LP3950

SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013

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The digitized input signal has a DC component that is removed by the digital DC REMOVER (-3 dB @ 400 Hz).

The automatic GAIN CONTROL adjusts the input signal to suitable range automatically. User can disable AGC

and the gain can be set manually with PROGRAMMABLE GAIN. The LP3950 has two audio synchronization

modes: amplitude and frequency. For amplitude based synchronization the PEAK DETECTION method is used.

For frequency based synchronization the three-way crossover FILTER separates high pass, low pass and band

pass signals. For both modes, a predefined lookup table (LUT) is used to match the audio visual effect. The

MODE SELECTOR selects the synchronization mode. Reaction speed can be selected using INTEGRATOR

speed variables. Finally PWM GENERATOR sets the driver FETs duty cycles.

Table 4. Audio Synchronization Characteristics

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Zin

Input Impedance of AD1, AD2, ASE pins

200

500

k

Ω

Audio Input Level Range (peak-to-peak),

0.1

V

Single Ended Audio

Audio Input Level Range (peak-to-peak),

0.1

V

Differential Audio

Crossover Frequencies (

−3 dB)

Narrow Frequency Response

Low Pass

0.5

Band Pass

1.0 and 1.5

High Pass

2.0

kHz

Wide Frequency Response

Low Pass

1.0

Band Pass

2.0 and 3.0

High Pass

4.0

CONTROL OF AUDIO SYNCHRONIZATION

The following table describes the controls required for audio synchronization. Note that these controls are

functionality is illustrated in Figure 27.

Table 5. Audio Synchronization Control

EN_SYNC

Audio synchronization enabled. Set EN_SYNC = 1 to enable audio synchronization or 0 to disable.

SYNC_MODE

Synchronization mode selector. Set SYNC_MODE = 0 for amplitude synchronization. Set SYNC_MODE

= 1 for frequency synchronization.

MODE_CTRL[1:0]

See below: Mode control

EN_AGC

Automatic gain control. Set EN_AGC = 1 to enable automatic control or 0 to disable. When EN_AGC is

disabled, the audio input signal gain value is defined by GAIN_SEL.

GAIN_SEL[2:0]

Input signal gain control. Gain has a range from 0 dB to 21 dB with 3 dB steps:

[000] ... 0 dB

[011] ... 9 dB

[110] ... 18 dB

[001] ... 3 dB

[100] ... 12 dB

[111] ... 21 dB

[010] ... 6 dB

[101] ... 15 dB

INPUT_SEL[1:0]

[00] ... Single ended input signal, ASE.

[01] ... Differential input signal, AD1 and AD2.

[10] ... Stereo input or single ended and differential input signal.

Note: Sum of input signals divided by 2.

[11] ... No input

Please see Figure 24 for wiring.

SPEED_CTRL[1:0]

Control for speed of the mapping. Sets the reaction speed (or "sampling rate") for the audio input signal:

[00] ... FASTEST

[01] ... FAST

[10] ... MEDIUM

[11] ... SLOW

24

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