LM4954

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SNAS292B – JUNE 2005 – REVISED APRIL 2013

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4954 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 LM4954 supply current draw will be minimized in idle

mode. While the device will be disabled with shutdown pin voltages less than 0.4VDC, the idle current may be

greater than the typical value of 0.01µA. (Idle current is measured with the shutdown pin tied to ground). The

LM4954 has an internal 75k

Ω pull-down resistor. If the shutdown pin is left floating the IC will automatically enter

shutdown mode.

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 LM4954 is tolerant of external component combinations,

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

The LM4954 is unity-gain stable which gives the designer maximum system flexibility. The LM4954 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

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 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 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 affected by the size of the input coupling

capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally

1/2VDD). 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.

Choosing CB equal to 2.2µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), should produce a

virtually clickless and popless shutdown function. While the device will function properly, (no oscillations or

motorboating), with CB equal to 0.1µF.

AUDIO POWER AMPLIFIER DESIGN

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.

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 3 and Equation 4.

(3)

AVD = (Rf/Ri) 2

(4)

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