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PDF LM4670ITL Data sheet ( Hoja de datos )
| Número de pieza | LM4670ITL | |
| Descripción | Filterless High Efficiency 3W Switching Audio Amplifier | |
| Fabricantes | National Semiconductor | |
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December 2004
LM4670
Filterless High Efficiency 3W Switching Audio Amplifier
General Description
The LM4670 is a fully integrated single-supply high efficiency
switching audio amplifier. It features an innovative modulator
that eliminates the LC output filter used with typical switching
amplifiers. Eliminating the output filter reduces external com-
ponent count, simplifies circuit design, and reduces board
area. The LM4670 processes analog inputs with a delta-
sigma modulation technique that lowers output noise and
THD when compared to conventional pulse width modula-
tors.
The LM4670 is designed to meet the demands of mobile
phones and other portable communication devices. Operat-
ing on a single 5V supply, it is capable of driving a 4Ω
speaker load at a continuous average output of 2.3W with
less than 1% THD+N. Its flexible power supply requirements
allow operation from 2.4V to 5.5V.
The LM4670 has high efficiency with speaker loads com-
pared to a typical Class AB amplifier. With a 3.6V supply
driving an 8Ω speaker, the IC’s efficiency for a 100mW
power level is 77%, reaching 88% at 600mW output power.
The LM4670 features a low-power consumption shutdown
mode. Shutdown may be enabled by driving the Shutdown
pin to a logic low (GND).
The gain of the LM4670 is externally configurable which
allows independent gain control from multiple sources by
summing the signals.
Key Specifications
j Efficiency at 3.6V, 100mW into 8Ω speaker 77% (typ)
j Efficiency at 3.6V, 600mW into 8Ω speaker 88% (typ)
j Efficiency at 5V, 1W into 8Ω speaker
87% (typ)
j Quiescent current, 3.6V supply
4.8mA (typ)
j Total shutdown power supply current 0.01µA (typ)
j Single supply range
2.4V to 5.5V
Features
n No output filter required for inductive loads
n Externally configurable gain
n Very fast turn on time: 1.35ms (typ)
n Minimum external components
n "Click and pop" suppression circuitry
n Micro-power shutdown mode
n Short circuit protection
n Available in space-saving microSMD and LLP packages
Applications
n Mobile phones
n PDAs
n Portable electronic devices
Typical Application
20089901
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2004 National Semiconductor Corporation DS200899
www.national.com
1 page  
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 = (TJMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4670, TJMAX = 150˚C.
The typical θJA is 220˚C/W for the microSMD package and 64˚C/W for the LLP package.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 5: Machine Model, 220pF–240pF discharged through all pins.
Note 6: Typical specifications are specified at 25˚C and represent the parametric norm.
Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: Shutdown current is measured in a normal room environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA. The Shutdown pin should
be driven as close as possible to GND for minimal shutdown current and to VDD for the best THD performance in PLAY mode. See the Application Information
section under SHUTDOWN FUNCTION for more information.
Note 10: The performance graphs were taken using the Audio Precision AUX-0025 Switching Amplifier Measurement Filter in series with the LC filter on the board.
External Components Description
(Figure 1)
Components
1. CS
2. RI
Functional Description
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.
Gain setting resistor. Differential gain is set by the equation AV = 2 * 150kΩ / Ri(V/V).
5 www.national.com
5 Page 
Application Information (Continued)
current usage while in the shutdown state. While the
LM4670 may be disabled with shutdown voltages in between
ground and supply, the idle current will be greater than the
typical 0.01µA value. Increased THD may also be observed
with voltages less than VDD on the Shutdown pin when in
PLAY mode.
The LM4670 has an internal resistor connected between
GND and Shutdown pins. The purpose of this resistor is to
eliminate any unwanted state changes when the Shutdown
pin is floating. The LM4670 will enter the shutdown state
when the Shutdown pin is left floating or if not floating, when
the shutdown voltage has crossed the threshold. To mini-
mize the supply current while in the shutdown state, the
Shutdown pin should be driven to GND or left floating. If the
Shutdown pin is not driven to GND, the amount of additional
resistor current due to the internal shutdown resistor can be
found by Equation (1) below.
(VSD - GND) / 300kΩ
(1)
With only a 0.5V difference, an additional 1.7µA of current
will be drawn while in the shutdown state.
PROPER SELECTION OF EXTERNAL COMPONENTS
The gain of the LM4670 is set by the external resistors, Ri in
Figure 1, The Gain is given by Equation (2) below. Best
THD+N performance is achieved with a gain of 2V/V (6dB).
AV = 2 * 150 kΩ / Ri (V/V)
(2)
It is recommended that resistors with 1% tolerance or better
be used to set the gain of the LM4670. The Ri resistors
should be placed close to the input pins of the LM4670.
Keeping the input traces close to each other and of the same
length in a high noise environment will aid in noise rejection
due to the good CMRR of the LM4670. Noise coupled onto
input traces which are physically close to each other will be
common mode and easily rejected by the LM4670.
Input capacitors may be needed for some applications or
when the source is single-ended (see Figures 3, 5). Input
capacitors are needed to block any DC voltage at the source
so that the DC voltage seen between the input terminals of
the LM4670 is 0V. Input capacitors create a high-pass filter
with the input resistors, Ri. The –3dB point of the high-pass
filter is found using Equation (3) below.
fC = 1 / (2πRi Ci ) (Hz)
(3)
The input capacitors may also be used to remove low audio
frequencies. Small speakers cannot reproduce low bass
frequencies so filtering may be desired . When the LM4670
is using a single-ended source, power supply noise on the
ground is seen as an input signal by the +IN input pin that is
capacitor coupled to ground (See Figures 5 – 7). Setting the
high-pass filter point above the power supply noise frequen-
cies, 217Hz in a GSM phone, for example, will filter out this
noise so it is not amplified and heard on the output. Capaci-
tors with a tolerance of 10% or better are recommended for
impedance matching.
DIFFERENTIAL CIRCUIT CONFIGURATIONS
The LM4670 can be used in many different circuit configu-
rations. The simplest and best performing is the DC coupled,
differential input configuration shown in Figure 2. Equation
(2) above is used to determine the value of the Ri resistors
for a desired gain.
Input capacitors can be used in a differential configuration as
shown in Figure 3. Equation (3) above is used to determine
the value of the Ci capacitors for a desired frequency re-
sponse due to the high-pass filter created by Ci and Ri.
Equation (2) above is used to determine the value of the Ri
resistors for a desired gain
The LM4670 can be used to amplify more than one audio
source. Figure 4 shows a dual differential input configuration.
The gain for each input can be independently set for maxi-
mum design flexibility using the Ri resistors for each input
and Equation (2). Input capacitors can be used with one or
more sources as well to have different frequency responses
depending on the source or if a DC voltage needs to be
blocked from a source.
SINGLE-ENDED CIRCUIT CONFIGURATIONS
The LM4670 can also be used with single-ended sources but
input capacitors will be needed to block any DC at the input
terminals. Figure 5 shows the typical single-ended applica-
tion configuration. The equations for Gain, Equation (2), and
frequency response, Equation (3), hold for the single-ended
configuration as shown in Figure 5.
When using more than one single-ended source as shown in
Figure 6, the impedance seen from each input terminal
should be equal. To find the correct values for Ci3 and Ri3
connected to the +IN input pin the equivalent impedance of
all the single-ended sources are calculated. The single-
ended sources are in parallel to each other. The equivalent
capacitor and resistor, Ci3 and Ri3, are found by calculating
the parallel combination of all Civalues and then all Ri val-
ues. Equations (4) and (5) below are for any number of
single-ended sources.
Ci3 = Ci1 + Ci2 + Cin ... (F)
(4)
Ri3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin ...) (Ω)
(5)
The LM4670 may also use a combination of single-ended
and differential sources. A typical application with one single-
ended source and one differential source is shown in Figure
7. Using the principle of superposition, the external compo-
nent values can be determined with the above equations
corresponding to the configuration.
11 www.national.com
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