LM4861

SNAS095C – MAY 1997 – REVISED MAY 2013

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POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply

rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as

possible. As displayed in the Typical Performance Characteristics, the effect of a larger half supply bypass

capacitor is improved low frequency THD+N due to increased half-supply stability. Typical applications employ a

5V regulator with 10

μF and a 0.1μF bypass capacitors which aid in supply stability, but do not eliminate the need

dependant upon desired low frequency THD+N, system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4861 contains a shutdown pin to externally turn off

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

shutdown pin. Upon going into shutdown, the output is immediately disconnected from the speaker. A typical

quiescent current of 0.6

μA results when the supply voltage is applied to the shutdown pin. 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 that when closed, is

connected to ground and enables the amplifier. If the switch is open, then a soft pull-up resistor of 47k

Ω will

disable the LM4861. There are no soft pull-down resistors inside the LM4861, so a definite shutdown pin voltage

must be applied externally, or the internal logic gate will be left floating which could disable the amplifier

unexpectedly.

HIGHER GAIN AUDIO AMPLIFIER

The LM4861 is unity-gain stable and requires no external components besides gain-setting resistors, an input

coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential

gain of greater than 10 is required, a feedback capacitor may be needed, as shown in Figure 3, to bandwidth

limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency

oscillations. Care should be taken when calculating the

VOICE-BAND AUDIO AMPLIFIER

Many applications, such as telephony, only require a voice-band frequency response. Such an application

usually requires a flat frequency response from 300Hz to 3.5kHz. By adjusting the component values of Figure 3,

Components Description .

(4)

Five times away from a

−3dB point is 0.17dB down from the flatband response. With this selection of

components, the resulting

frequency response of better than ±0.25dB with a rolloff of 6dB/octave outside of the passband. If a steeper

rolloff is required, other common bandpass filtering techniques can be used to achieve higher order filters.

SINGLE-ENDED AUDIO AMPLIFIER

Although the typical application for the LM4861 is a bridged monoaural amp, it can also be used to drive a load

single-endedly in applications, such as PC cards, which require that one side of the load is tied to ground.

coupled through a 470

μF capacitor, which blocks the half-supply DC bias that exists in all single-supply amplifier

0.1

it is good design practice to load the second output.

10

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