LP3950 Datasheet, PDF (27/38 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

LP3950
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SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013
MODE CONTROL IN THE DEFAULT MODE
One of the main benefits of LP3950 is the default mode, which enables user to build applications without I²C or
SPI control. The LP3950 is set to the default mode when DME pin is high. DME pin high –state forces registers
NSTBY and EN_SYNC to the high [1] state so that the start-up sequence get started (see start-up sequence on
Modes of Operation). Function of LP3950 in the default mode of operation is controlled by AMODE pin. If
AMODE is pulled low the LP3950 is in the amplitude synchronization mode. If the AMODE pin is pulled high the
LP3950 is in the frequency synchronization mode. In the default mode default control register values are used,
see Table 8. Please refer to Figure 32 in Typical Applications for wiring.
RGB OUTPUT SELECTOR
The usage of RGB outputs (RGB1 and RGB2) can be selected with RGB_SEL[1:0] control bits. Audio
synchronization and RGB pattern generator output can be connected to RGB ports as shown in the following
table.
RGB_SEL[0]
0
1
0
1
Table 6. RGB Output Control
RGB_SEL[1]
0
0
1
1
RGB1 Output Control
Pattern Generator
Audio Sync
Pattern Generator
Audio Sync
RGB2 Output Control
Pattern Generator
Pattern Generator
Audio Sync
Audio Sync
Recommended External Components
OUTPUT CAPACITOR, C_OUT
The output capacitor C_OUT directly affects the magnitude of the output ripple voltage. In general, the higher the
value of C_OUT, the lower the output ripple magnitude. Multilayer ceramic capacitors with low ESR (Equivalent
Series Resistance) are the best choice. At the lighter loads, the low ESR ceramics offer a much lower V_OUT ripple
than the higher ESR tantalums of the same value. At the higher loads, the ceramics offer a slightly lower V_OUT
ripple magnitude than the tantalums of the same value. However, the dv/dt of the V_OUT ripple with the ceramics is
much lower that the tantalums under all load conditions. Capacitor voltage rating must be sufficient, 10V is
recommended.
Some ceramic capacitors, especially those in small packages, exhibit a strong capacitance reduction
with the increased applied voltage. The capacitance value can fall to below half of the nominal
capacitance. Too low output capacitance can make the boost converter unstable.
INPUT CAPACITOR, C_IN
The input capacitor C_IN directly affects the magnitude of the input ripple voltage and to a lesser degree the V_OUT
ripple. A higher value C_IN will give a lower V_IN ripple. Capacitor voltage rating must be sufficient, 10V is
recommended.
OUTPUT DIODE, D₁
A Schottky diode should be used for the output diode. To maintain high efficiency the average current rating of
the schottky diode should be larger than the peak inductor current (1.0A). Schottky diodes with a low forward
drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse
breakdown of the schottky diode larger than the output voltage. Do not use ordinary rectifier diodes, since slow
switching speeds and long recovery times cause the efficiency and the load regulation to suffer.
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