MP3308 – STEP-UP WHITE LED DRIVER WITH DUAL PWM DIMMING INPUT CONTROL

MP3308 Rev. 1.02

www.MonolithicPower.com

11

7/19/2012

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Selecting the Inductor

The inductor forces a higher output voltage

driven from a lower input voltage. A larger

inductor value results in less ripple current, a

lower peak inductor current, and reduces stress

on the internal N-Channel MOSFET. However,

larger-value inductor has a larger physical

footprint, higher series resistance, and lower

saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. Calculate the

required inductance value using the following

equations:

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.

Over-Voltage Protection

Open Load protection will shut off the MP3308 if

the

output

voltage

reaches

over-voltage

protection (OVP) threshold. LED failures will

result in the feedback voltage falling to zero,

which will cause the part to run at maximum duty

cycle and boost the output voltage. If the output

voltage exceeds the adjusted OVP threshold:

+

OV

R2

V=0.9V(1

)

R3

,

The MP3308 will shut down and will not remain

off until the power cycles.

Layout Considerations

Layout requires special consideration to avoid

electromagnetic interference from the high-

frequency switching path. The high-frequency

ripper current in the loop from the SW pin,

through the output diode, and output capacitor to

GND requires a connection loop that is as short

as possible.

The IN pin must be locally bypassed, with a

highly-recommended RC filter to eliminate noise.

See Figure 3 for an example of this circuit.

VIN

C1 C2

L1

B0530

D1

0.1

C4

100

R5

JP1

Bias

PGND

2

LED

12

SW

1

ISET

13

IN

3

GND

14

U1 MP3308

Figure 3—Input Bias Filtering