LP39542 Datasheet, PDF (17/58 Pages) National Semiconductor (TI) – Advanced Lighting Management Unit

Audio Synchronization
The color LEDs connected to RGB outputs can be synchro-
nized to incoming audio with Audio Synchronization feature.
Audio Sync has 2 modes. Amplitude mode synchronizes
color LEDs based on input signal’s peak amplitude. In the
amplitude mode the user can select between 3 different am-
plitude mapping modes and 4 different speed configurations.
The frequency mode synchronizes the color LEDs based on
bass, middle and treble amplitudes (= low pass, band pass
and high pass filters). User can select between 2 different
frequency responses and 4 different speed configurations for
best audio-visual user experience. Programmable gain and
AGC function are also available for adjustment of input signal
amplitude to light response. The Audio Sync functionality is
described more closely below.
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. Sug-
gested cut-off frequency (-3 dB) should be around 2 kHz to 4
kHz and the stop-band attenuation at sampling frequency
should be around -48 dB 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_CTRL=01b selection is recommended when frequen-
cy synchronization mode is enabled. Application example 5
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 re-
quired.
AUDIO SYNCHRONIZATION SIGNAL PATH
LP39542 audio synchronization is mainly done digitally and it
consists of the following signal path blocks:
• Input Buffers
• AD Converter
• DC Remover
• Automatic Gain Control (AGC)
• Programmable Gain
• 3 Band Digital Filter
• Peak Detector
• Look-up Tables (LUT)
• Mode Selector
• Integrators
• PWM Generator
• Output Drivers
The digitized input signal has DC component that is removed
by digital DC REMOVER (-3 dB @ 400 Hz). Since the light
response of input audio signal is very much amplitude de-
pendent the AGC adjusts the input signal to suitable range
automatically. User can disable AGC and the gain can be set
manually with PROGRAMMABLE GAIN. LP39542 has 2 au-
dio synchronization modes: amplitude and frequency. For
amplitude based synchronization the PEAK DETECTION
method is used. For frequency based synchronization 3
BAND FILTER separates high pass, low pass and band bass
signals. For both modes the predefined LUT is used to opti-
mize the audio visual effect. MODE SELECTOR selects the
synchronization mode. Different response times to music
beat can be selected using INTEGRATOR speed variables.
Finally PWM GENERATOR sets the driver FET duty cycles.
INPUT SIGNAL TYPE AND BUFFERING
LP39542 supports single ended audio input as shown in the
figure below. The electric parameters of the buffer are de-
30008525
scribed in the Audio Synch table. The buffer is rail-to-rail input
operational amplifier connected as a voltage follower. DC lev-
el of the input signal is set by a simple resistor divider
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