LM4910MA National Semiconductor, LM4910MA Datasheet - Page 11

• Manufacturer Part Number: LM4910MA
• Description: Output Capacitor-less Stereo 35mW Headphone Amplifier
• Manufacturer: National Semiconductor

Application Information
headphone operation (32Ω impedance) using a 3.3V supply
the maximum power dissipation is only 138mW. Therefore,
power dissipation is not a major concern.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible.
Typical applications employ a 3.3V regulator with 10µF tan-
talum or electrolytic capacitor and a ceramic bypass capaci-
tor which aid in supply stability. This does not eliminate the
need for bypassing the supply nodes of the LM4910. A
bypass capacitor value in the range of 0.1µF to 1µF is
recommended for C
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4910’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4910’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 volt-
age that is higher than ground may increase the shutdown
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 LM4910 is tolerant of exter-
nal component combinations, consideration to component
values must be used to maximize overall system quality.
The LM4910 is unity-gain stable which gives the designer
maximum system flexibility. The LM4910 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. To a large extent, the band-
width is dictated by the choice of external components
i
and R
f
should be less than 1MΩ. Please refer to the
S
.
DD
. Connect the switch between the
(Continued)
rms
are available
11
shown in Figure 1. The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. 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.
USING EXTERNAL POWERED SPEAKERS
The LM4910 is designed specifically for headphone opera-
tion. Often the headphone output of a device will be used to
drive external powered speakers. The LM4910 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 LM4910 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.
Given:
Power Output
Load Impedance
Input Level
Input Impedance
i
. A high value capacitor can
www.national.com
30mWrms
i
, forms a
1Vrms
20kΩ
32Ω
(2)
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