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 100Hz to 150Hz. 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 LM4897 turns

on. The slower the LM4897’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

the range of 0.1µF to 0.39µF), 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

1.0µF is recommended in all but the most cost sensitive

designs.

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

20k

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)

5V is a standard voltage, in most applications, chosen for the

supply rail. Extra supply voltage creates headroom that al-

lows the LM4897 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)

A

From Equation 3, the minimum A

Since the desired input impedance was 20k

Ω, and with a

A

R

Ω and R

Ω. The final design step is to address

the bandwidth requirements which must be stated as a pair

of −3dB frequency points. Five times away from a −3dB point

is 0.17dB down from passband response which is better

f

f

As stated in the External Components section, R

junction with C

C

i

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

The high frequency pole is determined by the product of the

desired frequency pole, f

With a A

300kHz which is much smaller than the LM4897 GBWP of

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

design an amplifier with a higher differential gain, the

LM4897 can still be used without running into bandwidth

limitations.

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