LM4811
Preliminary
CMOS IC
APPLICATION INFORMATION(Cont.)
PROPER SELECTION OF EXTERNAL COMPONENTS
In applications, selection of external components when using integrated power amplifiers is critical for optimum
device and system performance. Moreover, the vales of external components are also important to consider as they
directly affect the overall system quality. The UTC
LM4811
is tolerant to a variety external component combinations.
The UTC
LM4811
is unity-gain stable, providing designers 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 reach 1Vrms or higher are available from sources such as audio codecs.
SELECTION OF INPUT AND OUTPUT CAPACITOR SIZE
Another consideration is the closed loop bandwidth besides gain. To a large extent, the bandwidth is dicated by
the choice of external components shown in Figure 1. Both the C
I
(input coupling capacitor) and the C
O
(output
coupling capacitor) form first order high pass filters which limit low frequency response. The values of these
capacitors 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.
Moreover, for the system cost and size, click and pop performance is affected by the size of the C
I
(input
coupling capacitor). 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 the pops are apt to be generated upon
device enable. Reducing the value of input coupling capacitor can minimize the turn on pops, which is based on
necessary low frequency response.
The value of bypass capacitor should be taken into consideration besides minimizing the input and output
capacitor values. For minimizing turn on pops, the bypass capacitor C
B
plays the most important role, since it
determines how fast the UTC
LM4811
turns on. The slower the UTC
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.
In addition, the type of capacitors should be given enough consideration since different types of capacitors
(tantalum, electrolytic, ceramic) have unique performance characteristics and may affect overall system
performance.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
7 of 9
QW-R502-560.a
UTC-IC [ UNISONIC TECHNOLOGIES ]
