MP3398E

– 4-STRING, 80V, STEP-UP, WLED CONTROLLER

MP3398E Rev. 1.02

www.MonolithicPower.com

11

7/31/2017

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

© 2017 MPS. All Rights Reserved.

OPERATION

The MP3398E is a programmable constant

frequency,

peak-current

mode,

step-up

converter with 4-channel regulated current

sources to drive an array of up to four strings of

white LEDs.

Internal 5.9V Regulator

The MP3398E

includes

an internal linear

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

this regulator outputs a 5.9V 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

MP3398E

features

under-voltage

lockout

(UVLO).

The

chip

is

disabled

until

VCC

exceeds the UVLO threshold. The UVLO

hysteresis is approximately 350mV.

System Start-Up

When enabled, the MP3398E checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to determine

if the Schottky diode is connected or if the boost

output is shorted to GND. An OVP voltage

higher than 55mV allows the chip to switch

normally; otherwise, switching is disabled. The

MP3398E also checks other safety limits,

including UVLO, over-temperature protection

(OTP), and over-current protection (OCP) after

passing the OVP test. If all protection tests pass,

the chip then begins boosting the step-up

converter with an internal soft start.

It is recommended that the enable signal occur

after the establishment of the input voltage and

PWM

dimming

signal

during

the

start-up

sequence to prevent a large inrush current.

Step-Up Converter

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET.

During normal operation, the minimum turn-on

time is around 150ns. 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 the summed

voltage reaches the output voltage of the error

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 chooses the lowest active LEDX

pin voltage automatically to provide a high

enough 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

skipping mode. In this mode, the MOSFET

turns on for a minimum on time, 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 MP3398E 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

PWM signal, and the average LED current is

cycle of the PWM dimming signal, and ISET is

the LED current amplitude.

For analog dimming, apply a PWM signal to

ADIM. An internal R-C filter (10M

Ω resistor and

100pF capacitor) is integrated to ADIM. This

PWM signal is filtered to the DC voltage by the

internal R-C filter. The LED current amplitude is

cycle of the PWM dimming signal, and ISET is

the LED current amplitude. A PWM signal

20kHz or higher is recommended for better

filtering.

Operation Switching Frequency

The

converter

operating

frequency

is

set

through an external resistor on OSC. This helps

optimize

both

the

size

of

the

external

components and the system efficiency

.