MP4603 – WHITE LED DRIVER FOR LARGE SIZE TV BACKLIGHTING

MP4603 Rev. 1.0

www.MonolithicPower.com

11

4/27/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.MP4603

APPLICATION INFORMATION

The MP4603 is a buck-boost LED driver. Its

novel

power

leverage

control

technology

provides a highly efficient, low-cost solution for

LCD TV LED drivers. It has a high bus voltage

and a low supply voltage (typically 12V or 24V,

up to 60V) to drive LED backlight strings

of >350V for LCD TVs measuring 60-inch or

more.

Setting the LED Current

current as per:

FB

LED

0.200V

R

I



Setting the Switching Frequency



S

FST

60k

f0.95MHz

R

This equation applies to a programmable

frequency range between 200kHz and 2MHz.

switching frequency is set to default value of

0.9MHz.

Setting the Slope Compensation

The

MP4603

employs

peak-current–mode

control, which needs slope compensation to

avoid sub-harmonic oscillation when the duty

cycle exceeds 50%.

The current loop has effective sense resistance

of 0.4Ω. Given a desired input/output voltage

relationship, estimate the sense current ramp-

down slope as:

L

DOWN

V

S0.4V/ s

L





volt; and L is the inductor value in μH.

Ensuring current loop stability requires a

compensation slope of at least half of the ramp-

down slope:

SC

DOWN

1

SS

2



compensation for the current loop as per:

SC

SLOPE

60k

V

S= 0.6

*

s R



The equation is effective only for a resistor range

400k

, the slope compensation is set to default

value of 0.5V/μs.

Selecting the Inductor

The input voltage, output voltage, and LED

current factor into inductor selection. In addition,

select the inductor so that the circuit always

operates in continuous current mode (CCM).

Estimate the inductor value using the following

equation:

IN

OUT

SIN

OUT

L

VV

L

f(V

V

)

I







 

Where

ripple. Design

60% of the inductor average current, which is:

OUT

L _ AVG

LED

IN

V

II

(1

)

V

 

.

Select an inductor that does not saturate at the

maximum peak current, which is:

L_PK

L _ AVG

L

II

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. Use ceramic capacitors

with X5R or X7R dielectrics because they have

low

ESR

values

and

small

temperature

coefficients.

Select

a

sufficiently-large

capacitance to limit the input voltage ripple

(

DC value.

L_ AVG

OUT

IN

sIN

IN

OUT

IV

C

fV

(V

V

)









