LM4879, LM4879MMBD, LM4879SDBD

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SNAS142G – SEPTEMBER 2001 – REVISED MAY 2013

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4879 contains a shutdown pin to externally turn off

the amplifier's bias circuitry. This shutdown feature turns the amplifier off when a logic low is placed on the

shutdown pin. By switching the shutdown pin to ground, the LM4879 supply current draw will be minimized in idle

greater than the typical value of 0.1µA. (Idle current is measured with the shutdown pin tied to ground).

In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to

provide 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

LM4879. This scheme ensures 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 LM4879 is tolerant of external component combinations,

consideration to component values must be used to maximize overall system quality.

The LM4879 is unity-gain stable which gives the designer maximum system flexibility. The LM4879 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 1

Vrms 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

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 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 100 Hz to

150 Hz. 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

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.

0.39 µF), should produce a virtually clickless and popless shutdown function. While the device will function

designs.

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