MP4688 – 80V INPUT, 1A HIGH POWER LED DRIVER

MP4688 Rev. 1.12

www.MonolithicPower.com

8

3/28/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the LED Current

The LED current I

resistor R

and connected to GND. The voltage on the

sensing resistor R

FB

LED

FB

V

I

R

=

For example, for a 700mA LED current, R

287mΩ.

Inductor Selection and Frequency Setting

current to the LED. The inductor value is related

(

)

()

OUT

IN

OUT

s

8

IN

IN

LED

VV

V

f

V

8 10

V

L 15% I

−

⋅−

=

⋅⋅

⋅

+ ⋅

⋅

output voltage to drive the LEDs. A larger value

inductor

will

result

in

smaller

switching

frequency.

The peak inductor current is about:

8

IN

LP

LED

LED

410

V

II

1.075

LI

−

⎛⎞

⋅⋅

=⋅

+

⎜⎟

⋅

⎝⎠

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 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 whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum diode current.

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 especially under 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 by:

OUT

OUT

LED

IN

sIN

IN

IN

VV

I

V1

fC

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Choose the input capacitor with enough RMS

current rating and enough capacitance for small

input voltage ripple.

When electrolytic or tantalum capacitors are

applied, a small, high quality ceramic capacitor,

i.e. 0.1μF, should be placed as close to the IC

as possible.

MP4688. The LED current ripple (peak-to-peak

value) is about 15% of the LED DC current.

Output capacitor can be used to further reduce

the LED current ripple. Low ESR capacitors are

preferred to keep the output voltage ripple low

so that the AC ripple current through the LEDs

is small. The output voltage ripple can be

estimated by:

OUT

OUT

OUT

ESR

SO

IN

S

OUT

VV

1

V1

R

fL

V

8 f

C

⎛⎞

⎛⎞

Δ=

× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠