MP3386- 50V, 6 STRINGS WHITE LED DRIVERS

MP3386 Rev. 1.0

www.MonolithicPower.com

9

1/19/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

The MP3386 employs a constant frequency,

peak current mode step-up converter and 6-

channels regulated current sources architecture

to regulate the array of 6 strings white LEDs. The

operation of the MP3386 can be understood by

referring to the block diagram of Figure 1.

Internal 5V Regulator

The MP3386 includes an internal linear regulator

(VCC). When VIN is greater than 5.5V, this

regulator offers a 5V power supply for the internal

MOSFET switch gate driver and the internal

control circuitry. The VCC voltage drops to 0V

when the chip shuts down. In the application of

VIN smaller than 5.5V, tie VCC and VIN together

and connect them to an external 5V power

supply. The MP3386 features Under Voltage

Lockout. The chip is disabled until VCC exceeds

the UVLO threshold. And the hysteresis of UVLO

is approximately 200mV.

System Startup

When the MP3386 is enabled, the chip checks

the topology connection first. The VFAULT pin

drives the external Fault Disconnection PMOS to

turn on slowly. And after 400us delay, the chip

monitors the OVP pin to see if the Schottky diode

is not connected or the boost output is short to

GND. If the OVP voltage is lower than 70mV, the

chip will be disabled and the external PMOS is

turned off together. The MP3386 will also check

other safety limit, including UVLO and OTP after

the OVP test is passed. If they are all in function,

it then starts boosting the step-up converter with

an internal soft-start.

It is recommended on the start up sequence that

the enable signal comes after input voltage and

PWM dimming signal established.

Step-up Converter

The converter operation frequency is selectable

(1.25MHz or 625kHz), which is helpful for

optimizing the external components sizes and

improving the efficiency.

At the beginning of each oscillator cycle, the

power FET is turned on. To prevent sub-

harmonic oscillations at duty cycles greater than

50 percent, a stabilizing ramp is added to the

output of the current sense amplifier and the

result is fed into the PWM comparator. When this

voltage equals to the output voltage of the error

The voltage at the output of the internal error

amplifier is an amplified signal of the difference

between the 600mV reference voltage and the

feedback voltage. The converter automatically

chooses the lowest active LEDX pin voltage for

providing enough bus voltage to power all the

LED arrays.

If the feedback voltage drops to smaller than the

600mV reference, the output of the error amplifier

increases. This results in more current flowing

through the power FET, thus increasing the

power delivered to the output. In this way it forms

a close loop to make the output voltage in

regulation.

At light-load or Vout near to Vin operation, the

converter runs into the pulse-skipping mode, the

FET is turned on for a minimum on-time of

approximately 100ns, and then the converter

discharges the power to the output in the

remnant period. The FET will keep off until the

output voltage needs to be boosted again.

Dimming Control

The MP3386 provides several PWM dimming

methods: external PWM signal from PWMI pin or

DC input PWM Dimming mode (see Figure 2).

These methods results in PWM chopping of the

current in the LEDs for all 6 channels to provide

an average LED current.

+

-

DPWM

Oscillator

DPWM

Comparator

400 k?

1.2V

0.2V

PWMO

PWMI

FSET

Ex-PWM Input

DPWM Output

1.2V

0.2V

DC Input

Figure 2—PWM Dimming Method