Freq = 1kHz
0.1 %
45 dB (min)
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mVP-P sine wave
Freq = 100Hz, OCL
66
dB
Note 1: 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 2: The maximum power dissipation is dictated by TJMAX, θJA, and the ambient temperature TA and must be derated at elevated temperatures. The maximum
allowable power dissipation is PDMAX = (TJMAX − TA)/θJA. For the LM4924, TJMAX = 150˚C. For the θJAs, please see the Application Information section or the
Absolute Maximum Ratings section.
Note 3: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 4: Machine model, 220pF–240pF discharged through all pins.
Note 5: All voltages are measured with respect to the ground (GND) pins unless otherwise specified.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Note 9: Output power is measured at the device terminals.
5 www.national.com
5 Page 
Application Information (Continued)
pin. When active, the LM4924’s micro-power shutdown fea-
ture turns off the amplifier’s bias circuitry, reducing the sup-
ply current. The trigger point is 0.4V (max) for a logic-low
level, and 1.5V (min) for a logic-high level. The low 0.1µA
(typ) shutdown current is achieved by applying a voltage that
is as near as ground as possible to the SHUTDOWN pin. A
voltage that is higher than ground may increase the shut-
down current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
SHUTDOWN pin and VDD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by opening the switch. Closing the switch connects the
SHUTDOWN pin to ground, activating micro-power shut-
down. The switch and resistor guarantee that the SHUT-
DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or microcontrol-
ler, use a digital output to apply the control voltage to the
SHUTDOWN pin. Driving the SHUTDOWN pin with active
circuitry eliminates the pull-up resistor.
SELECTING EXTERNAL COMPONENTS
Selecting proper external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the LM4924 is tolerant of exter-
nal component combinations, consideration to component
values must be used to maximize overall system quality.
The LM4924 is unity-gain stable which gives the designer
maximum system flexibility. The LM4924 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. The input coupling capacitor,
Ci, forms a first order high pass filter which limits low fre-
quency response. This value should be chosen based on
needed frequency response and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifiying 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.
USING EXTERNAL POWERED SPEAKERS
The LM4924 is designed specifically for headphone opera-
tion. Often the headphone output of a device will be used to
drive external powered speakers. The LM4924 has a differ-
ential output to eliminate the output coupling capacitors. The
result is a headphone jack sleeve that is connected to VO3
instead of GND. For powered speakers that are designed to
have single-ended signals at the input, the click and pop
circuitry will not be able to eliminate the turn-on/turn-off click
and pop. Unless the inputs to the powered speakers are fully
differential the turn-on/turn-off click and pop will be very
large.
AUDIO POWER AMPLIFIER DESIGN
A 30mW/32Ω Audio Amplifier
Given:
Power Output
30mWrms
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.
Since 3.3V is a standard supply voltage in most applications,
it is chosen for the supply rail in this example. Extra supply
voltage creates headroom that allows the LM4924 to repro-
duce peaks in excess of 30mW without producing audible
distortion. At this time, the designer must make sure that the
power supply choice along with the output impedance does
no violate the conditions explained in the Power Dissipa-
tion section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 2.
(2)
From Equation 2, the minimum AV is 0.98; use AV = 1. Since
the desired input impedance is 20kΩ, and with AV equal to 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 last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired ±0.25dB
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ±0.25dB
desired limit. The results are an
and an
fL = 100Hz/5 = 20Hz
fH = 20kHz x 5 = 100kHz
(3)
(4)
11 www.national.com
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LM4924.PDF ] | |
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