LM4991

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SNAS217A – MAY 2004 – REVISED APRIL 2013

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4991 contains a shutdown pin to externally turn off

the amplifier's bias circuitry. 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- supply. It is best to

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

VDD, the LM4991 supply current draw will be minimized in idle mode. While the device will be disabled with

shutdown pin voltages less then 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 definite voltage to avoid unwanted state changes.

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

provides 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 LM4991. 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 LM4991 is tolerant of external component combinations,

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

The LM4991 is unity-gain stable which gives a designer maximum system flexibility. The LM4991 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 3. 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 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

LM4991 turns on. The slower the LM4991'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 clickless and popless shutdown function. While the device will function

properly, (no oscillations or motorboating), 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.

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