MP3386- 50V, 6 STRINGS WHITE LED DRIVERS

MP3386 Rev. 1.0

www.MonolithicPower.com

12

1/19/2012

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

© 2012 MPS. All Rights Reserved.

VCC

C1

FSET

PWMI

PWMO

MP3386

PWM Dimming

100Hz~2kHz

R1

100 k

Figure 5— Direct PWM Dimming

with Negative Logic

4, DC input PWM dimming

To apply DC input PWM dimming, apply an

analog signal (range from 0.2V to 1.2V) at the

PWMO pin to modulate the LED current directly.

If the PWMO is applied with a DC voltage<0.2V,

the PWM duty cycle will be 100%. If the PWMO

pin is applied with a DC voltage>1.2V, the output

will be 0% (See Figure 6). The capacitor on

FSET pin set the frequency of internal triangle

waveform.

C1

FSET

PWMI

PWMO

MP3386

Analog Dimming

0.2V~1.2V

Figure 6—DC input PWM dimming

Selecting the Inductor

A 10μH (for 1.25MHz switching frequency) /22uH

(for 625kHz switching frequency) inductor with a

DC current rating of at least 40% higher than the

maximum input current is recommended for most

applications. For highest efficiency, the inductor’s

DC resistance should be as small as possible.

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

high frequency switching current from passing

through the input. Ceramic capacitors with X5R

or X7R dielectrics are highly recommended

because of their low ESR and small temperature

coefficients. For most applications, a 4.7μF

capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance should 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 will be sufficient.

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 electromagnetic interference

problems. The loop of MP3386 SW to PGND pin

(U1), output diode (D1), and output capacitor (C2)

is flowing with high frequency pulse current. it

must be as short as possible (See Figure 7).

Figure 7—Layout Consideration

The IC exposed pad is internally connected to

GND pin, and all logic signals are refer to the

GND. The PGND should be externally connected

to GND and is recommended to keep away from

the logic signals.