MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER

MP3388S Rev.1.01

www.MonolithicPower.com

12

10/29/2012

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© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the Switching Frequency

The user can select either a 625kHz or a 1.25MHz

step-up converter switching frequency. A bi-level

switching frequency selection input (the OSC pin)

sets the internal oscillator frequency. Tie the OSC

pin to GND corresponds for 625kHz, and to VCC

or float for 1.25MHz.

Setting the LED Current

The ISET pin sets the LED string currents to the

same level as per the equation below:

SET

LED

R

V

.

I

21

1

1000





not leave the ISET pin open.

Setting the Over-Voltage Protection Threshold

The open string protection is achieved through

OVP. In some cases, an LED string failure results

in a 0V feedback voltage. The MP3388S then

continues boosting the output voltage. OVP

triggers if the output voltage reaches the

programmed OVP threshold.

To ensure proper chip function, set the OVP

setting resistor divider with appropriate values.

Select an OVP threshold about 1.3x higher than

the output voltage for normal operation.

2

2

1

23

1

R

R

R

.

V

OVP







Selecting the Dimming-Control Mode

The MP3388S provides four different dimming

methods:

PWM Dimming Mode with Internal Triangle

Waveform Generator

Apply a 100Hz-to-50kHz square waveform to the

PWMI pin. The internal 400kΩ resistor and the

external capacitor on the PWMO pin filters the

dimming signal to a DC voltage (0.2V to 1.2V).

Then an internal PWM dimming signal whose

frequency is set by a capacitor on the FPWM pin

modulates the DC voltage, where:

FPWM

DPWM

C

F

.

f





5

3

The

minimum

recommended

PWM

signal

amplitude is 2.1V (See Figure 6).

Figure 6: PWM Dimming with Internal Triangle

Waveform Generator

Direct PWM Dimming with Positive Logic

To use a direct external PWM dimming signal,

connect a 100kΩ resistor from the FPWM pin to

GND and apply a 100Hz-to-30kHz (high SW

frequency) PWM dimming signal to the PWMI pin.

The minimum recommended amplitude of the

PWM signal is 1.5V (See Figure 7).

Figure 7: Direct PWM Dimming with Positive

Logic

Table 1 shows the PWM dimming duty range with

different PWM dimming frequencies.