LM4912MM National Semiconductor, LM4912MM Datasheet - Page 13

• Manufacturer Part Number: LM4912MM
• Description: Stereo 40mW Low Noise Headphone Amplifier
• Manufacturer: National Semiconductor

Application Information
MUTE
When in C-CUPL mode, the LM4912 also features a mute
function that is independent of load impedance and enables
extremely fast turn-on/turn-off with a minimum of output pop
and click. The mute function leaves the outputs at their bias
level, thus resulting in higher power consumption than shut-
down mode, but also provides much faster turn on/off times.
Mute mode is enabled by providing a logic high signal on the
MUTE pin in the opposite manner as the shutdown function
described above. Threshold voltages and activation tech-
niques match those given for the shutdown function as well.
Additionally, Mute should not be enabled during shutdown or
while entering or returning from shutdown. This is not a valid
operation condition and may result in much higher pop and
click values.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4912 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4912 is unity-gain stable which gives the designer
maximum system flexibility. The LM4912 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
shown in Figures 2 and 3. The input coupling capacitor, C
forms a first order high pass filter which limits low frequency
response. This value should be chosen based on needed
frequency response and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying 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 C
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
i
and R
f
should be less than 1MΩ. Please refer to the
i
. A high value capacitor can
(Continued)
rms
i
. A larger input
are available
i
,
13
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of C
0.39µF), is recommended.
AUDIO POWER AMPLIFIER DESIGN
A 25mW/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.
3V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4912 to reproduce peak in excess of 25mW
without producing audible distortion. At this time, the de-
signer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required gain can be determined from Equation 2.
From Equation 4, the minimum A
the desired input impedance is 20kΩ, and with a A
1, a ratio of 1:1 results from Equation 1 for R
values are chosen with R
design step is to address the bandwidth requirements 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 which is better than the required
fied.
As stated in the External Components section, R
junction with C
The high frequency pole is determined by the product of the
desired frequency pole, f
an A
which is much smaller than the LM4912 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4912 can still
be used without running into bandwidth limitations.
Given:
Power Output
Load Impedance
Input Level
Input Impedance
f
f
C
L
H
i
= 100Hz/5 = 20Hz
V
= 20kHz * 5 = 100kHz
≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.
= 1 and f
i
creates a
H
= 100kHz, the resulting GBWP = 100kHz
H
i
, and the differential gain, A
= 20kΩ and R
i
V
(in the range of 0.1µF to
is 0.89; use A
f
= 20kΩ. The final
±
0.25dB speci-
V
f
www.national.com
25mWrms
to R
= 1. Since
V
i
1Vrms
gain of
in con-
V
20kΩ
i
. With
. The
32Ω
(3)