MP2487 – 55V, 1A, HIGH-POWER LEDS DRIVER

MP2487 Rev. 1.0

www.MonolithicPower.com

9

9/22/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the LED Current

The LED current is set using a sensing resistor,

which is in series with the LEDs and connected

to GND. The voltage on the sensing resistor is

connected to FB pin.

FB

LED

FB

V

I

R

=

287mΩ.

Inductor L1

The inductor is required to supply constant

current to the output load while being driven by

the switched input voltage. A larger value

inductor will help to reduce the output filter

capacitance for the same LED current ripple.

However, the larger value inductor will have a

larger physical size, higher series resistance,

and/or lower saturation current.

A good rule for determining the inductance to use

is to allow the peak-to-peak ripple current in the

inductor to be approximately 30% to 40% of the

LED current. Also, make sure that the peak

inductor current is below the maximum switch

current limit. The inductance value can be

calculated by:

OUT

OUT

sL

IN

VV

L1=

(1-

)

f

ΔIV

×

×

,

OUT

F

Vn V

=×

is the peak-to-peak inductor ripple current.

Choose an inductor that will not saturate under

the maximum inductor peak current. The peak

inductor current can be calculated by:

OUT

OUT

LP

LED

sIN

VV

II

(1

)

2f

L1

V

=+

× −

××

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 forward voltage

and recovery times, use a Schottky diode.

Choose a diode whose maximum reverse voltage

rating is greater than the maximum input voltage,

and whose current rating is greater than the

maximum load current. Table 1 lists example

Schottky diodes and manufacturers.

Table 1—Diode Selection Guide

Diodes

Voltage/Current

rating

Manufacture

B180-7-F

80V/1A

Diodes Inc.

CMSH2-

100M

100V/2A

Central Semi

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR capacitors for the best performance.

Ceramic capacitors are preferred, but tantalum or

low-ESR electrolytic capacitors may also suffice.

For simplification, choose the input capacitor with

RMS current rating greater than half of the

maximum load current. The input capacitor (C1)

can be electrolytic, tantalum or ceramic.

When using electrolytic or tantalum capacitors, a

small, high quality ceramic capacitor, i.e. 0.1μF,

should be placed as close to the IC as possible.

When using ceramic capacitors, make sure that

they have enough capacitance to provide

sufficient charge to prevent excessive voltage

ripple at input. The input voltage ripple caused by

capacitance can be estimated by:

OUT

OUT

LED

IN

sIN

IN

IN

VV

I

V1

fC

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠