MP4603 – WHITE LED DRIVER FOR LARGE SIZE BACKLIGHTING

MP4603 Rev1.0

www.MonolithicPower.com

15

4/27/2012

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© 2012 MPS. All Rights Reserved.MP4603

Figure 5: LED Driver Schematic Used for Evaluation (EV4603-S-00A)

Setting Current Sense Resistor

FB

LED

0.200V

R

I



Use two 3.32Ω resistor in parallel (R15 and R16)

as the LED current sensor resistor.

Setting the Frequency Set Resistor

The

following

equation

determines

the

frequency set resistor, R18:

S

0.95MHz 60k

R18

f





Select 300kΩ to set the switching frequency to

about 200kHz.

Selecting the Inductor

Select an inductor such that the circuit always

operates in CCM as per the following equation:







 



IN_min

OUT_max

OUT_max

S

IN_min

OUT_max

LED

IN_min

VV

L1

V

f(V

V

) η I(1

)

V

Where

 is about 40% to 60%. Select an

inductance of 100μH and the inductor peak-to-

peak current of 0.49A.

Select an inductor that does not saturate at the

maximum peak current, which is:

L_ PK

L_ AVG

L

II

0.5

I



 

Select L1=100μH with a saturation current of

1.5A

Setting the Slope Compensation

The voltage across the inductor in the buck-

boost converter is the output voltage. The

maximum slope compensation is:





OUT_max

DOWN

V

S0.4V/ s

L

To ensure current-loop stability, select a

compensation slope that is at least half of the

needed ramp-down slope:

SC

DOWN

1

SS

2



for the current loop as per the following

equation:

SC

V

0.6

s

R19 60kΩ

S





Based

on

this

equation,

the

slope

compensation resistor cannot exceed 330kΩ.