MP3388- 50V, 8-STRING WHITE LED DRIVERS

MP3388 Rev. 1.0

www.MonolithicPower.com

9

12/16/2011

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

OPERATION

The MP3388 employs a constant frequency, peak

current mode step-up converter and 8-channels

regulated current sources to regulate the array of

8 strings white LEDs. The operation of the

MP3388 can be understood by referring to the

block diagram of Figure 1.

Internal 5V Regulator

The MP3388 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.

The MP3388 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 MP3388 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 MP3388 will also check

other safety limits, 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 cycle, the power FET is

turned with the internal clock. 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

result voltage reaches the output voltage of the

off.

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 below the 600mV

reference, the output of the error amplifier

increases. It 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 MP3388 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 8 channels to provide

LED control.

+

-

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