width is dictated by the choice of external components

shown in Figure 1. The input coupling capacitor, C

first order high pass filter which limits low frequency re-

sponse. 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 attenu-

ation. 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 perfor-

mance.

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 (nominally 1/2 V

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 consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4878 turns

on. The slower the LM4878’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a virtually

clickless and popless shutdown function. While the device

will function properly, (no oscillations or motorboating), with

C

to turn-on clicks and pops. Thus, a value of C

1.0 µF is recommended in all but the most cost sensitive

designs.

LOW VOLTAGE APPLICATIONS ( BELOW 3.0 V

The LM4878 will function at voltages below 3 volts but this

mode of operation requires the addition of a 1k

Ω resistor

from each of the differential output pins ( pins 8 and 4 )

directly to ground. The addition of the pair of 1k

Ω resistors (

R4 & R5 ) assures stable operation below 3 Volt Vdd opera-

tion. The addition of the two resistors will however increase

the idle current by as much as 5mA. This is because at 0v

input both of the outputs of the LM4878’s 2 internal opamps

go to 1/2 V

current to flow through the 1K resistors from output to

ground. See fig 4.

Jumper options have been included on the reference design,

Fig. 4, to accommodate the low voltage application. J2 & J3

connect R4 and R5 to the outputs.

AUDIO POWER AMPLIFIER DESIGN

A 1W/8

Ω AUDIO AMPLIFIER

Given:

Power Output

1 Wrms

Load Impedance

8

Ω

Input Level

1 Vrms

Input Impedance

20 k

Ω

Bandwidth

A designer must first determine the minimum supply rail to

obtain the specified output power. By extrapolating from the

Output Power vs Supply Voltage graphs in the Typical Per-

formance Characteristics section, the supply rail can be

easily found. A second way to determine the minimum sup-

ply rail is to calculate the required V

and add the output voltage. Using this method, the minimum

supply voltage would be (V

V

age vs Supply Voltage curve in the Typical Performance

Characteristics section.

(2)

Using the Output Power vs Supply Voltage graph for an 8

Ω

load, the minimum supply rail is 4.6V. But since 5V is a

standard voltage in most applications, it is chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4878 to reproduce peaks in excess of 1W with-

out producing audible distortion. 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.

Once the power dissipation equations have been addressed,

the required differential gain can be determined from Equa-

tion 3.

(3)

R

From Equation 3, the minimum A

Since the desired input impedance was 20 k

Ω, and with a

A

allocation of R

Ω and R

Ω. The final design step

is to address the bandwidth requirements which must be

stated as a pair of −3 dB frequency points. Five times away

from a −3 dB point is 0.17 dB down from passband response

f

f

As stated in the External Components section, R

junction with C

C

i

≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF

The high frequency pole is determined by the product of the

desired frequency pole, f

With a A

150 kHz which is much smaller than the LM4878 GBWP of

4 MHz. This figure displays that if a designer has a need to

design an amplifier with a higher differential gain, the

LM4878 can still be used without running into bandwidth

limitations.

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