PAM2846 datasheet(10/14 Pages) PAM

10 / 14 page

Inductor Selection

Capacitor Selection

Diode Selection

Methods for Setting LED Current

The inductance, peak current rating, series

resistance, and physical size should all be

considered when selecting an inductor. These

factors affect the conver ter's operating mode,

efficiency, maximum output load capability,

transient response time, output voltage ripple,

and cost.

The maximum output current, input voltage,

output voltage, and switching f requency

determine the inductor v alue. Ve ry high

inductance minimizes the current rip ple, and

therefore reduces the peak current, which

decreases core losses in the inductor and I R

losses in the entire power path. However, large

inductor values also require more energy storage

and more turns of wire, which increases physical

size and I R copper losses in the inductor. Low

inductor values decrease the physical size, but

increase the current ripple and peak current.

Finding the best inductor involves the

compromises among circuit efficiency, inductor

size, and cost.

When choosing an inductor, the first step is to

determine the operating mode: continuous

conduction mode (CCM) o r discontinuous

conduction mode (DCM). When CCM mode is

chosen, the ripple current and the peak current of

the inductor can be minimized. If a small-size

inductor is required, DCM mode can be chosen.

In DCM mode, the inductor value and size can be

mini mized but the inductor ripple current and

peak current are higher than those in CCM.

An input capacitor is required to reduce the input

ripple and noise for proper operation of the

PAM2846. For good input decoupling, low ESR

(equivalent series resistance) capacitors should

be used at the input. At least 2.2 F input

capacitor is recommended for most applications.

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 c omponents: 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 i mportant 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

(50V) capacitor is chosen.

PAM2846 is high switching frequency convertor,

which demands high speed rectifier. It's

indispensable to use a Schottky diode rated at

2A, 60V with the PAM2846. Using a Schottky

diode with a lower forward voltage drop can

improve efficiency of the convertor. The voltage

rating of the diode should be greater than the

output voltage.

There a re three m ethods for s etting and

adjusting the LED current outlined here. The

methods are:

RSET

PWM Input at PWMD

Single wire logic signal at ENA

only

Power Analog Microelectronics Inc

www.poweranalog.com

PAM2846

Integrated 6 String LED Boost Type Driver

Application Information

200 s typ

()

ENA

SOFT-START

>10ms

LED_

t >500ns

500ns<t <10 s

32/32

31/32

30/32

29/32

28/32

32/32

1/32

2/32

3/32

4/32

5/32

shutdown

Figure 1. Timing diagram for EN

07/2008 Rev 1.2

PAM2846 Datasheet(PDF) 10 Page - Power Analog Micoelectronics

PAM2846 Datasheet(HTML) 10 Page - Power Analog Micoelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Datasheet Download

Link URL