MP3387L ― STEP UP, 6STRINGS,WHITE LED DRIVER

MP3387L Rev. 1.11

www.MonolithicPower.com

11

3/29/2016

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

© 2016 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended from 500kHz to 1.2MHz for

most of application. An oscillator resistor on

FREQ pin sets the internal oscillator frequency

for the step-up converter according to the below

equation:

SW

OSC

62500

F(kHz)

R(k )

=

Ω

to 625kHz.

Setting the LED Current

The current of each string is identical and set

through the current setting resistor on the ISET

pin.

SET

SET

1.21(V)

I1000

R(k)

=×

The ISET pin can not be 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

impedance at the switching frequency should be

less than the input source impedance to prevent

the

high-frequency

switching

current

from

passing through to the input. Use ceramic

capacitors with X5R or X7R dielectrics for their

low ESR and small temperature coefficients. For

most applications, a 4.7μF ceramic capacitor is

sufficient.

Selecting the Inductor

The MP3387L requires an inductor to supply a

higher output voltage while being driven by the

input voltage. A larger value inductor results in

less ripple current, lower peak inductor current

and less stress on the internal N-channel

MOSFET. However, the larger value inductor has

a larger physical size, higher series resistance,

and lower saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. Select the

minimum inductor value to ensure that the boost

converter works in continuous conduction mode

with high efficiency and good EMI performance.

Calculate the required inductance value using the

equation:

××

× −

≥

××

2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I

=−

IN

OUT

V

D1

V

the LED load current, and η is the efficiency.

With given inductor value, the inductor DC

current rating is at least 40% higher than the

maximum input peak inductor current for most

applications. The inductor’s DC resistance should

be as small as possible to get higher efficiency.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance must be low at

the switching frequency. Ceramic capacitors with

X7R dielectrics are recommended for their low

ESR characteristics. For most applications, a

2.2μF ceramic capacitor is sufficient.

Setting the Over Voltage Protection

The open string protection is achieved through

the detection of the voltage on the OVP pin. In

some unexpected cases, the part keeps boosting

the output voltage higher and higher. If the output

voltage reaches the programmed OVP threshold,

the protection will be triggered.

To ensure the chip functions properly, select the

resistor values for the OVP resistor divider to

provide

an

appropriate

set

voltage.

The

recommended OVP point is about 1.1 to 1.2

times higher than the output voltage for normal

operation.

HIGH

OVP

LOW

R

V1.2(V) (1

)

R

=× +