MP2480 – 36V INPUT, 3A HIGH POWER LED DRIVER

MP2480 Rev. 1.01

www.MonolithicPower.com

10

1/25/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the LED Current

FB

LED

FB

V

I

R

=

Inductor Selection and Frequency Setting

The inductor L supplies a constant current to

the LED. The inductor value is related to the

IN

OUT

OUT

s

IN

LED

(V

V

) V

f

VL I

25%

−⋅

=

⋅⋅

⋅

output voltage that drives the LEDs. A larger-

value inductor will result in smaller switching

frequency.

The peak inductor current is about:

LP

LED

I

I

1.125

=⋅

Choose an inductor that will not saturate under

the maximum inductor peak current.

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode’s forward

voltage and recovery times, use a Schottky

diode.

The RMS current through the diode is about:

OUT

DLED

IN

V

II

1

V

=⋅

−

Choose a diode with a maximum reverse

voltage rating greater than the maximum input

voltage, and a current rating greater than the

maximum diode current.

The input current to the step-down converter is

discontinuous,

and

therefore

requires

a

capacitor to supply the AC current to the step-

down converter and maintain the DC input

voltage. Use low-ESR capacitors for best

performance, especially for high-switching–

frequency applications.

The RMS current through the input capacitor is

about:

OUT

DLED

IN

V

II

V

=⋅

’

With low ESR capacitors, the input voltage

ripple can be estimated as:

OUT

OUT

LED

IN

sIN

IN

IN

VV

I

V1

fC

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Choose the input capacitor with a high-enough

RMS current rating and enough capacitance for

a small input-voltage ripple.

Include a small high-quality ceramic capacitor

(0.1μF) placed as close to the IC as possible

when using electrolytic or tantalum capacitors.

reduce the LED ripple current: The LED ripple

current (peak-to-peak value) is about 15% of

the LED DC current. However, the output

capacitor can be used to further reduce the

LED ripple current. Use low-ESR capacitors to

limit the output ripple voltage. The output ripple

voltage can be estimated by:

OUT

OUT

OUT

ESR

SIN

S

OUT

VV

1

V1

R

fL

V

8 f

C

⎛⎞

⎛⎞

Δ=

×

−

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

(ESR) value of the output capacitor.