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LM4941 Scheda tecnica(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Il numero della parte LM4941
Spiegazioni elettronici  1.25 Watt Fully Differential Audio Power Amplifier With RF Suppression and Shutdown
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Produttore elettronici  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM4941 Scheda tecnica(HTML) 9 Page - National Semiconductor (TI)

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Application Information (Continued)
device as possible. Typical applications employ a 5V regu-
lator with 10µF and 0.1µF bypass capacitors that increase
supply stability. This, however, does not eliminate the need
for bypassing the supply nodes of the LM4941. The LM4941
will operate without the bypass capacitor C
B, although the
PSRR may decrease. A 1µF capacitor is recommended for
C
B. This value maximizes PSRR performance. Lesser val-
ues may be used, but PSRR decreases at frequencies below
1kHz. The issue of C
B selection is thus dependant upon
desired PSRR and click and pop performance as explained
in the section Proper Selection of External Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4941 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. The device may then be placed
into shutdown mode by toggling the Shutdown Select pin to
logic low. The trigger point for shutdown is shown as a typical
value in the Supply Current vs Shutdown Voltage graphs in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for maximum perfor-
mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current may
be greater than the 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 when optimizing
device and system performance. Although the LM4941 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4941 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4941 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize 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. Please refer to the
Audio Power Amplifier Design section for a more complete
explanation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4941
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
DD. Exact
allowable input common-mode voltage levels are actually a
function of V
DD,Ri, and Rf and may be determined by
Equation 5:
V
CMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri /2Rf)
(5)
-R
F /RI =AVD
(6)
When using DC coupled inputs, special care must be taken
to match the values of the input resistors (R
i1 and Ri2)to
each other. Because of the balanced nature of differential
amplifiers, resistor matching differences can result in net DC
currents across the load. This DC current can increase
power consumption, internal IC power dissipation, reduce
PSRR, and possibly damaging the loudspeaker. The chart
below demonstrates this problem by showing the effects of
differing values between the feedback resistors while as-
suming that the input resistors are perfectly matched. The
results below apply to the application circuit shown in Figure
1, and assumes that V
DD =5V, RL =8
Ω, and the system has
DC coupled inputs tied to ground.
Tolerance
R
i1
R
i2
V
02 -V01
I
LOAD
20%
0.8R
1.2R
-0.500V
62.5mA
10%
0.9R
1.1R
-0.250V
31.25mA
5%
0.95R 1.05R
-0.125V
15.63mA
1%
0.99R 1.01R
-0.025V
3.125mA
0%
R
R
0
0
Since the same variations can have a significant effect on
PSRR and CMRR performance, it is highly recommended
that the input resistors be matched to 1% tolerance or better
for best performance.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8
Ω Audio Amplifier
Given:
Power Output
1Wrms
Load Impedance
8
Input Level
1Vrms
Input Impedance
20k
Bandwidth
100Hz–20kHz ± 0.25dB
A designer must first determine the minimum supply rail to
obtain the specified output power. The supply rail can easily
be found by extrapolating from the Output Power vs Supply
Voltage graphs in the Typical Performance Characteris-
tics section. A second way to determine the minimum supply
rail is to calculate the required V
OPEAK using Equation 7 and
add the dropout voltages. Using this method, the minimum
supply voltage is (Vopeak + (V
DO TOP +(VDO BOT )), where
V
DO BOT
and V
DO TOP are extrapolated from the Dropout
Voltage vs Supply Voltage curve in the Typical Perfor-
mance Characteristics section.
(7)
Using the Output Power vs Supply Voltage graph for an 8
load, the minimum supply rail just about 5V. Extra supply
voltage creates headroom that allows the LM4941 to repro-
duce peaks in excess of 1W without producing audible dis-
tortion. At this time, the designer must make sure that the
power supply choice along with the output impedance does
not violate the conditions explained in the Power Dissipa-
tion section. Once the power dissipation equations have
been addressed, the required differential gain can be deter-
mined from Equation 8.
www.national.com
9


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