Datasheet

LM4876
SNAS054E –FEBRUARY 2000–REVISED MAY 2013 www.ti.com
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4876's performance requires properly selecting external components. Though the LM4876
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4876 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 Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input coupling capacitor (C
i
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
150 Hz. Applications using speakers with this limited low frequency response reap little improvement by using a
large input capacitor.
Besides affecting system cost and size, C
i
also affects the LM4876's click and pop performance. When the
supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero
to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor's size. Higher value
capacitors need more time to reach a quiescent DC voltage (usually V
CC
/2) when charged with a fixed current.
The amplifier's output charges the input capacitor through the feedback resistor, R
f
. 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 7.
f
-3dB
= 2πR
IN
C
I
(7)
As an example when using a speaker with a low frequency limit of 150Hz, Equation 7 gives a value of C
i
equal to
0.1µF. The 0.22µF C
i
shown in Figure 1 allows for a speaker whose response extends down to 75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of, C
B
, the capacitor
connected to the BYPASS pin. Since C
B
determines how fast the LM4876 settles to quiescent operation, its
value is critical when minimizing turn-on pops. The slower the LM4876'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 along with a small value of
C
i
(in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As discussed above,
choosing C
i
as small as possible helps minimize clicks and pops.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Power Output 1W
RMS
Load Impedance 8Ω
Input Level 1V
RMS
Input Impedance 20kΩ
Bandwidth 100Hz–20kHz ± 0.25dB
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