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

MP3388S Rev.1.01

www.MonolithicPower.com

10

10/29/2012

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© 2012 MPS. All Rights Reserved.

(a)

(b)

(c)

Figure 4: Recommended Power-Off Sequence

Step-Up Converter

The converter operation frequency is selectable

(625kHz or 1.25MHz), which can optimize for

external component sizes and improve efficiency.

At the beginning of each cycle, the internal clock

turns on the power MOSFET. To prevent

sub-harmonic oscillations at D>50%, add a

stabilizing ramp to the output of the current-sense

amplifier; the output goes to the positive input of

the

PWM

comparator.

When

the

PWM

comparator’s positive input voltage equals the

power MOSFET turns off.

The

internal

error

amplifier

amplifies

the

difference between the 600mV reference voltage

and the feedback voltage. The converter

automatically chooses the lowest active LEDx pin

voltage to provide the bus voltage to power all the

LED arrays.

If the feedback voltage drops below the 600mV

reference, the output of the error amplifier

increases to increase the current flowing through

the power MOSFET, thus increasing the power

delivered to the output. This forms a closed loop to

regulate the output voltage.

At

light-load

operation—or

converter enters pulse-skipping mode where the

MOSFET turns on for a minimum ON-time of

approximately

100ns

before

the

converter

discharges the power to the output. The MOSFET

repeats this cycle until the output voltage requires

a boost.

Dimming Control

The MP3388S provides two PWM dimming

methods: external PWM signal from the PWMI pin

or DC-input PWM dimming mode (see Figure 5).

Figure 5: PWM Dimming

When applying a PWM signal to the PWMI pin, the

MP3388S generates a DC voltage on the PWMO

pin that is proportional to the duty cycle of the

PWMI signal. By comparing the PWMO pin signal

to the FPWM pin triangle waveform, the converter

gets a choppy low-frequency signal with a duty

cycle the same as the input’s. This choppy

low-frequency signal modulates the LED current.

Directly applying a DC analog signal to the PWMO

pin also modulates the LED current: the DC signal

is converted to a DPWM signal set at the

oscillation frequency. The polarity is negative. The

brightness of the LED array is proportional to the

duty cycle of the DPWM signal. The capacitor at

the FPWM pin sets the DPWM signal frequency.