MP3387A ― 6-STRINGS, MAX 80MA/STRING, STEP-UP, WHITE LED DRIVER

MP3387A Rev. 1.0

www.MonolithicPower.com

12

5/17/2017

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© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is recommended to be between 500kHz and

1.2MHz for most applications. An oscillator

resistor on FREQ sets the internal oscillator

frequency for the step-up converter according to

Equation (1):

SW

OSC

62500

F(kHz)

R(k )





(1)

625kHz.

Setting the LED Current

The current of each string is identical and set

through the current setting resistor on ISET,

which can be calculated with Equation (2):

SET

SET

1.24(V)

I

1000

R(k)



(2)

cannot be open.

Selecting the Input Capacitor

The input capacitor reduces surge current drawn

from the input supply and 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 MP3387A 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

also has a larger physical size, higher series

resistance, and lower saturation current.

Choose an inductor that will not saturate under

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

Equation (3) and Equation (4):



 





2

OUT

SW

LOAD

η V

D (1 D)

L

2

f

I

(3)



IN

OUT

V

D1

V

(4)

the LED load current, and η is the efficiency.

With a 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 achieve a 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

Open-string protection is achieved through the

detection of the voltage on OVP. In some

unexpected cases, the part continues boosting

the output voltage higher and higher. If the output

voltage reaches the programmed OVP threshold,

the protection triggers.

To ensure that 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. The OVP voltage can be calculated

with Equation (5):

HIGH

OVP

LOW

R

V1.2(V) (1

)

R

 

(5)