ceeded momentarily, due to the discharging time constant of

the bias-pin voltage. This time constant is established by the

two 50k resistors (in parallel) with the series bypass capaci-

tor value.

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, en-

abling the headphone function and disabling the LM4870

amplifier. The headphone amplifier then drives the head-

phones, whose impedance is in parallel with resistor R2.

Since the typical impedance of headphones are 32

Ω, resis-

tor R2 has negligible effect on the output drive capability.

Also shown in

Figure 3 are the electrical connections for the

headphone jack and plug. A 3-wire plug consists of a Tip,

Ring, and Sleave, where the Tip and Ring are signal carrying

conductors and the Sleave is the common ground return.

One control pin contact for each headphone jack is sufficient

to indicate to control inputs that the user has inserted a plug

into a jack and that another mode of operation is desired.

For a system implementation where the headphone amplifier

is designed using a split supply, the output coupling cap, C

C

and resistor R2 of

Figure 3, can be eliminated. The function-

ality described earlier remains the same, however.

In addition, the HP-SENSE pin, although it may be con-

nected to the SHUTDOWN pin as shown in

Figure 2, may

still be used as a control flag. It is capable of driving the input

to another logic gate or approximately 2 mA without serious

loading.

AUDIO POWER AMPLIFIER DESIGN

Design a 500 mW/8

Ω Audio Amplifier

Given:

Power Output:

1.0Wrms

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. The Output Power vs Supply

Voltage graphs in the Typical Performance Characteris-

tics section show how much power the LM4870 will output

given different supply voltage. According to the graph, 4.6V

DS100094-7

FIGURE 2. HP-SENSE Pin to

SHUTDOWN Pin Connection

DS100094-8

FIGURE 3. Typical Headphone Control Input Circuitry

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