Datasheet

LM4811
SNAS119D –DECEMBER 2000–REVISED APRIL 2013 www.ti.com
The LM4811's unity-gain stability allows a designer to maximize system performance. Low gain settings
maximize signal-to-noise performance and minimizes THD+N. 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.
Selection of Input and Output Capacitor Size
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 4. 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 based on the desired frequency response weighed against the following:
Large value input and output capacitors are both expensive and space consuming 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
150Hz. Thus large input and output capacitors may not increase system performance.
In addition to system cost and size, click and pop performance 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 (nominally
1/2 V
DD
). This charge comes from the output via the feedback and is apt to create pops upon device enable.
Turn on pops can be minimized by reducing C
i
value based on necessary low frequency response.
Besides minimizing the input and output capacitor values, 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 LM4811 turns on. The slower the LM4811's outputs ramp to their quiescent DC voltage (nominally
1/2 V
DD
), the smaller the turn on pop. While the device will function properly, (no oscillations or motorboating),
with C
B
equal to 1µF, the device will be much more susceptible to turn on clicks and pops. Thus, a value of C
B
equal to 1µF or larger is recommended in all but the most cost sensitive designs.
Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system.
Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may
affect overall system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32Ω Audio Amplifier
Given:
Power Output 70mW
Load Impedance 32Ω
Input Level 1Vrms (max)
Input Impedance 33kΩ (min)
Bandwidth 100 Hz–20 kHz ± 0.50dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from Figure 26 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 3 and add the dropout
voltage. For a single-ended application, the minimum supply voltage can be approximated by (2V
OPEAK
+ (V
ODTOP
+ V
ODBOT
)), where V
ODBOT
and V
ODTOP
are extrapolated from Figure 29 in Typical Performance Characteristics.
(3)
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