Inductor Selection

Capacitor Selection

Diode Selection

LED Current Setting

3 Modes Cycling

Low Voltage Startup and Soft Start

The PAM2805 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 PAM2805 applications. In typical high

c u r r e n t

w h i t e

L E D

a p p l i c a t i o n s ,

i t

i s

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

PAM2805. 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

r e c o m m e n d e d

u n d e r

n o r m a l

o p e r a t i n g

conditions, while a 10uF-22uF capacitor may be

req uired f or high er p owe r L ED curren t. A

r e a s o n a b l e v a l u e o f t h e o u t p u t c a p a c i t o r

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 PAM2805 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 PAM2805. 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.

SS2 2 is reco m m en d ed Sch ot tky d iod e f o r

rectifier.

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/R1. It's recommended to

use a 1% or better precision resistor for the

better LED current accuracy. The formula for Rs

selection is shown as follows:

Rs(Ohm)=95mV/I

(mA) at VIN=3V.

Typically, for 1W(330mA) and 3W(750mA) LED

light applications, the Rs are 0.288ohm and

0.127Ohm respectively.

T h e

PA M 2 8 0 5

h a s

t h r e e

m o d e s :

1 0 0 %

brightness, 25% brightness and blinking(typical

8.5Hz).

The mode change is triggered by power on/off

actions and cycles in the following sequence:

bright, dimming, blinking and back to bright

mode.

The PAM2805 will reset to the bright mode after

being powe r off for more than 5 seconds.

The PAM2805 has a build-in low voltage startup

circuit for the best battery life solution. It can

start up at 0.9V VIN typically when the preset

LED current is 200mA.

The soft-start function is made by clamping the

output voltage of error amplifier with another

voltage source which increases slowly from zero

t o n e a r V I N d u r i n g t h e s o f t - s t a r t p e r i o d .

Therefore, the duty cycle of the PWM will be

increased

from

zero

to

maximum

in

this

period. The charging time of the inductor will be

limited by the smaller duty so that the inrush

current

can

be

reduced

to

an acceptable

value.`

2

LED

Over Voltage Protection

The output voltage of PAM2805 is monitored by

Over Voltage Protection circuit. Once VOUT

goes over V

, typically 4.5V, the power NMOS

is

turned

off

and

SW

pin

stops switching.

Then, the VOUT is clamped to around V

.

OVP

OVP

Application Information

6

,

Power Analog Microelectronics Inc

www.poweranalog.com

11/2011 Rev1.0

PAM2805

3-Modes/3W WLED Driver with 1 or 2 Cell Battery