MP3398D – 4-STRING, 350MA/STRING, STEP-UP, WHITE LED CONTROLLER

MP3398D Rev. 1.0

www.MonolithicPower.com

12

5/23/2016

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

© 2016 MPS. All Rights Reserved.

OPERATION

The MP3398D is a step-up converter with peak-

current mode control. It employs four channels

of current sources to drive up to four strings of

white LEDs.

Internal 6V Regulator

The MP3398D 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

MP3398D

also

has

under-voltage

lockout

(UVLO).

The

chip

is

disabled

until

VCC

exceeds the UVLO threshold. The UVLO

hysteresis is approximately 200mV.

System Start-Up

When enabled, the MP3398D checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) to determine if

the Schottky diode is not connected or if the

boost output is shorted to GND. An OVP

voltage less than 75mV will disable the chip.

The MP3398D also checks other safety limits

after passing the OVP test, including UVLO,

over-temperature protection (OTP), and over-

current protection (OCP). If all protection tests

pass, the chip starts boosting the step-up

converter with an internal soft start.

It is recommended that the enable signal occurs

after the establishment of the input voltage and

the PWM dimming signal during the start-up

sequence to prevent a large inrush current.

Step-Up Converter

The

converter

operating

frequency

is

programmable from 100kHz to 500kHz with an

external resistor connected to OSC. This helps

optimize efficiency and the size of the external

components.

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET.

During normal operation, the minimum turn on

time is 200ns. A stabilizing ramp added to the

output of the current sense amplifier prevents

subharmonic oscillations for duty cycles greater

than 50%. This result is fed into the PWM

comparator.

When

this

resulting

voltage

reaches the output voltage of the error amplifier

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 chooses the lowest active LEDX

pin voltage automatically to provide a sufficient

bus voltage to power all of the LED arrays.

If

the feedback voltage drops

below the

reference, the output of the error amplifier

increases. This results in more current flowing

through the MOSFET, increasing the power

delivered to the output and forming a closed

loop that regulates the output voltage.

Under light-load operation, especially in the

≈ VIN, the converter runs in pulse-

skipping mode. In this mode, the MOSFET

turns

on

for

a

minimum

on

time

of

approximately 200ns, 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 MP3398D provides two dimming methods:

PWM and analog dimming mode.

For PWM dimming, apply a PWM signal to

PWM. The LED current is chopped by this

signal, and the average LED current is equal to

PWM dimming signal.

For analog dimming, a PWM signal is applied to

ADIM. When a PWM signal is applied to ADIM,

this signal is filtered by the internal RC filter.

The LED current amplitude is equal to ISET x

signal. A PWM signal 20kHz or higher is

recommended for better filtering.