PAM2803 Datasheet, PDF (6/8 Pages) Power Analog Micoelectronics – 3W High Power White LED Driver

PAM2803
3W High Power White LED Driver
Application Information
Inductor Selection
The PAM2803 can use small value inductors due
to its switching frequency of 1 MHz. The value of
inductor will focus in the range of 2.2uH to 4.7uH
for most PAM2803 applications. In typical high
current white LED applications, it is
recommended to use a 4.7uH inductor. The
inductor should have low DCR (DC resistance) to
minimize the I²R power loss, and it requires a
current rating of 2A to handle the peak inductor
current without saturating.
Capacitor Selection
An input capacitor is required to reduce the input
ripple and noise for proper operation of the
PAM2803. For good input decoupling, Low ESR
(equivalent series resistance) capacitors should
be used at the input. At least 2.2uF input
capacitor is recommended for most applications.
A minimum output capacitor value of 6.8uF is
recommended under normal operating
conditions, while a 10uF-22uF capacitor may be
required for higher power LED current. A
reasonable value of the output capacitor
depends on the LED current. The ESR of the
output capacitor is the important parameter to
determine the output voltage ripple of the
converter, so low ESR capacitors should be used
at the output to reduce the output voltage ripple.
The small size of ceramic capacitors is an
excellent choice for PAM2803 applications. The
X5R and X7R types are preferred because they
maintain capacitance over wide voltage and
temperature ranges.
Diode Selection
It's indispensable to use a Schottky diode rated
at 2A with the PAM2803. Using a Schottky diode
with a lower forward voltage drop is better to
improve the power LED efficiency, and its voltage
rating should be greater than the output voltage.
In application, the ON Semiconductor
MBRA210LT3G is recommended.
LED Current Setting
The LED current is set by the single external Rs
resistor connected to the FB pin as shown in the
typical application circuit on page 1. The typical
FB reference is internally regulated to 95mV. The
LED current is 95mV/Rs. It's recommended to
use a 1% or better precision resistor for the
better LED current accuracy. The formula and
table 1 for Rs selection are shown as follows:
Rs=95mV/I_LED
Table 1. Rs Resistor Value Selection
Standard Value(Ω)
0.18
I_LED(mA)
528
0.15
633
0.12
792
0.10
950
PCB Layout Guidelines
As for all switching power supplies, the layout
and components placement of the PAM2803 is
an important step in the design; especially at
high peak currents and high switching
frequencies.
The input capacitor and output capacitor should
be placed respectively as close as possible to
the input pin and output pin of the IC; the inductor
and schottky diode should be placed as close as
possible to the switch pin by using wide and short
traces for the main current path; the current
sense resistor should be placed as close as
possible between the ground pin and feedback
pin.
Power Analog Microelectronics,Inc
www.poweranalog.com
6
08/2007