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CM8662 Datasheet(PDF) 6 Page - Bookly Micro electronic

No. de Pieza. CM8662
Descripción  1.7W CMOS AUDIO AMP
Descarga  8 Pages
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Fabricante  BOOKLY [Bookly Micro electronic]
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CM8662 Datasheet(HTML) 6 Page - Bookly Micro electronic

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Proper Selection of External Components
Proper selection of external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the CM8662 is tolerant of external
component combinations, consideration to component values
must be used to maximize overall system quality.
The CM8662 is unity-gain stable which gives a designer
maximum system flexibility. The CM8662 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 1Vrms are available
from sources such as audio codecs. Please refer to the
section, 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, Cj, 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 design. 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 100-150Hz. Thus using a large input
capacitors 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, Cj. A
larger input coupling capacitor requires more charge to reach
its quiescent DC voltage (nominally 1/2 VDD). 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, CB, is the most critical component to
minimize turn-on pops since it determines how fast the
CM8662 turns on. The slower the CM8662's, outputs ramp to
their quiescent DC voltage (nominally 1/2 VDD), the smaller the
turn-on pop. Choosing CB equal to 1.0μF along with a small
value of Cj (in the range of 0.1μF to 0.39μF), should produce
virtually clickless and popless shutdown function. While the
device will function properly, (no oscillations or motorboating),
with CB equal to 0.1μF, the device will be much more
susceptible to turn-on clicks and pops. Thus, a value of CB
equal to 0.1μF or larger is recommended in all but the most
cost sensitive designs.
Audio Power Amplifier Design
Design a 500mW/8Ω Audio Amplifier
Power Output
Load Impedance
Input Level
1 Vrms
Input Impedance
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical
Performance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum supply
rail is to calculate the required Vopeak using equation 3 and add
the dropout voltage. Using this method, the minimum supply
voltage would be (Vopeak + (2*VOD)), where VOD is extrapolated
from the Dropout Voltage vs Supply Voltage curve in the
Typical Performance Characteristics section.
Vopeak =
(2R 0
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 CM8662 to reproduce peaks in excess of 500mW
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 the
Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equation
(R 0
/(VIN) = Vorms/Vinrms
For Equation 4, the minimum AVD is 2; use AVD=2. Since the
desired input impedance was 20kΩ, and with a AVD of 2, a
ratio of 1:1 of Rf to Ri results in an allocation of Ri=Rf= 20kΩ.
The final design step is to address the bandwidth
requirements which must be stated as a pair of -3dB
frequency points. Five times away from a -3dB point is 0.17dB
down from passband response which is better than the
required ±0.25dB specified. This fact results in a low and high
frequency pole of 20Hz and 100kHz respectively. As stated in
the External Components section, Rj in conjunction with Cj
create a highpass filter.
Cj≧1/(2π*20 kΩ*20Hz)=0.397μF; use 0.39μF
The high frequency pole is determined by the product of the
desired high frequency pole, fH, and the differential gain, AVD.
With an AVD=2 and fH=100kHz, the resulting GBWP of
12.5MHz. This figure displays that if a designer has a need to
design an amplifier with a high differential gain, the CM8662
can still be used without running into bandwidth problems.

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