MP3391—8-CHANNEL, 120mA/CH STEP-UP WLED DRIVER CONTROLLER

MP3391 Rev. 1.11

www.MonolithicPower.com

10

3/14/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3391 employs a programmable constant

frequency, peak current mode step-up converter

and 8-channels regulated current sources to

regulate the array of 8 strings white LEDs. The

number of LED string is selected by NUMSEL pin

for 8/6/4 LED strings. The operation of the

MP3391 can be understood by referring to the

block diagram of Figure 1.

Internal Regulator

The MP3391 includes two internal linear

regulators (VCC and VDD). VCC regulator offers

a 5V power supply for the internal control circuitry.

VDD regulator offers a 10V power supply for the

external MOSFET switch gate driver. The VCC

and VDD voltage drops to 0V when the chip

shuts down. The MP3391 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 MP3391 is enabled, 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 80mV, the chip will

be disabled. The MP3391 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

programmable (from 150kHz to 500kHz) with a

external set resistor on OSC pin, which is helpful

for optimizing the external components sizes and

improving the efficiency.

At the beginning of each cycle, the external

MOSFET 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

MOSFET is turned off.

The voltage at the output of the internal error

amplifier is an amplified signal of the difference

between the 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 450mV

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 remain

period. The external MOSFET will keep off until

the output voltage needs to be boosted again.

Dimming Control

The MP3391 provides two PWM dimming

methods: external PWM signal or DC input PWM

Dimming mode (see Figure 2). Both methods

results in PWM chopping of the current in the

LEDs for all 8 channels to provide LED control.

Figure 2—PWM Dimming Method

When bias the BOSC pin to a DC level, applying

a PWM signal to the DBRT pin to achieve the

PWM dimming. A DC analog signal can be

directly applied to the DBRT pin to modulate the

LED current with a capacitor on BOSC pin. And

the DC signal is then converted to a DPWM

dimming

signal

at

the

setting

oscillation

frequency.

The brightness of the LED array is proportional to

the duty cycle of the DPWM signal. The DPWM

signal frequency is set by the cap at the BOSC

pin.