MP3384- 50V, 4-STRING WHITE LED DRIVER

MP3384 Rev. 1.01

www.MonolithicPower.com

10

1/29/2013

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APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

can be selected to 1.25MHz or 625kHz. A bi-level

Switching Frequency Selection (OSC) input sets

the internal oscillator frequency for the step-up

converter. When connects OSC pin to GND, the

switching frequency is set to 625kHz, when floats

the OSC pin or connects it to VCC, the frequency

is set to 1.25MHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin.

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 to a

proper value. The recommended OVP point is

about 1.3 times of the output voltage for normal

operation.

PWM

Dimming Mode

The MP3384 provides PWM dimming mode with

external PWM signal on PWM pin.

An external PWM dimming signal is directly

employed to achieve PWM dimming control.

Apply the 100Hz to 2kHz PWM dimming signal to

PWM pin. The minimum recommended amplitude

of the PWM signal is 1.5V (See Figure 2).

VCC

C1

PWM

MP3384

PWM Dimming

100Hz~2kHz

Figure 2— PWM Dimming

Selecting the Inductor

A 22μH (for 1.25MHz switching frequency) /47uH

(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 is sufficient.