Datasheet

LM4881
SNAS001D –SEPTEMBER 1997–REVISED MAY 2013 www.ti.com
The LM4881 is unity gain stable and this gives a designer maximum system flexibility. The LM4881 should be
used in low gain configurations to minimize THD+N values, and maximum 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 1
Vrms are available from sources such as audio codecs. 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 dicated by the choice of external components shown in Figure 1. Both the input coupling capacitor,
C
i
, and the output coupling capacitor, C
o
, form first order high pass filters which limit low frequency response.
These values should be chosen based on needed frequency response for a few distinct reasons.
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and space hungry for portable designs. Clearly a certain
sized capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the
speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150
Hz. Thus using large input and output capacitors may not increase system performance.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling
capacitor, C
i
. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally
1/2 V
DD
). This charge comes from the output via the feedback and is apt to create pops upon device enable.
Thus, by minimizing the capacitor size based on necessary low frequency response, turn on pops can be
minimized.
Besides minimizing the input and output capacitor sizes, careful consideration should be paid to the bypass
capacitor value. Bypass capacitor C
B
is the most critical component to minimize turn on pops since it determines
how fast the LM4881 turns on. The slower the LM4881's outputs ramp to their quiescent DC voltage (nominally
1/2 V
DD
), the smaller the turn on pop. Thus choosing C
B
equal to 1.0 µF along with a small value of C
i
(in the
range of 0.1 µF to 0.39 µF), the shutdown function should be virtually clickless and popless. While the device will
function properly, (no oscillations or motorboating), with C
B
equal to 0.1 µF, the device will be much more
susceptible to turn on clicks and pops. Thus, a value of C
B
equal to 0.1 µF or larger is recommended in all but
the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 200mW/8Ω Audio Amplifier
Given:
Power Output 200 mWrms
Load Impedance 8Ω
Input Level 1 Vrms (max)
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz ± 0.50 dB
A designer must first determine the needed supply rail to obtain the specified output power. Calculating the
required supply rail involves knowing two parameters, V
OPEAK
and also the dropout voltage. The latter is typically
530 mV and can be found from the graphs in the Typical Performance Characteristics. V
OPEAK
can be determined
from Equation 3.
(3)
For 200 mW of output power into an 8Ω load, the required V
OPEAK
is 1.79 volts. A minimum supply rail of 2.32V
results from adding V
OPEAK
and V
OD
. Since 5V is a standard supply voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that allows the LM4881 to reproduce peaks in excess of
200 mW without clipping the signal. 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.
Remember that the maximum power dissipation point from Equation 1 must be multiplied by two since there are
two independent amplifiers inside the package.
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