MP4600 – WHITE LED DRIVER FOR UP TO 55 INCH TV BACKLIGHTING

MP4600 Rev. 0.91

www.MonolithicPower.com

6

1/6/2012

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Setting the LED Current

maximum LED current through the use of the

equation:

LED

SENSE

I

0.198V

R

=

Setting the Switching Frequency

following curve:

Frequency vs. RSET

0

200

400

600

800

1000

1200

1400

0

50

100

150

200

250

Series1

If RSET pin is floating, the setting frequency is

the default value 1.35MHz.

Selecting the Inductor

Inductor selection is related to the input voltage,

output voltage, and LED current. Select the

inductor to make the circuit always operate in

continuous

current

mode

(CCM).

The

inductance is designed as:

I N

O U T

S

I N

O U T

L

V

V

L

f ( V

V

)

I

⋅

=

⋅+

⋅ Δ

,

where

ripple. Design

inductor average current, which is:

O U T

L _

A V G

L E D

I N

V

I

I

( 1

)

V

=⋅ +

.

Make sure the inductor is not saturated at the

maximum peak current, which is:

L _

P K

L _

A V G

L

I

I

0 . 5

I

=+

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. Ceramic capacitors with

X5R

or

X7R

dielectrics

are

highly

recommended because of their low ESR and

small

temperature

coefficients.

Enough

capacitance is required for limited input voltage

ripple

5~10% of the DC value.

L _

A V G

O U T

I N

s

I N

I N

O U T

I

V

C

f

V

( V

V

)

⋅

>

⋅Δ

⋅

+

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple

the DC value) and ensures feedback loop

stable. The output capacitor impedance should

be low at the switching frequency. Ceramic

capacitors with X5R or X7R dielectrics are

recommended for their low ESR characteristics.

L E D

O U T

O U T

s

O U T

I N

O U T

I

V

C

f

V

( V

V

)

⋅

>

⋅Δ

⋅

+

PC Board Layout

The high current paths (VSS, VDD and SW)

should be aced very close to the device with

short, direct and wide traces. The input

capacitor needs to be as close as possible to

the VDD and VSS pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switch node traces short and away

from the feedback network.