FAN7024 Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – 675mW CMOS Mono Power Amplifier with Shutdown

FAN7024
Power Dissipation
Power dissipation is a major concern when designing any power amplifier and must be thoroughly uderstood to ensure a suc-
cessful design. Equation (2) states the maximum power dissipation point for a bridged amplifier operating at a given supply
voltage and driving a specified output load.
P_DMAX
=
4 ⋅ ----^V----^D2---^D-----
2π²R_L
(2)
Since the FAN7024 is driving a bridged amplifier, the internal maximum power dissipation point of the FAN7024 results from
equation (2). Even with the large internal power dissipation, the FAN7024 does not require heat sinking over a wide range of
ambient temperature. From equation (2), assuming a 5V power supply and an 8Ω load, the maximum power dissipation point
is 633mW. The maximum power dissipation point obtained from equation (2) must not be greater than the power dissipation
that results from equation (3) :
P_DMAX
=
-⁽--^T----^J--^M----^A----^X----–-----^T----^A---⁾
R_thja
(3)
For package 8MSOP(FAN7024MU), Rthja=190°C/W, TJMAX=150°C for the FAN7024.
Depending on the ambient temperature, TA, of the system surroundings, equation (3) can be used to find the maximum internal
power dissipation supported by the IC packaging. If the result of equation (2) is greater than that of equation (3), then decrease
the supply voltage, increase the load impedance, or reduce the ambient temperature, TA. If these measures are insufficient, a
heat sink can be added to reduce TA. For the typical application of a 5V power supply, with 8Ω load, the maximum ambient
temperature possible without violating the maximum junction temperature is approximately 30°C provided that device opera-
tion is around the power dissipation point. Internal power dissipation is a function of output power and thus, if typical opera-
tion is not around the maximum power dissipation point, the ambient temperature can be increased. Refer to the Performance
Characteristics curves for power dissipation information for lower output powers.
Proper Selection of External Components
Selection of external components in applications using integrated power amplifiers is critical to optimize device and system
performance. While the FAN7024 is tolerant of external component combinations, consideration to component values must be
used to maximize overall system quality. The FAN7024 is unity-gain stable and this gives a designer maximum system flexi-
bility. The FAN7024 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. Besides gain, one of the major con-
siderations is the closed-loop bandwidth of the amplifier. 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 rea-
sons.
Selection of Capacitor Size
In the typical application, an input capacitor, CI, is required to allow the amplifier to bias the input signal to the proper DC
level for optimum operation. In this case, CI and RI form a high-pass filter with the corner frequency
f_c = ₂----_π----_R1----_I--_C----_I
(4)
The value of CI is important to consider, as it directly affects the bass(low frequency) performance of the circuit. Clearly a cer-
tain 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 using large input
capacitor may not increase systme performance. In addition to systme cost and size, click and pop performance is affected by
the size of the input coupling capacitor, CI. A larger input coupling capacitor requires more charge to reach its quiescent DC
voltage(normally VDD/2). This charge comes from the output via 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 sizes, 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 FAN7024 turns on.
The slower the FAN7024’s outputs ramp to their quiescent DC voltage(normally VDD/2), the smaller the turn-on pop. Thus
choosing CB equal to 1.0uF along with a small value of CI(in the range of 0.1uF to 0.39uF), should produce a clickless and
popless shutdown function. While the device will function properly, (no oscillations or motorboating), with CB equal to 0.1uF,
the device will be much more susceptible to turn-on clicks and pops. Thus, a value of CB equal to 1uF or larger is recom-
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