The operation of this circuit is rather simple. With the switch

closed, R

gate voltage of less than 5 mV. This gate voltage keeps the

NMOS inverter off and R

LM4880 to the supply voltage. This places the LM4880 in

shutdown mode which reduces the supply current to 0.7 µA

typically. When the switch is open, the opposite condition is

produced. Resistor R

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 op-

eration.

This type of circuit is clearly valuable in portable products

where battery life is critical, but is also benefical for power

conscious designs such as “Green PC’s”.

AUTOMATIC SWITCHING CIRCUIT

A circuit closely related to the Automatic Shutdown Circuit

is the Automatic Switching Circuit of

Figure 3. The Auto-

matic 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 3 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 in-

ternal 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 com-

ponent combinations, care must be exercised when choos-

ing 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. In-

put signals equal to or greater than 1 Vrms are available

from sources such as audio codecs. Please refer to the sec-

tion, 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 1. Both the input coupling capacitor, C

the output coupling capacitor, C

filters which limit low 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 se-

vere 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 us-

ing large input and output capacitors may not increase sys-

tem performance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

C

reach its quiescent DC voltage (normally 1/2 V

charge comes from the output via the feedback and is apt to

create pops upon device enable. Thus, by minimizing the ca-

pacitor size based on necessary low frequency response,

turn-on pops can be minimized.

Besides minimizing the input and output capacitor sizes,

careful consideration should be paid to the bypass capacitor

size. The bypass capacitor, C

nent to minimize turn-on pops since it determines how fast

the LM4880 turns on. The slower the LM4880’s outputs

ramp to their quiescent DC voltage (nominally 1/2 V

smaller the turn-on pop. Choosing C

with a small value of C

should produce a virtually clickless and popless shutdown

function. While the device will function properly, (no oscilla-

tions or motorboating), with C

will be much more susceptible to turn-on clicks and pops.

Thus, a value of C

mended in all but the most cost sensitive designs.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 200 mW/8

Ω Audio Amplifier

Given:

Power Output:

200 mWrms

Load Impedance:

8

Ω

Input Level:

1 Vrms (max)

Input Impedance:

20 k

Ω

Bandwidth:

A designer must first determine the needed supply rail to ob-

tain the specified output power. Calculating the required sup-

ply rail involves knowing two parameters, V

dropout voltage. As shown in the Typical Performance

Curves, the dropout voltage is typically 0.5V. V

determined from

Equation (3).

(3)

For 200 mW of output power into an 8

Ω load, the required

V

minimum supply voltage is twice the sum of V

Since 5V is a standard supply voltage in most applications, it

is chosen for the supply rail. Extra supply voltage creates

headroom that allows the LM4880 to reproduce peaks in ex-

cess of 200 mW without clipping the signal. 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. Remember that

the maximum power dissipation value from

Equation (1)

must be multiplied by two since there are two independent

amplifiers inside the package.

Once the power dissipation equations have been addressed,

the required gain can be determined from

Equation (4).

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