PAM2801SWAF095 datasheet(6/9 Pages) PAM

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Inductor Selection

Capacitor Selection

Diode Selection

LED Current Setting

LED Dimming Control

Dimming Control Using a DC Voltage to FB Pin

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 I R power loss, and it requires a

current rating of 2A to handle the peak inductor

current without saturating.

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.

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.

application,

the

O N

S emiconductor

MBRA210LT3G is recommended.

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

Table 1. Rs Resistor Value Selection

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

ranges from 0V to 2V, the resistors in the Figure

will set the LED current from 0mA to 350mA.

LED

Figure 1. Dimming Control Using a DC Voltage to

FB Pin

Application Information

PAM2801

0.27

WLED

22KΩ

1KΩ

08/2007

PAM2801

1W High Power White LED Driver

Standard Value(Ω

)

ILED(mA)

195

0.47

202

0.39

243

0.33

288

0.27

352

0.24

396

Power Analog Microelectronics Inc

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