LM4917

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SNAS238G – AUGUST 2004 – REVISED MAY 2013

Input Capacitor Value Selection

capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however,

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

150Hz. Applications using speakers with this limited frequency response reap little improvement by using high

value input and output capacitors.

magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by

selecting an input capacitor value that is no higher than necessary to meet the desired

−3dB frequency.

frequency that is found using Equation (3).

(4)

Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system.

Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may

affect overall system performance.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 90mW/16

Ω Audio Amplifier

Given:

Power Output

90mW

Load Impedance

16

Ω

Input Level

1Vrms (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.50dB

The design begins by specifying the minimum supply voltage necessary to obtain the specified output power.

One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical

Performance Characteristics section. Another way, using Equation 5, is to calculate the peak output voltage

necessary to achieve the desired output power for a given load impedance. For a single-ended application, the

result is Equation 5.

(5)

(6)

The Output Power vs Supply Voltage graph for a 16

Ω load indicates a minimum supply voltage of 3.1V. This is

easily met by the commonly used 3.3V supply voltage. The additional voltage creates the benefit of headroom,

allowing the LM4917 to produce peak output power in excess of 90mW without clipping or other audible

distortion. The choice of supply voltage must also not create a situation that violates maximum power dissipation

as explained above in the POWER DISSIPATION section. Remember that the maximum power dissipation point

from Equation 2 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 7.

(7)

Thus, a minimum gain of 1.2 allows the LM4917 to reach full output swing and maintain low noise and THD+N

impedance set at 20k

Ω, the feedback resistor is found using Equation (7).

where

•

(8)

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