LM4862M Datasheet, PDF (10/18 Pages) Texas Instruments – LM4862 675 mW Audio Power Amplifier with Shutdown

LM4862
SNAS102F – MAY 1997 – REVISED MAY 2013
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The LM4862 is unity-gain stable which gives a designer maximum system flexibility. The LM4862 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 1
Vrms are available from sources such as audio codecs. Please refer to 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
band-width is dictated by the choice of external components shown in Figure 1. 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 for a few distinct reasons.
Selection of Input Capacitor Size
Large input 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 100–150 Hz.
Thus using a large input capacitor 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
½ 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 capacitor size, 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
LM4862 turns on. The slower the LM4862's outputs ramp to their quiescent DC voltage (nominally ½ V_DD), the
smaller the turn-on pop. 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), should produce a virtually clickless and popless shutdown function. 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 1.0 μF or larger is recommended in all but the most cost
sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a 500 mW/8Ω Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
500 mWrms
8Ω
1 Vrms
20 kΩ
100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from Figure 10, Figure 11, and Figure 12 in Typical Performance Characteristics, the supply rail can be easily
found. A second way to determine the minimum supply rail is to calculate the required V_opeak using Equation 4
and add the dropout voltage. Using this method, the minimum supply voltage would be (V_opeak + (2*V_OD)), where
V_OD is extrapolated from the Figure 16 in Typical Performance Characteristics.
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Using the Output Power vs Supply Voltage graph for an 8Ω load, the minimum supply rail is 4.3V. But 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 LM4862 to reproduce peaks in excess of 500 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 POWER DISSIPATION.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation 5.
R_f/R_i = A_VD/2
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