MAX9775 datasheet(24/38 Pages) AD

24 / 38 page

if the right and left volume controls are set for a maxi-

mum gain 0dB (11111 in Table 7, IN+6dB = 0 from Table

10) before the 3D effect is activated, the volume control

should be programmed to -6dB (11001 in Table 7)

immediately after the 3D effect has been activated.

Signal Path

The audio inputs of the MAX9775/MAX9776—INA, INB,

and INC—are preamplified and then mixed by the input

mixer to create three internal signals: Left (L), Right (R),

and Mono (M). Tables 5a and 5b show how the inputs

are mixed to create L, R, and M. These signals are then

independently volume adjusted by the L, R, and M vol-

ume control and routed to the output mixer. The output

mixer mixes the internal L, R, and M signals to create a

variety of audio mixes that are output to the headphone

speaker and mono receiver amplifiers. Figure 6 shows

the signal path that the audio signals take.

Signal amplification takes place in three stages. In the

first stage, the inputs (INA, INB, and INC) are pre-

amplified. The amount by which each input is amplified

is determined by the bits INA+20dB (B4 in the Input

Mode Control Register) and IN+6dB (B3 in the Global

Control Register). After preamplification, they are mixed

in the Input Mixer to create the internal signals L, R,

and M.

In the second stage of amplification, the internal L, R,

and M signals are independently volume adjusted.

Finally, each output amplifier has its own internal gain.

The speaker, headphone, and mono receiver amplifiers

have fixed gains of 12dB, 3dB, and 3dB, respectively.

Current-Limit and Thermal Protection

The MAX9775/MAX9776 feature current limiting and

thermal protection to protect the device from short cir-

cuits and overcurrent conditions. The headphone

amplifier pulses in the event of an overcurrent condition

with a pulse every 100µs as long as the condition is

present. Should the current still be high, the above

cycle is repeated. The speaker amplifier current-limit

protection clamps the output current without shutting

down the output. This can result in a distorted output.

Current is limited to 1.6A in the speaker amplifiers and

170mA in the headphone and mono receiver amplifiers.

The MAX9775/MAX9776 have thermal protection that

disables the device at +150°C until the temperature

decreases to +120°C.

2 x 1.5W, Stereo Class D Audio Subsystem

with DirectDrive Headphone Amplifier

______________________________________________________________________________________

INPUT

MIXER

INPUT

PREAMPLIFIER

INPUT A:

0dB, 6dB, 20dB

-75dB TO 0dB

0dB TO 6dB

MVOL

MONO+6dB

LVOL

-75dB TO 0dB

RVOL

INPUT B AND C:

0dB, 6dB

OUTPUT

MIXER

3dB

12dB

SPEAKER

HEADPHONE

RECEIVER

MONO

Figure 7. Signal Path

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