MP3393—8-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3393 Rev. 1.05

www.MonolithicPower.com

13

3/26/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is programmable from 100kHz to 500kHz. A

resistor on the OSC pin sets the internal

oscillator frequency for the step-up converter

according to the equation:

160kHz.

Setting the LED Current

The LED string currents are identical and set

through the current sense resistor on the FB pin.

The FB pin cannot be left open.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. The input capacitor’s

impedance at the switching frequency should be

less than the input source impedance to prevent

high-frequency switching current from passing

through the input. Use ceramic capacitors with

X5R or X7R dielectrics for their low ESR and

small

temperature

coefficients.

For

most

applications, use a 4.7μF ceramic capacitor in

parallel with a 220µF electrolytic capacitor.

Selecting the Inductor and Current Sensing

Resistor

The device requires an inductor to force the

output voltage higher while being driven by the

input voltage. A larger-value inductor results in

less ripple current, resulting in lower peak

inductor current and reducing stress on the

external

N-Channel

MOSFET

of

step-up

converter. However, the larger-value inductor is

physically larger, has a higher series resistance

and lower saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. The minimum

value of inductor is selected to ensure that the

boost converter works in continuous conduction

mode for high efficiency and good EMI

performance.

Calculate the required inductance value by the

equation:

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I

××

× −

≥

××

OUT

IN

V

V

1

D

−

=

the LED load current, and η is the efficiency.

The switch current is usually used for the peak

current mode control. In order to avoid hitting the

current limit, the voltage across the sensing

SENSE

SENSE

L(PEAK)

0.8 V

R

I

×

=

OUT

LOAD

IN

OUT

IN

L(PEAK)

IN

SW

OUT

VI

V

(V

-V )

I

ηV

2 L f

V

××

=+

××

×

276

262

248

234

220

206

192

177

163

10

20

30

40

50

60

70

80

90

Current Limit vs.

Duty Cycle

0

50

100

150

200

250

300

0

20

40

60

80

100

Selecting External Bipolar Junction

Transistor

Each LEDs string has a bipolar junction transistor

(BJT) in order to regulate LED current. Voltage

rating and current rating of BJT should exceed