LM4811LDBD National Semiconductor, LM4811LDBD Datasheet - Page 14

• Manufacturer Part Number: LM4811LDBD
• Description: BOARD EVALUATION LM4811LD
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4811LDBD

• Amplifier Type: Class AB
• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 105mW x 2 @ 16 Ohm
• Voltage - Supply: 2 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4811
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
logic high level, and 0.4V maximum for a logic low level. It is
best to switch between ground and V
shutdown operation. By switching the SHUTDOWN pin to
V
idle mode. Whereas the device will be disabled with shut-
down voltages less than V
than the typical value of 0.3µA. In either case, the SHUT-
DOWN pin should be tied to a fixed voltage to avoid un-
wanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry. This provides
a quick, smooth shutdown transition. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the
SHUTDOWN pin is connected to ground and enables the
amplifier. If the switch is open, the external pull-up resistor,
R
the SHUTDOWN pin will not float, thus preventing unwanted
state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated
power amplifiers is critical for optimum device and system
performance. While the LM4811 is tolerant of external com-
ponent combinations, consideration must be given to the
external component values that maximize overall system
quality.
The LM4811’s unity-gain stability allows a designer to maxi-
mize system performance. Low gain settings maximize
signal-to-noise performance and minimizes THD+N. Low
gain configurations require large input signals to obtain a
given output power. Input signals equal to or greater than 1
Vrms are available from sources such as audio codecs.
Please refer to the section, Audio Power Amplifier Design,
for a more complete explanation of proper gain selection.
Selection of Input and Output Capacitor Size
Besides gain, one of the major considerations is the closed
loop bandwidth of the amplifier. To a large extent, the band-
width is dicated by the choice of external components shown
in Figure 1. Both the input coupling capacitor, C
output coupling capacitor, C
filters which limit low frequency response. These values
should be based on the desired frequency response
weighed against the following:
Large value input and output capacitors are both expensive
and space consuming for portable designs. Clearly a certain
sized capacitor is needed to couple in low frequencies with-
out severe attenuation. But in many cases the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Thus large
input and output capacitors may not increase system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is affected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (nominally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Turn on pops can be
minimized by reducing C
frequency response.
Besides minimizing the input and output capacitor values,
careful consideration should be paid to the bypass capacitor
value. Bypass capacitor C
DD
PU
i
. A larger input coupling capacitor requires more charge to
, the LM4811 supply current draw will be minimized in
, will disable the LM4811. This scheme guarantees that
DD
B
i
value based on necessary low
is the most critical component to
, the idle current may be greater
o
, form first order high pass
DD
to ensure optimal
(Continued)
i
, and the
DD
). This
14
minimize turn on pops since it determines how fast the
LM4811 turns on. The slower the LM4811’s outputs ramp to
their quiescent DC voltage (nominally 1/2 V
the turn on pop. While the device will function properly, (no
oscillations or motorboating), with C
vice will be much more susceptible to turn on clicks and
pops. Thus, a value of C
mended in all but the most cost sensitive designs.
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32Ω Audio Amplifier
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. A second way to determine the minimum sup-
ply rail is to calculate the required V
and add the dropout voltage. For a single-ended application,
the minimum supply voltage can be approximated by
(2V
are extrapolated from the Dropout Voltage vs Supply Voltage
curve in the Typical Performance Characteristics section.
Using the Output Power vs Supply Voltage graph for a 32Ω
load, the minimum supply rail is 4.8V. Since 5V is a standard
supply voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4811 to reproduce peaks in excess of 70mW
without clipping the signal. 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 Dissipation section. Remember that the maxi-
mum power dissipation point from Equation 1 must be mul-
tiplied by two since there are two independent amplifiers
inside the package.
The final design step is to address the bandwidth require-
ments 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 assuming a single pole roll-off. As
stated in the External Components section, C
create first order highpass filters. Thus to obtain the desired
frequency low response of 100Hz within
must be taken into consideration. The combination of two
single order filters at the same frequency forms a second
order response. This results in a signal which is down
0.34dB at five times away from the single order filter −3dB
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
OPEAK
+ (V
OD TOP
+ V
OD BOT
B
equal to 1µF or larger is recom-
)), where V
100 Hz–20 kHz
OPEAK
B
equal to 1µF, the de-
±
using Equation (3)
OD BOT
0.5dB, both poles
DD
1Vrms (max)
), the smaller
33kΩ (min)
and V
±
i
0.50dB
and C
70mW
OD TOP
32Ω
(3)
o