graphs in the Typical Performance Characteristics section. It is best to switch between ground and supply for maximum
performance. While the device may be disabled with shutdown voltages in between ground and supply, the idle current may be
greater than the typical value of 0.1μA. In either case, the shutdown pin should be tied to a definite voltage to avoid unwanted
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry, which provides a
quick, smooth transition to shutdown. Another solution is to use a single-throw switch in conjunction with an external pull-up
resistor (or pull-down, depending on shutdown high or low application). This scheme guarantees that the shutdown pin will not
float, thus preventing unwanted state changes.
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 CM8600 is tolerant of external component combinations, consideration to component values
must be used to maximize overall system quality.
The CM8600 is unity-gain stable which gives the designer maximum system flexibility. The CM8600 should be used in low
gain configurations to minimize THD+N+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 codec’s. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain
Besides gain, one of the major considerations is the closed loop bandwidth of the amplifier. To a large extent, the bandwidth is
dictated by the choice of external components shown in Figure 1. The input coupling capacitor, Ci, 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
Selection of Input Capacitor Size
Large input capacitors are both expensive and space hungry 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 100Hz to 150Hz. Thus, using a large input capacitor may not increase
actual system performance.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling capacitor, Ci. 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 CM8600 turns on. The
slower the CM8600’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 Ci (in the range of 0.1μF to 0.39μF), should produce a virtually click less and
peoples shutdown function. While the device will function properly, (no oscillations or motor boating), 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 1.0μF is recommended in all
but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
100Hz–20kHz ± 0.25dB
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.
5V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows
the CM8600 to reproduce peaks in excess of 1W without producing audible distortion. 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 2.
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