LM3464_1 Datasheet, PDF (1/12 Pages) National Semiconductor (TI) – DHC 25W Reference Designs Dynamic Headroom Control (DHC)

LM3464 and LM5021 DHC
25W Reference Designs
National Semiconductor
Application Note 2102
David Zhang
March 31, 2011
Introduction
This evaluation board consists of a National Semiconductor
LM5021 based 120Vac 24W AC/DC flyback converter and a
LM3464 based dynamic head room control (DHC) four chan-
nel LED driver IC. This demonstration platform was devel-
oped so that the customers could evaluate the LM3464 with
an AC/DC supply. Once familiar with the operation of the
LM3464 and its control of the AC/DC converter's control loop,
the type of AC/DC power supply and the converter's output
power becomes less relevant. The evaluation board contains
eight CREE XLamp MC-E LEDs. Each XLamp MC-E LED has
four separate LEDs in each package for a total of 32 LEDs.
The CREE MC-E LED allows for an easy demonstration of
the LM3464 four channel operation with eight packaged
LEDs. each LED string is regulated to 200mA. A heat sink is
mounted on the back of the PCB to dissipate the heat gener-
ated by the LEDs.The LM3464 regulates each LED string
current and adjust the secondary output voltage of the AC/DC
power supply 0.9V above the LED string forawrd voltage.The
LM3464 monitors the output voltage and adjusts the feedback
voltage to the LM5021 primary control. This evaluation also
demonstreates the LM3464 thermal fold-back feature which
reduces the average LED current when the LED temperature
is over 80°C. The AC/DC flyback power supply works in the
discontinuous conduction mode and achieves power factor
(PF) greater than 0.98.
The schematic, bill of materials, layout and measured perfor-
mance characteristics are included.
Key Features
• Drive four strings of LEDs from 120Vac
• Overall Efficiency greater than 82%
• DC/LED Efficiency greater than 90%
• Power Factor greater than 0.98
• Dynamic Headroom Control (DHC)
• Thermal foldback control
Performance Specifications
Symbol
V_IN
V_OUT
I_LED
P_OUT
f_sw
Parameter
Input voltage
LED string voltage
LED string average current
per string
Output power
Switching frequency
Min
90 V_RMS
22 V
-
-
-
Typ
120 V_RMS
26 V
200 mA
21 W
67 kHz
Max
132 V_RMS
30 V
-
-
-
© 2011 National Semiconductor Corporation 301439
www.national.com