PAM2845 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

PAM2845. 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 10uF is

recommended

under

normal

operating

conditions, while a 22uF 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 10uF, voltage

rating(50V) capacitor is chosen.

PAM2845 is high switching frequency convertor,

which demands high speed rectifier. It's

indispensable to use a Schottky diode rated at

2A, 60V with the PAM2845. 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.

There a re three m ethods for s etting and

adjusting the LED current outlined here. The

methods are:

RSET

PWM Input at PWMD

PWM Input at ENA

only

Power Analog Microelectronics Inc

www.poweranalog.com

Application Information

200us typ

()

ENA

SOFT-START

>10ms

LED_

t >500ns

500ns<t <10us

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

08/2008 Rev 1.0

PAM2845

40V High Power Boost Convertor

with 6 Channel Constant Current Source

PAM2845 Datasheet(PDF) 10 Page - Power Analog Micoelectronics

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