LP3950 Datasheet, PDF (19/31 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

Audio Synchronization
The LEDs connected to the RGB outputs can be synchro-
nized 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 be-
low.
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 15 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 compo-
nents from audio signal, MODE_CONTROL=[01] selection is
recommended when frequency synchronization mode is en-
abled. Figure 23 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, con-
nect 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.
Schematic Diagram
FIGURE 15. Wiring Diagram for LP3950 Audio Inputs
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