LM4911LDX National Semiconductor, LM4911LDX Datasheet - Page 19

• Manufacturer Part Number: LM4911LDX
• Description: IC AMP AUDIO PWR .145W AB 10LLP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4911LDX

• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 145mW x 2 @ 16 Ohm
• Voltage - Supply: 2 V ~ 5.5 V
• Features: Mute, Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 10-LLP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

cussed in greater detail in the Mute section as well as shown
in Figure 5.
MUTE
When in C-CUPL mode, the LM4911 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 consump-
tion (
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 signal
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 LM4911 is used to
drive high impedance loads. This is because the LM4911 re-
lies on a typical headphone load (16-32Ω) to reduce input
signal feedthrough through the input and feedback resistors.
Mute attenuation can thus be calculated by the following for-
mula:
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 LM4911 is op-
erating in OCL mode since the shutdown function operates
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 Figure 5. X1 occurs immediately following a return from
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 Figure 5. Mute should not be toggled during these time pe-
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 LM4911 is tolerant of external
component combinations, consideration to component values
must be used to maximize overall system quality.
The LM4911 is unity-gain stable which gives the designer
maximum system flexibility. The LM4911 should be used in
low gain configurations to minimize THD+N values, and max-
imize the signal to noise ratio. Low gain configurations require
large input signals to obtain a given output power. Input sig-
nals equal to or greater than 1V
such as audio codecs. Very large values should not be used
for the gain-setting resistors. Values for R
less than 1MΩ. Please refer to the section, Audio Power
Amplifier Design, for a more complete explanation of proper
gain selection
Mute Attenuation (dB) = 20Log(R
≤
occurred
100µA). The mute function leaves the outputs at their
B
WU
for OCL) and lasts for 100ms±25%. The timing
) and lasts 40ms±25%. X2 occurs after the
(2.2*400k*C
B
rms
are available from sources
for
L
/ (R
i
cap-coupled
+R
i
and R
F
)
f
should be
and
19
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 Figure 2 and Figure 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 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 LM4911 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 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
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
f
L
H
= 100Hz/5 = 20Hz
= 20kHz * 5 = 100kHz
i
= 20kΩ and R
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
,