MP3331-2A SYNCHRONOUS BOOST FLASH LED DRIVER WITH I

MP3331 Rev. 1.02

www.MonolithicPower.com

23

5/3/2016

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APPLICATION INFORMATION

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. A

higher value capacitor helps reduce the input

voltage ripple and noise. For most applications, a

10μF ceramic capacitor will suffice.

Selecting the Inductor

The converter 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, resulting in lower peak

inductor current and reducing stress on the

internal N-channel MOSFET. However, the larger

value inductor has a larger physical size, a higher

series resistance, and a 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

(CCM) with high efficiency and good EMI

performance.

Calculate the required inductance value using

Equation (1) and Equation (2):

2

OUT

SW

LOAD

η V

D (1D)

L

2

f

I



 





(1)

IN

OUT

V

D1

V



(2)

the LED load current, and η is the efficiency.

The switching current is used for the peak current

mode control. In order to avoid reaching the

current limit, the worst-case inductor peak current

For most applications, 0.47uH~2uH inductor will

suffice.

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. Please note that ceramic

capacitance is dependent on the voltage rating; a

DC bias voltage and the value can lose as much

as 50% of its capacitance at its rated voltage

rating. Please leave a large enough voltage

rating margin when selecting the component. In

addition, too low a capacitance value will cause

loop instability. For most applications, a 10μF

ceramic capacitor will suffice.