Datasheet

LM3464, LM3464A

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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 R

and R

. The LM3464/64A influences the output

voltage of the AC/DC converter by sinking current from the junction of the voltage divider (R

and R

) 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

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

Product Folder Links: LM3464 LM3464A