lm4911qmm National Semiconductor Corporation, lm4911qmm Datasheet - Page 19

• Manufacturer Part Number: lm4911qmm
• Description: Stereo Headphone Amplifier For Automotive
• Manufacturer: National Semiconductor Corporation

period. These periods are shown as X1 and X2 and are dis-
cussed in greater detail in the Mute section as well as shown
in
MUTE
When in C-CUPL mode, the LM4911QMM also features a
mute function that enables extremely fast turn-on/turn-off with
a minimum of output pop and click with a low current con-
sumption (
their bias level, thus resulting in higher power consumption
than shutdown mode, but also provides much faster turn on/
off times. Mute mode is enabled by providing a logic high sig-
nal on the MUTE pin in the opposite manner as the shutdown
function described above. Threshold voltages and activation
techniques match those given for the shutdown function as
well.
Mute may not appear to function when the LM4911QMM is
used to drive high impedance loads. This is because the
LM4911QMM relies on a typical headphone load (16-32Ω) to
reduce input signal feedthrough through the input and feed-
back resistors. Mute attenuation can thus be calculated by the
following formula:
Parallel load resistance may be necessary to achieve satis-
factory Mute levels when the application load is known to be
high impedance.
The mute function is not necessary when the LM4911QMM
is operating in OCL mode since the shutdown function oper-
ates quickly in OCL mode with less power consumption than
mute.
Mute may be enabled during shutdown transitions, but should
not be toggled for a brief period immediately after exiting or
entering shutdown. These brief time periods are labeled X1
(time after returning from shutdown) and X2 (time after en-
tering shutdown) and are shown in the timing diagram given
in
shutdown (T
part is placed in shutdown and the decay of the bias voltage
has
2.2*100k*C
of these transition periods relative to X1 and X2 is also shown
in
riods, but may be made during the shutdown transitions or
any other time the part is in normal operation (while in cap-
coupled mode - Mute is not valid in OCL mode). Failure to
operate mute correctly may result in much higher click and
pop values or failure of the device to mute at all.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the LM4911QMM is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system quality.
The LM4911QMM is unity-gain stable which gives the de-
signer maximum system flexibility. The LM4911QMM should
be used in low gain configurations to minimize THD+N values,
and maximize the signal to noise ratio. Low gain configura-
tions require large input signals to obtain a given output
power. Input signals equal to or greater than 1V
able from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
R
i
Mute Attenuation (dB) = 20Log(R
Figure
Figure
and R
Figure
occurred
f
should be less than 1MΩ. Please refer to the section,
5. Mute should not be toggled during these time pe-
5.
5. X1 occurs immediately following a return from
≤
B
100µA). The mute function leaves the outputs at
WU
for OCL) and lasts for 100ms±25%. The timing
) and lasts 40ms±25%. X2 occurs after the
(2.2*400k*C
B
for
L
/ (R
i
cap-coupled
+R
F
)
rms
are avail-
and
19
Audio Power Amplifier Design, for a more complete expla-
nation 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
forms a first order high pass filter which limits low frequency
response. This value should be chosen based on needed fre-
quency response and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high value
input coupling capacitor, C
expensive and may compromise space efficiency in portable
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 fre-
quency 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
pling capacitor requires more charge to reach its quiescent
DC voltage. This charge comes from the output via the feed-
back Thus, by minimizing the capacitor size based on nec-
essary low frequency response, turn-on time can be mini-
mized. A small value of C
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 LM4911QMM to reproduce peak in excess of
25mW without producing audible distortion. At this time, the
designer must make sure that the power supply choice along
with the output impedance does not violate the conditions ex-
plained 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
a ratio of 1:1 results from Equation 1 for R
are chosen with R
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 ± 0.25dB specified.
Given:
Power Output
Load Impedance
Input Level
Input Impedance
f
Figure 2
L
= 100Hz/5 = 20Hz
and
Figure
i
= 20kΩ and R
3. The input coupling capacitor, C
i
(in the range of 0.1µF to 0.39µF),
i
. A high value capacitor can be
V
f
is 0.89; use A
= 20kΩ. The final design
i
. A larger input cou-
f
to R
i
. The values
www.national.com
V
25mWrms
V
= 1. Since
gain of 1,
1Vrms
20kΩ
32Ω
(4)
i
,