LM4880

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SNAS103C – NOVEMBER 1995 – REVISED MAY 2013

AUTOMATIC SHUTDOWN CIRCUIT

As shown in Figure 3, the LM4880 can be set up to automatically shutdown when a load is not connected. This

circuit is based upon a single control pin common in many headphone jacks. This control pin forms a normally

closed switch with one of the output pins. The output of this circuit (the voltage on pin 5 of the LM4880) has two

states based on the state of the switch. When the switch is open, signifying that headphones are inserted, the

LM4880 should be enabled. When the switch is closed, the LM4880 should be off to minimize power

consumption.

shutdown pin of the LM4880 to the supply voltage. This places the LM4880 in shutdown mode which reduces the

supply current to 0.7

the gate of the NMOS high which turns on the inverter and produces a logic low signal on the shutdown pin of

the LM4880. This state enables the LM4880 and places the amplifier in its normal mode of operation.

This type of circuit is clearly valuable in portable products where battery life is critical, but is also beneficial for

power conscious designs such as “Green PC's”.

AUTOMATIC SWITCHING CIRCUIT

A circuit closely related to Automatic Shutdown Circuit is Automatic Switching Circuit. Automatic Switching Circuit

utilizes both the input and output of the NMOS inverter to toggle the states of two different audio power

amplifiers. The LM4880 is used to drive stereo single ended loads, while the LM4861 drives bridged internal

speakers.

In this application, the LM4880 and LM4861 are never on at the same time. When the switch inside the

headphone jack is open, the LM4880 is enabled and the LM4861 is disabled since the NMOS inverter is on. If a

headphone jack is not present, it is assumed that the internal speakers should be on and thus the voltage on the

LM4861 shutdown pin is low and the voltage at the LM4880 pin is high. This results in the LM4880 being

shutdown and the LM4861 being enabled.

Only one channel of this circuit is shown in Figure 4 to keep the drawing simple but the typical application would

a LM4880 driving a stereo external headphone jack and two LM4861's driving the internal stereo speakers. If

only one internal speaker is required, a single LM4861 can be used as a summer to mix the left and right inputs

into a single mono channel.

PROPER SELECTION OF EXTERNAL COMPONENTS

Selection of external components when using integrated power amplifiers is critical to optimize device and

system performance. While the LM4880 is tolerant of external component combinations, care must be exercised

when choosing component values.

The LM4880 is unity-gain stable which gives a designer maximum system flexibility. The LM4880 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 AUDIO POWER AMPLIFIER DESIGN for

a more complete explanation of proper gain selection.

Besides gain, one of the major design 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 2. Both the input

frequency response. These values should be chosen based on needed frequency response for a few distinct

reasons.

Selection of Input and Output Capacitor Size

Large input and output 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

transducers used in portable systems, whether internal or external, have little ability to reproduce signals below

100 Hz–150 Hz. Thus using large input and output capacitors may not increase system performance.

Copyright © 1995–2013, Texas Instruments Incorporated

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Product Folder Links: LM4880