LP3950 Datasheet, PDF (22/38 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

LP3950
SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013
AUDIO SYNCHRONIZATION
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The LEDs connected to the RGB outputs can be synchronized to incoming audio signal with Audio
Synchronization feature. Audio Synchronization has two modes. Amplitude mode synchronizes LEDs based on
the peak amplitude of the input signal. In the amplitude mode the user can select one of three amplitude
mapping options. The frequency mode synchronizes the LEDs based on bass, middle and treble amplitudes (=
low pass, band pass and high pass filters). The user can select between two different responses of frequency for
best audio-visual user experience. Both of the modes provide a control for speed of the mapping with four
different speed configurations. Programmable gain and AGC (Automatic Gain Control) function are also available
for adjustment of the optimum audio signal mapping. The Audio Synchronization functionality is described more
closely below.
INPUT SIGNAL TYPE
The LP3950 support four types of analog audio input signals for audio synchronization
1. Single ended audio
2. Differential audio
3. Stereo
4. Single ended and differential audio.
Figure 24 shows how to wire the LP3950 audio inputs case by case (NC = Not Connected).
USING A DIGITAL PWM AUDIO SIGNAL AS AN AUDIO SYNCHRONIZATION SOURCE
If the input signal is a PWM signal, use a first or second order low pass filter to convert the digital PWM audio
signal into an analog waveform. There are two parameters that need to be known to get the filter to work
successfully: frequency of the PWM signal and the voltage level of the PWM signal. Suggested cut-off frequency
(-3dB) should be around 2 kHz to 4 kHz and the stop-band attenuation at sampling frequency should be around -
48dB or better. Use a resistor divider to reduce the digital signal amplitude to meet the specification of the analog
audio input. Because a low-order low-pass filter attenuates the high-frequency components from audio signal,
MODE_CONTROL=[01] selection is recommended when frequency synchronization mode is enabled. Figure 33
shows an example of a second order RC-filter for 29 kHz PWM signal with 3.3V amplitude. Active filters, such as
a Sallen-Key filter, may also be applied. An active filter gives better stop-band attenuation and cut-off frequency
can be higher than for a RC-filter.
To make sure that the filter rolls off sufficiently quickly, connect your filter circuit to the audio input(s), turn on the
audio synchronization feature, set manual gain to maximum, apply the PWM signal to the filter input and keep an
eye on LEDs. If they are blinking without an audio signal (modulation), a sharper roll-off after the cut-off
frequency, more stop-band attenuation, or smaller amplitude of the PWM signal is required.
10 nF _ASE
BUFFERS
SEL
10 nF _ASE
BUFFERS
SEL
AD1
NC
NC ^AD2
SINGLE-ENDED
ASE
NC
BUFFERS
MIX/
MUX
SEL
10 nF _AD1
AD2
10 nF
STEREO
10 nF _ASE
BUFFERS
MIX/
MUX
SEL
10 nF _AD1
MIX/
MUX
10 nF _AD1
MIX/
MUX
AD2
10 nF
DIFFERENTIAL
AD2
10 nF
SINGLE ENDED AND DIFFERENTIAL
Figure 24. Wiring Diagram for LP3950 Audio Inputs
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