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LM4970 Arkusz danych(PDF) 11 Page - Texas Instruments

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Numer części LM4970
Szczegółowy opis  Audio Synchronized Color LED Driver
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LM4970
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SNVS312D – JANUARY 2005 – REVISED MAY 2013
After the last bit of the address bit is sent, the master checks for the LM4970’s acknowledge. The master
releases the data line high (through a pullup resistor). Then the master sends a clock pulse. If the LM4970 has
received the address correctly, then it holds the data line low during the clock pulse. If the data line is not low,
then the master should send a “stop” signal (discussed later) and abort the transfer.
The 8 bits of data are sent next, most significant bit first. Each data bit should be valid while the clock level is
stable high.
After the data byte is sent, the master must check for another acknowledge to see if the LM4970 received the
data.
If the master has more data bytes to send to the LM4970, then the master can repeat the previous two steps
until all data bytes have been sent.
The “stop” signal ends the transfer. To signal “stop”, the data signal goes high while the clock signal is high. The
data line should be held high when not in use.
AUDIO SYNCHRONIZATION MODE
The LM4970 features an audio synchronization mode where each PWM color LED driver output is dependent on
the audio input signal. The audio synchronization mode allows each LED output to react to the amplitude of the
audio input signal, according to the LED output's assigned frequency band. Audio synchronization mode is
activated by clearing the I2C_SEL bit in the Pattern Select Register.
The audio synchronization filter separates the mixed audio signal into three frequency bands: lowband, midband,
and highband. Each frequency band is assigned to a particular PWM LED output, with lowband controlling the
duty cycle of the LED1 output, midband controlling the duty cycle of the LED2 output, and highband controlling
the duty cycle of the LED3 output. This occurs whenever the audio synchronization randomizer is not turned on.
The operation of the audio synchronization randomizer is explained in the AUDIO SYNCHRONIZATION
RANDOMIZER section. The duty cycle of any given LED output is dependent upon the amplitude of the audio
signal for its particular frequency band. An increase in the amplitude of the audio signal will increase the duty
cycle of the PWM LED driver. LEDs driven with a higher duty cycle results in a brighter lighting effect.
The LM4970 has three single-ended analog audio inputs designated MIN, LIN, and RIN, where mono voice data is
routed to MIN and stereo MP3 or stereo FM radio data is routed to LIN and RIN. Audio signals coupled in from MIN,
LIN, and RIN are mixed together by an audio input summing amplifier. The gain of the audio input summing
amplifier is programmed by the SGAIN<2:0> bits of the Gain Select Register. Increasing the gain of the audio
input summing amplifier will increase the intensity of the LEDs in audio synchronization mode.
The pole of the low pass filter band is set by the filter cap, Cfilt, and an internal 4k
Ω resistor. The pole of the high
pass filter band is internally set by programming the HPF_F<1:0> bits of the Frequency Select Register. The
midband frequency band is a function of the lowband and highband poles. The gain response of the midband
frequency band can be set by programming the MGAIN<2:0> bits of the Gain Select Register.
To minimize LED leakage between audio bands, care should be taken when selecting input gain, midband gain,
Cfilt, and LED current drive. There is a trade off between LED brightness and LED leakage in other audio bands.
Leakage can be minimized by reducing LED current drive and input gain. Please refer to the frequency response
graphs found in the Typical Performance Characteristics section as a guideline to minimize LED leakage.
AUDIO SYNCHRONIZATION RANDOMIZER
The LM4970 features a randomizer block that randomizes the frequency band assigned to each PWM LED driver
during audio synchronization operation. The randomizer is activated by setting the RAND bit in the Mode Select
Register. Clearing the RAND bit will disable the randomizer. The randomizer can only be activated when the
LM4970 is programmed to audio synchronization mode. The interval at which randomizer assigns a new
frequency band is set to occur once every 3.2 seconds. The randomizer ensures that all the colored LEDs will
light up over a long duration even if the audio input has a fixed frequency.
Copyright © 2005–2013, Texas Instruments Incorporated
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