MP3309L Rev. 1.01

www.MonolithicPower.com

9

7/18/2017

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

© 2017 MPS. All Rights Reserved.

APPLICATION INFORMATIO

Selecting the Switching Frequency

The switching frequency of the step-up

converter can be programmed from 300kHz to

1.2MHz. A resistor on the OSC pin sets the

internal oscillator frequency for the step-up

converter as per the equation below:

SW

OSC

120000

f(kHz)

R(k )





frequency is to 600kHz.

Setting the LED Current

Set the LED current through the current-setting

resistor on the FB pin.

REF

LED

FB

V(V)

I(mA)

1000

R( )





LED current is 20mA. Do not leave the FB pin

floating.

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, use a 2.2µF-to-10μF

ceramic capacitor.

Selecting the Inductor

The MP3309L requires an inductor to boost its

output voltage. A larger value inductor results in

less ripple current, lowering both the peak-

inductor current and the stress on the internal

N-channel MOSFET. However, the larger

inductor is physically larger, has 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 with high efficiency and good EMI

performance.

Calculate the required inductance value using

the equation:

2

OUT

SW

LOAD

η V

D (1D)

L

2

f

I



 





IN

OUT

V

D1

V



the LED load current, and η is the efficiency.

The switching current for peak-current mode.

To avoid hitting the current limit, the worst-case

inductor peak current should be less than 80%

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. Selection must also

account for the capacitance’s dependence on

the voltage rating; with a DC bias voltage, the

capacitor can lose as much as 50% of its

capacitance at its rated voltage rating. Leave a

sufficient voltage rating margin when select the

component.

Too-low or too-high capacitance will cause loop

instability. For most applications, select a

capacitor in the range of 0.1µF to 1μF.

Layout Considerations

Careful attention must be paid to the PCB

board layout and components placement.

Proper layout of the high frequency switching

path is critical to prevent noise and limit

electromagnetic

interference.

The

loop

consisting of MP3309L’s internal low-side

MOSFET, synchronous MOSFET, and output

capacitor contains a high-frequency ripple

current—minimize this loop. Place the input and

output capacitor should as to the IC as possible.