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參數(shù)資料
| 型號(hào)： | LM4671ITLX/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2.5 W, 1 CHANNEL, AUDIO AMPLIFIER, PBGA9 |
| 封裝： | MICRO, SMD-9 |
| 文件頁(yè)數(shù)： | 2/10頁(yè) |
| 文件大?。?/td> | 589K |
| 代理商： | LM4671ITLX/NOPB |
ference. It is essential to keep the power and output traces
short and well shielded if possible. Use of ground planes,
beads, and micro-strip layout techniques are all useful in pre-
venting unwanted interference.
As the distance from the LM4671 and the speaker increase,
the amount of EMI radiation will increase since the output
wires or traces acting as antenna become more efficient with
length. What is acceptable EMI is highly application specific.
Ferrite chip inductors placed close to the LM4671 may be
needed to reduce EMI radiation. The value of the ferrite chip
is very application specific.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion ratio (PSRR). The capacitor (C
S) location should be as
close as possible to the LM4671. Typical applications employ
a voltage regulator with a 10F and a 0.1F bypass capacitors
that increase supply stability. These capacitors do not elimi-
nate the need for bypassing on the supply pin of the LM4671.
A 1F tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4671 contains shutdown circuitry that reduces current
draw to less than 0.01A. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics Ta-
bles and in the Shutdown Hysteresis Voltage graphs found in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for minimum current
usage while in the shutdown state. While the LM4671 may be
disabled with shutdown voltages in between ground and sup-
ply, the idle current will be greater than the typical 0.01A
value.
The LM4671 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 LM4671 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 minimize 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 Equa-
tion (1) below.
(V
SD - GND) / 60k
(1)
With only a 0.5V difference, an additional 8.3A of current will
be drawn while in the shutdown state.
PROPER SELECTION OF EXTERNAL COMPONENTS
The gain of the LM4671 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).
A
V = 2 * 150 k / Ri (V/V)
(2)
It is recommended that resistors with 1% tolerance or better
be used to set the gain of the LM4671. The Ri resistors should
be placed close to the input pins of the LM4671. 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 LM4671. Noise coupled onto input traces
which are physically close to each other will be common mode
and easily rejected by the LM4671.
Input capacitors may be needed for some applications or
when the source is single-ended (see Figures 3, 5). Input ca-
pacitors are needed to block any DC voltage at the source so
that the DC voltage seen between the input terminals of the
LM4671 is 0V. Input capacitors create a high-pass filter with
the input resistors, R
i. The –3dB point of the high-pass filter
is found using Equation (3) below.
f
C = 1 / (2πRi Ci ) (Hz)
(3)
The input capacitors may also be used to remove low audio
frequencies. Small speakers cannot reproduce low bass fre-
quencies so filtering may be desired . When the LM4671 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. Capacitors
with a tolerance of 10% or better are recommended for
impedance matching.
DIFFERENTIAL CIRCUIT CONFIGURATIONS
The LM4671 can be used in many different circuit configura-
tions. 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 R
i 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 C
i capacitors for a desired frequency re-
sponse due to the high-pass filter created by C
i and Ri.
Equation (2) above is used to determine the value of the R
i
resistors for a desired gain
The LM4671 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 R
i resistors for each input and
Equation (2). Input capacitors can be used with one or more
sources as well to have different frequency responses de-
pending on the source or if a DC voltage needs to be blocked
from a source.
SINGLE-ENDED CIRCUIT CONFIGURATIONS
The LM4671 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 application
configuration. The equations for Gain, Equation (2), and fre-
quency 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 C
i3 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 resis-
tor, C
i3 and Ri3, are found by calculating the parallel combi-
nation of all C
ivalues and then all Ri values. Equations (4) and
(5) below are for any number of single-ended sources.
C
i3 = Ci1 + Ci2 + Cin ... (μF)
(4)
www.national.com
10
LM4671
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