MP3388- 50V, 8-STRING WHITE LED DRIVERS

MP3388 Rev. 1.0

www.MonolithicPower.com

11

12/16/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is alternative for 1.25MHz or 625kHz. A bi-level

Switching Frequency Selection (OSC) input sets

the internal oscillator frequency for the step-up

converter. Tie OSC pin to GND corresponds to

the frequency 625kHz and tie OSC pin to VCC or

floating corresponds to 1.25MHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin.

19.9mA. The ISET pin can not be open.

Setting the Over Voltage Protection

The open string protection is achieved through

the over voltage protection (OVP). In some cases,

an LED string failure results in the feedback

voltage always zero. The part then keeps

boosting the output voltage higher and higher. If

the output voltage reaches the programmed OVP

threshold, the protection will be triggered.

To make sure the chip functions properly, the

OVP setting resistor divider must be set with a

proper value. The recommended OVP point is

about 1.3 times higher than the output voltage for

normal operation.

Selecting Dimming Control Mode

The MP3388 provides 4 different dimming

methods

1. PWM dimming mode with internal triangle

waveform generator

Apply a 100Hz to 50kHz square waveform to the

PWMI pin. The internal 400kΩ and external

capacitor on PWMO pin filters the dimming signal

to a DC voltage(0.2V~1.2V).Then the DC voltage

is modulated to an internal PWM dimming signal

whose frequency is set via the capacitor on FSET

pin according to the equation:

The minimum recommended amplitude of the

PWM signal is 2.1V (See Figure 3)

C1

FSET

PWMI

PWMO

MP3388

PWM Dimming

100Hz~50kHz

C2

Figure 3—PWM Dimming

with Internal Triangle Waveform Generator

2. Direct PWM dimming with positive logic

An external PWM dimming signal is directly

employed to achieve PWM dimming control.

Connect a 100kΩ resistor from FSET pin to GND

and apply the 100Hz to 2kHz PWM dimming

signal to PWMI pin. The minimum recommended

amplitude of the PWM signal is 1.5V (See Figure

4).

VCC

C1

FSET

PWMI

PWMO

MP3388

PWM Dimming

100Hz~2kHz

R1

100 kO

Figure 4—Direct PWM Dimming

with Positive Logic

3. Direct PWM dimming with negative logic

It is similar to method 2. Apply a 100Hz to 2 kHz

external square waveform to the PWMO pin for

negative logic PWM dimming. The minimum

recommended amplitude of the PWM signal is

1.5V (See Figure 5),

VCC

C1

FSET

PWMI

PWMO

MP3388

PWM Dimming

100Hz~2kHz

R1

100 kO

Figure 5— Direct PWM Dimming

with Negative Logic