PAM2842 datasheet(10/17 Pages) PAM

10 / 17 page

PAM2842

High Power LED Driver

Power Analog Microelectronics Inc

www.poweranalog.com

09/2008 Rev 1.1

Application Information

Topology Selection

Table-1: Voltage condition Vs Topology

Inductor Selection

When maximum power supply voltage is below

than minimum load voltage, select the boost

topology. When minimum power supply voltage is

high than maximum load voltage, select buck

topology. When load voltage range is small and

between the power supply voltage, select sepic

topology.

The inductance, peak current rating, series

resistance, and physical size should all be

considered when selecting an inductor. These

factors affect the converter'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 frequency

determine the inductor value. Large inductance

can minimizes the current ripple, and therefore

reduces the peak current, which decreases core

losses in the inductor and I2R 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 I2R

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

minimized but the inductor ripple current and peak

current are higher than those in CCM.

For the large power application, if chose DCM, the

peak current will be very large, it will have great

electrical stress on the components, so we chose

CCM.

When work in CCM mode, a reasonable ripple

current is chosen to

I =0.4I

For the boost topology,

D: duty cycle, Io: output current, F: switching

frequency.

From above equation we can get the inductance:

The inductor's current rating should be higher

than

For the buck topology, I =I

For the sepic topology, L1=L2

I=I

Condition

Topology

min

max

Vin

Boost

max

min

Vin

Buck

Vin

Sepic

OIN

V(V

V )

LFV

2.5V (V

V )

FI V

V )V

LFV

OIN

2.5V (V

V )

FI V

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