PAM2801 Datasheet, PDF (6/9 Pages) Power Analog Micoelectronics – 1W High Power White LED Driver

Application Information
Inductor Selection
The PAM2801 can use a small value inductor
due to its switching frequency of 1MHz. The
value of inductor focuses in the range of 1.5uH to
4.7uH for most PAM2801 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
PAM2801. 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 4.7uF 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 PAM2801 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 PAM2801. 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
PAM2801
1W High Power White LED Driver
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)
1
I_LED(mA)
95
0.47
202
0.39
243
0.33
288
0.27
352
0.24
396
LED Dimming Control
Dimming Control Using a DC Voltage to FB Pin
One method for dimming the LEDs is to apply a
variable DC voltage through a resistor to the FB
pin of the PAM2801. The dimming control with a
DC voltage is shown in the Figure1. The DC
voltage artificially raises the FB pin voltage, with
the DC voltage increasing, the voltage across R2
increases and the voltage across Rs decreases,
which therefore lowers the LED current. The
values of resistor R1 and R2 should be large
enough to make the current from the DC source
much smaller than the LED current while much
larger than the FB leakage current. When V_DC
ranges from 0V to 2V, the resistors in the Figure
will set the LED current from 0mA to 350mA.
PAM2801
FB
R1
V_DC
22KΩ
R2
1KΩ
1W
WLED
R_S
0.27Ω
Figure 1. Dimming Control Using a DC Voltage to
FB Pin
Power Analog Microelectronics,Inc
www.poweranalog.com
6
08/2007