Datasheet

LM4808
SNAS051D –FEBRUARY 2000–REVISED MAY 2013 www.ti.com
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4808's performance requires properly selecting external components. Though the LM4808
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4808 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more
than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-
noise ratio. These parameters are compromised as the closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources
such as audio CODECs have outputs of 1V
RMS
(2.83V
P-P
). Please refer to the AUDIO POWER AMPLIFIER
DESIGN section for more information on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input and output coupling capacitors (C
I
and C
O
in
Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In
many cases, however, the speakers used in portable systems, whether internal or external, have little ability to
reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little
improvement by using high value input and output capacitors.
Besides affecting system cost and size, C
i
has an effect on the LM4808's click and pop performance. The
magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by
selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency.
As shown in Figure 1, the input resistor, R
I
and the input capacitor, C
I
, produce a −3dB high pass filter cutoff
frequency that is found using Equation 3. In addition, the output load R
L
, and the output capacitor C
O
, produce a
-3db high pass filter cutoff frequency defined by Equation 4.
f
I-3db
=1/2πR
I
C
I
(3)
f
O-3db
=1/2πR
L
C
O
(4)
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.
Bypass Capacitor Value
Besides minimizing the input capacitor size, careful consideration should be paid to the value of the bypass
capacitor, C
B
. Since C
B
determines how fast the LM4808 settles to quiescent operation, its value is critical when
minimizing turn-on pops. The slower the LM4808's outputs ramp to their quiescent DC voltage (nominally 1/2
V
DD
), the smaller the turn-on pop. Choosing C
B
equal to 1.0µF or larger, will minimize turn-on pops. As discussed
above, choosing C
i
no larger than necessary for the desired bandwidth helps minimize clicks and pops.
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