PAM2842 datasheet(11/17 Pages) PAM

11 / 17 page

PAM2842

High Power LED Driver

Power Analog Microelectronics Inc

www.poweranalog.com

09/2008 Rev 1.1

Chose

I =0.4I

An input capacitor is required to reduce the input

ripple and noise for proper operation of the

PAM2842. For good input decoupling, Low ESR

(equivalent series resistance) capacitors should

be used at the input. At least 10 F input capacitor

is recommended for most applications. And close

the IC Vin-Pin we should add a bypass capacitor,

usually use a 1 F capacitor.

A minimum output capacitor value of 10 F is

recommended

under

normal

operating

conditions, while a 22 F or higher capacitor may

be required for higher power LED current. A

reasonable value of the output capacitor depends

on the LED current. The total output voltage ripple

has two components: the capacitive ripple caused

by the charging and discharging on the output

capacitor, and the ohmic ripple due to the

capacitor's equivalent series resistance. 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

v oltage

rating

and

temperature

characteristics of the Output capacitor must also

be considered. So a value of 10 F,

voltage

rating capacitor is chosen.

Consider from discharge aspect: Ix t=Cx V

In boost and sepic topology,

In buck topology,

: Output voltage allowable ripple.

Consider from equivalent series resistance:

V=I

In sepic topology, there is a series capacitor Cs

between L1 and L2 (see application schematic), it

flows the current:

The ripple voltage is

The voltage rating must be higher than input

voltage.

Because the Cs capacitor will flow the large RMS

current, so this topology is suitable for small

power application.

PAM2842 is a high switching frequency converter

which demands high speed rectifier. It's

indispensable to use a Schottky diode rated at 3A,

40V with the PAM2842. Using a Schottky diode

with a lower forward voltage drop is better to

improve the power LED efficiency.

In boost topology, the voltage rating should be

higher than Vout and in buck topology, the voltage

rating higher than Vin, the peak current is

in sepic topology, the voltage rating should be

higher than Vin+Vout, the peak current is

I=I

The average current of the diode equals to Io.

PAM2842 working frequency is decided by

resistor connect to the RT pin, it can be calculated

by follow equation:

From the equations, we can see when working

frequency is high, the inductance can be small.

It's important in some size limit application. But

we should know when the working frequency is

higher, the switching loss is higher too. We must

pay attention to thermal dissipation in this

application.

There are two methods for setting and adjusting

the LED current:

Rsense only

PWM signal with external components

Use the COMP pin

Use the Sense pin

ΔΔ

LL1

RIPPLE

ripple-esr

co.ripple

oesr

DMAX

L1peak

L2peak

Capacitor Selection

Diode Selection

Work frequency selection

Methods for Setting LED Current

50V

LF(V

V )

OIN

2.5V

FI (V

V )

RIPPLE

I(1 D)

Cs(RMS)

DMAX

F(Hz)

24 (RT 12K)

´+

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