PAM2841 datasheet(9/14 Pages) PAM

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Application Information

PAM2841

1.5A SW Current, 40V Precision WLED Driver

Power Analog Microelectronics Inc

www.poweranalog.com

Inductor Selection

Schottky Diode Selection

Input and Output Capacitor Selection

Input Capacitor

Output Capacitor

Dimming Control

1). Using an External PWM Signal to EN Pin

The selection of the inductor affects steady state

operation as well as transient behavior and loop

stability. These factors make it the most important

component in power regulator design. There are

three important inductor specifications, inductor

value, DC resistance and saturation current.

Considering inductor value alone is not enough.

The inductor value determines the inductor ripple

current. Choose an inductor that can handle the

necessary peak current without saturation, the

inductor DC current given by:

Iin_dc=Vout*Iout/(Vin* )

=efficiency.

Inductor values can have ±20% tolerance with no

current bias. W hen the i nductor c urrent

approaches saturation level, its inductance can

decrease 20% to 35% from the 0A value

depending on how the inductor vendor defines

saturation current. Using an inductor with a

smaller inductance value causes discontinuous

PWM when the inductor current ramps down to

zero before the end of each switching cycle. This

reduces the boost converter's maximum output

current, causes large input voltage ripple and

reduces efficiency. Large inductance value

provides much more output current and higher

conversion efficiency. For these reasons, an

inductor within

value range is

recommended.

The high switching frequency of the PAM2841

demands a high-speed rectification for optimum

efficiency. Ensure that the diode average and

peak current rating exceeds the average output

current and peak inductor current. In addition, the

diode's reverse breakdown voltage must exceed

the open protection voltage.

At least a 1 F input capacitor is recommended to

reduce the input ripple and switching noise for

normal operating conditions. Larger value and

lower ESR (Equivalent Series Resistance) may

be needed if the application require very low input

ripple. It follows that ceramic capacitors are a

good choice for applications. Note that the input

capacitor should be located as close as possible

to the device.

The output capacitor is mainly selected to meet

the requirement for the output ripple and loop

stability. This ripple voltage is related to the

capacitor's capacitance and its equivalent series

resistance (ESR). A output capacitor of 1μF

is recommended and maybe need a

larger capacitor. 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 (ESR):

V=V

1/2*(L/(C

*((V

-V

))))*(I

V=I

Where

is the peak inductor current.

Multilayer ceramic capacitors are an excellent

choice as they have extremely low ESR and are

available in small footprints. Capacitance and

ESR variation with temperature should be

considered for best performance in applications

with wide operating temperature ranges.

There are 4 different types of dimming control

methods:

With the PWM signal applied to the EN pin, the

PAM2841 is alternately turned on or off by the

PWM signal. The LEDs operate at either zero or

full current. The average LED current changes

proportionally with the duty cycle of the PWM

signal. A 0% duty cycle turns off the PAM2841 and

leads to zero LED current. A 100% duty cycle

generates full current.Also the recommend

dimming frequency is between 100Hz and 200Hz.

* (T

–T

)/(T +T

)

where

: on time of a period

: 0.85ms

: off time of a period

: on state current (full current)

ηη

4.7μH to 22μH

minimum

≈

RIPPLE

RIPPLE(C)

RIPPLE(ESR)

RIPPLE(C)

OUT

OUT(MAX)

IN(MIN)

OUT

RIPPLE(ESR)

PEAK

ESR(COUT)

AVE

STATE

STARTUP

OFF

PEAK

STARTUP

OFF

STATE

08/2008 Rev 1.3

PAM2841 Datasheet(PDF) 9 Page - Power Analog Micoelectronics

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