Datasheet

LM4911, LM4911Q
www.ti.com SNAS152M –MAY 2004–REVISED JUNE 2011
OCL) and lasts for 100ms±25%. The timing of these transition periods relative to X1 and X2 is also shown in
Figure 63. Mute should not be toggled during these time periods, 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/LM4911Q is tolerant of external component combinations,
consideration to component values must be used to maximize overall system quality.
The LM4911/LM4911Q is unity-gain stable which gives the designer maximum system flexibility. The
LM4911/LM4911Q 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
rms
are available from sources such as audio codecs. Very large values should
not be used for the gain-setting resistors. Values for R
i
and R
f
should be less than 1MΩ. Please refer to the
section, Audio Power Amplifier Design, for a more complete 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
bandwidth is dictated by the choice of external components shown in Figure 2 and Figure 3. The input coupling
capacitor, C
i
, 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
i
. A high value capacitor
can be 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 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
i
. A larger
input coupling capacitor requires more charge to reach its quiescent 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 C
i
(in the range of 0.1µF to 0.39µF), is recommended.
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32Ω AUDIO AMPLIFIER
Given:
Power Output 25mWrms
Load Impedance 32Ω
Input Level 1Vrms
Input Impedance 20kΩ
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 Performance 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/LM4911Q 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 1.
(4)
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