Serial-Pulse Gain Adjustment (ENAMP)

The audio amplifier has an internal gain adjustment,

controlled by a serial-pulse interface. The gain setting

of the input amplifier is adjustable from -9dB to +18dB

in 3dB steps. This allows the amplifier to be used for

both hands-free and for receiver mode, without any

external components.

Driving ENAMP high turns on the amplifier and sets the

internal register to -9dB. Adjust the gain by pulsing

ENAMP low (500ns to 500µs pulse width). Each rising

edge sets the gain as shown in Table 3. Once the

desired setting is reached, hold ENAMP high for at

least 3.2ms to set the internal register and change the

gain. To set a new current level, repeat the previous

sequence from the beginning. To shut down the ampli-

fier, hold ENAMP low for at least 3.2ms. Figure 3 is a

timing diagram for ENAMP.

Applications Information

BTL Amplifier

The MAX8678 is designed to drive a load differentially,

a configuration referred to as bridge-tied load (BTL).

The BTL configuration offers advantages over the sin-

gle-ended configuration, where one side of the load is

connected to ground. Driving the load differentially

doubles the output voltage, thus quadrupling output

power compared to a single-ended amplifier under

similar configurations.

Since the differential outputs are biased at midsupply,

there is no net DC voltage across the load. This elimi-

nates the need for DC-blocking capacitors required for

single-ended amplifiers. These capacitors can be

large, expensive, consume board space, and degrade

low-frequency performance.

White LED Charge Pump with

1.1W Audio Amplifier

10

______________________________________________________________________________________

OPTIONAL FILTER

CAPACITORS

DIFFERENTIAL

AUDIO INPUT

MONO

CLASS AB

AUDIO AMP

EARPIECE /

LOUDSPEAKER

A+

A-

OUT+

OUT-

MAX8678

Figure 4. Differential Audio Input with Optional Filter Capacitors

SINGLE-ENDED

AUDIO INPUT

MONO

CLASS AB

AUDIO AMP

EARPIECE /

LOUDSPEAKER

A+

A-

OUT+

OUT-

MAX8678

NOTE: FILTER CAPACITORS REQUIRED FOR SINGLE-ENDED AUDIO OUTPUT.

Figure 5. Single-Ended Audio Input

Ω)

ENAMP

RISING

EDGES

GAIN

SETTING

(dB)

MIN

TYP

MAX

1

-9

69.1

99.0

140.0

2

-6

62.4

89.2

116.0

3

-3

54.9

78.4

102.0

4

0

46.8

66.9

87.0

5

3

38.8

55.5

72.1

6

6

31.3

44.7

58.1

7

9

24.6

35.1

45.6

8

12

18.8

26.9

35.0

9

15

14.1

20.2

26.2

10

18

10.5

15.0

19.5

Table 3. Audio Amplifier-Input Impedance