MP3397—4-STRING, MAX 350mA/STRING WHITE LED DRIVER

MP3397 Rev. 1.01

www.MonolithicPower.com

9

9/4/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

The MP3397 employs a programmable constant-

frequency, peak-current–mode step-up converter

with 4 channels or regulated current sources to

drive an array of up to 4 strings of white LEDs.

Internal 6V Regulator

The MP3397 includes an internal linear regulator

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

regulator outputs a 6V power supply to the

external MOSFET switch gate driver and the

internal control circuitry. The VCC voltage drops

to 0V when the chip shuts down. The MP3397

features under-voltage lockout (UVLO). The chip

is disabled until VCC exceeds the UVLO

threshold. The UVLO hysteresis is approximately

200mV.

System Startup

When enabled, the MP3397 checks the topology

connection first. The chip monitors the over-

voltage protection (OVP) pin to see if the

Schottky diode is not connected or if the boost

output is shorted to GND. An OVP voltage of less

than 70mV will disable the chip. The MP3397

also checks other safety limits, including UVLO

and over-temperature protection (OTP) after

passing the OVP test. If all the protection tests

pass, the chip then starts boosting the step-up

converter with an internal soft-start.

The enable signal must occur after the

establishment of the input voltage and PWM

dimming signal during the start-up sequence.

Step-Up Converter

The

converter

operating

frequency

is

programmable (from 150kHz to 500kHz) with an

external set resistor on the OSC pin. This

flexibility helps to optimize the size of external

components and improve the efficiency.

At the beginning of each cycle, the internal clock

turns on the external MOSFET. A stabilizing

ramp added to the output of the current sense

amplifier prevents sub-harmonic oscillations for

duty cycles greater than 50 percent. This result is

fed into the PWM comparator. When this

resulting voltage rises to the level of the error

MOSFET turns off.

The output voltage 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 to provide a high-

enough bus voltage to power all the LED arrays.

If the feedback voltage drops below the

reference, the output of the error amplifier

increases. This result in more current flowing

through the MOSFET, thus increasing the power

delivered to the output. This forms a closed loop

that regulates the output voltage.

the converter runs in pulse-skipping mode where

the MOSFET turns on for a minimum on-time of

approximately 100ns, and then the converter

discharges the power to the output for the

remaining period. The external MOSFET remains

off until the output voltage needs to be boosted

again.

Dimming Control

The MP3397 provides two PWM dimming

methods: external PWM signal or DC-input PWM

dimming mode (see Figure 2).

+

-

DPWM

Oscillator

DPWM

Comparator

DBRT

BOSC

Ex-PWM Input

DPWM Output

Figure 2—PWM Dimming Method