lm4924sdx National Semiconductor Corporation, lm4924sdx Datasheet - Page 11

• Manufacturer Part Number: lm4924sdx
• Description: 2 Cell Battery, 40mw Per Channel Output Capacitor-less Ocl Stereo Headphone Audio Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
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 V
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 1V
from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
R
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,
C
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, C
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.
i
i
, forms a first order high pass filter which limits low fre-
and R
f
should be less than 1MΩ. Please refer to the
DD
. Connect the switch between the
i
. A high value capacitor can
(Continued)
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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 V
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
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.
From Equation 2, the minimum A
the desired input impedance is 20kΩ, and with A
a ratio of 1:1 results from Equation 1 for R
are chosen with R
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired
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
desired limit. The results are an
and an
Given:
Power Output
Load Impedance
Input Level
Input Impedance
f
H
i
f
= 20kΩ and R
= 20kHz x 5 = 100kHz
L
= 100Hz/5 = 20Hz
V
f
is 0.98; use A
= 20kΩ.
f
to R
i
V
. The values
V
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30mWrms
equal to 1,
= 1. Since
±
±
1Vrms
0.25dB
0.25dB
20kΩ
32Ω
(2)
(3)
(4)
O3