MP3399-12-STRING, CASCADING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3399 Rev. 1.0

www.MonolithicPower.com

10

3/27/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3399 employs a programmable constant

frequency, peak current mode step-up converter

and 12-channels regulated current sources to

regulate the array of 12 strings white LEDs. The

operation of the MP3399 can be understood by

referring to the block diagram of Figure 1.

Internal 6V Regulator

The MP3399 includes an internal linear regulator

(VCC). When VIN is greater than 6.5V, this

regulator offers a 6V 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 6.5V, tie VCC and VIN together.

The MP3399 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 MP3399 is enabled, the chip checks

the topology connection first. The VFAULT pin

drives the external Fault Disconnection PMOS to

turn on slowly. Then 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 MP3399 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 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 remain

period. The external MOSFET will keep off until

the output voltage needs to be boosted again.

Dimming Control

The MP3399 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 12 channels to provide LED control.

+

-

DPWM

Oscillator

DPWM

Comparator

DBRT

BOSC

Ex-PWM Input

DPWM Output

Figure 2—PWM Dimming Method