LM4901

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SNAS139F – DECEMBER 2001 – REVISED DECEMBER 2001

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4901 contains shutdown circuitry that is used to

turn off the amplifier's bias circuitry. In addition, the LM4901 contains a Shutdown Mode pin, allowing the

designer to designate whether the part will be driven into shutdown with a high level logic signal or a low level

logic signal. This allows the designer maximum flexibility in device use, as the Shutdown Mode pin may simply

low" device, respectively. The device may then be placed into shutdown mode by toggling the Shutdown pin to

the same state as the Shutdown Mode pin. For simplicity's sake, this is called "shutdown same", as the LM4901

enters shutdown mode whenever the two pins are in the same logic state. The trigger point for either shutdown

high or shutdown low is shown as a typical value in the Supply Current vs Shutdown Voltage graphs in the

Typical Performance Characteristics section. It is best to switch between ground and supply for maximum

performance. While the device may be disabled with shutdown voltages in between ground and supply, 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 to shutdown. Another solution is to use a single-throw switch in conjunction

with an external pull-up resistor (or pull-down, depending on shutdown high or low application). 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 LM4901 is tolerant of external component combinations,

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

The LM4901 is unity-gain stable which gives the designer maximum system flexibility. The LM4901 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.

Copyright © 2001, Texas Instruments Incorporated

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