MP4601 – WHITE LED DRIVER FOR LARGE SIZE TV BACKLIGHTING

MP4601 Rev. 1.0

www.MonolithicPower.com

12

1/28/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

MP4601 is a Buck-boost mode LED driver. Its

novel

power

leverage

control

technology

provides a high efficiency, low cost solution for

not recommend higher than 60V), MP4601 can

drive over 60 inch or larger size panel for over

350V LED string voltage.

Setting the LED Current

maximum LED current through the use of the

equation:

FB

LED

0.200V

R

I

=

Setting the Switching Frequency

equation:

S

FST

60k

f0.95MHz

R

=⋅

The equation is effective only for 200kHz to

2MHz frequency programming range.

500k

Ω, the setting frequency is the default

value 0.95MHz.

Setting the Current Limit

of the equation:

LIM

OCP

100k

I4.1A

1.1A

R

=⋅

−

The equation is effective only for a range from

500k

Ω, the current limit is the default value of

2.5A.

Setting the Slope Compensation

MP4601 employs peak current mode control,

which will need slope compensation to avoid

sub-harmonic oscillation when duty cycle

exceeds 50%.

The current loop has effective sense resistor of

0.4Ω. Given a desired input, output voltage

relationship, one can estimate the sense

current ramp down slope as:

L

DOWN

V

S0.4

L

=⋅

V/µs

Where VL is the voltage across the inductor,

and L is the inductor value.

To

ensure

current

loop

stability,

a

compensation slope of at least half of the ramp

down slope is needed:

SC

DOWN

1

SS

2

>=

the slope compensation for the current loop

through the use of the equation:

SC

SLOPE

60k

V

S1.2

s R

=⋅

µ

The equation is effective only for a range from

20k to 500k for the programming resistor

500k

Ω, the slope compensation is set to default

value of 1.2V/µs.

FAULT condition output

MP4601 has an open drain output to indicate

FAULT condition. During normal operation,

FAULT is high-Z output and user can pull it up

to any desired voltage with an external resistor.

However, if for some reason, FB is greater than

600mV above VSS, or FB is greater than

300mV above VSS for a pre-determined time of

level. The Rdson for this pull down switch is

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:

IN

OUT

SIN

OUT

L

VV

L

f(V

V

)

I

⋅

=

⋅

+⋅ ∆