LM3464 Datasheet, PDF (11/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
www.ti.com
SNVS652F – APRIL 2010 – REVISED MAY 2013
System Operation
In order to provide failure protection to the LEDs, the rail voltage is pulled up by the LM3464/64A from a
relatively low voltage level at system startup until the rail voltage reaches certain preset level. Figure 16 shows
the change of the rail voltage of the LM3464/64A LED lighting system upon the primary power source is
powered.
The Lm3464/64A can be interfaced to an off-the-shelf converter to form a LED lighting system with simple
connections. Figure 14 shows the typical application circuit of a lighting system using the LM3464/64A with a fly-
back AC/DC converter. In this application, the output voltage of the AC/DC converter is mainly governed by a
voltage reference IC, LM431 and a voltage divider consists of R₁ and R₂. The LM3464/64A influences the output
voltage of the AC/DC converter by sinking current from the junction of the voltage divider (R₁ and R₂) to realize
dynamic headroom control.
The operation of the LM3464/64A upon startup can be divided into several phases according to the changes of
the rail voltage as shown in Figure 16. When the AC/DC converter is powered, the rail voltage increases and
stays steady when its native nominal output voltage, V_RAIL(nom) is reached. This voltage is defined by the output
voltage feedback resistor divider of the AC/DC converter. At this voltage level, the LM3464/64A is powered
already. After certain delay defined by C_DHC, the LM3464/64A starts to push the rail voltage up by sinking current
into the OutP pin from the voltage feedback node of the AC/DC converter until the rail voltage reaches
V_{DHC_READY}. V_{DHC_READY} is the highest rail voltage in normal operation and should be enough to turn on all the
LED strings with current regulation (defined by R_SNSx). As V_RAIL reaches V_{DHC_READY}, the LM3464/64A turns on all
the output channels. This discharges the output capacitor of the primary power supply and causes the rail
voltage to decrease to certain level that system efficiency is maximized (V_LED).
Figure 16. Changes of Rail Voltage Upon Power Up
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