FAN7005M 查看數據表（PDF） - Fairchild Semiconductor

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FAN7005M

200mW Stereo Power Amplifier with Shutdown

Fairchild Semiconductor 

FAN7005

Application Informations

Power Supply Bypassing

Selection of proper power supply bypassing is critical to obtaining lower noise as well as higher power supply rejection.

Capacitors of the largest possible size may help to increase immunity to supply noise. However, taking into account

economical design, attaching 10µF electrolytic capacitor or tantalum capacitor with 0.1µF ceramic capacitor as closely as

possible to the VDD pin is sufficient to obtain a good supply noise rejection.

Single Ended Mode of Operation

The FAN7005 offers SE(Single Ended) operation. SE mode is adequate for head-phone load. The output power of SE mode is

expressed as follows :

PSE = 8---V--⋅---R-P2----L--

(1)

To use the amplifier in SE mode, the output DC voltage must be blocked not to increase power consumption. Thus, the load is

tied to the output via the output DC blocking capacitor. Capacitor size can be chosen using above f-3dB equation. For example,

assuming the load impedance is 32Ω, a 248.8µF capacitor guarantees 20Hz signal transmission to the load without gain

reduction. Refer to the Typical Performance Characteristics curves.

Shutdown Function

In order to reduce power consumption while not in use, the FAN7005 contains a shutdown pin(pin#3 @8MSOP) to turn off the

amplifier’s bias circuitry externally. This shutdown feature turns the amplifier off when a logic high is placed on the

shutdown pin. The trigger point between a logic low and logic high level is typically half the supply voltage. It is best to switch

between ground and supply to provide maximum device performance. By switching the shutdown pin to the VDD, the

FAN7005’s supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages

less than VDD, the idle current may be greater than the typical value of 0.1µA. In either case, the shutdown pin should be tied

to a defined voltage because leaving the shutdown pin floating may result in an unwanted shutdown. In many applications, a

micro controller or microprocessor output is used to control the shutdown circuitry, providing a quick, smooth transition into

shutdown. Another solution is to use a single pole, single throw switch in conjunction with an external pull up resistor.

When the switch is closed, the shutdown pin is connected to ground and enables the amplifier. If the switch is open, then the

external pull up resistor will disable the FAN7005. This scheme guarantees that the shutdown pin will not float, which will

prevent unwanted state changes.

Adaptive Q-Current Control Circuit

Among the different several kinds of analog amplifier, a class-AB satisfies moderate total harmonic distortion(THD) and

power efficiency. In general, distortion proportionally reduces to the quiescent current(Q-current) of the output stage, but

power efficiency is inversely proportional to it. To satisfy both needs, an adaptive Q-current control(AQC) technique is

proposed. The AQC circuit increases the Q-current with respect to the amount of output distortion, whereas it is not activated

when no input signal is applied and no output distortion is sensed.

Power Dissipation

Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a success-

ful design. Equation 2 states the maximum power dissipation point for a single-ended amplifier operating at a given supply

voltage and driving a specified output load.

PDMAX

=

-------V-----D2----D----------

2 ⋅ π2 ⋅ RL

(2)

Since the FAN7005 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that

of the number which results from equation(2). Even with a large internal power dissipation, the FAN7005 does not require a

heatsink over a large range of ambient temperature. From equation(2), assuming a 5V power supply and an 8Ω load, the max-

imum power dissipation point is 158.8mW per amplifier. Thus the maximum package dissipation point is 316.6mW. The max-

imum power dissipation point obtained must not be greater than the power dissipation that results from equation(3) :

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