STEP UP, 4 STRINGS, MAX. 350MA/STRING ANALOG AND PWM DIMMING, WHITE LED CONTROLLER

MP3398A Rev. 1.04

www.MonolithicPower.com

12

5/26/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP3398A is a step-up converter with peak-

current mode. It employs 4 channels of current

sources to drive up to 4 strings of white LEDs.

Internal 6V Regulator

The MP3398A

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 MP3398A

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 MP3398A 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 75mV will disable the chip. The

MP3398A also checks other safety limits,

including

UVLO

and

over-temperature

protection (OTP), over-current protection 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.

It is recommended that the enable signal occurs

after the establishment of the input voltage and

PWM

dimming

signal

during

the

start-up

sequence to avoid large inrush current.

Step-Up Converter

The

converter

operating

frequency

is

programmable (from 100kHz to 500kHz) with a

external resistor connected to the OSC pin.

This helps to optimize the size of external

components and the efficiency.

At the beginning of each switching cycle, the

internal clock turns on the external MOSFET (In

normal operation, the minimum turn on time is

200ns). 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

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 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 results 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.

Under light-load operation, especially in the

pulse-skipping mode where 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 MP3398A provides two dimming methods:

PWM or analog dimming mode.

For PWM dimming, apply a PWM signal to

PWM pin. The LED current is chopped by this

PWM signal and the average LED current is

equal to Iset*D_dim where D_dim is the duty

cycle of PWM dimming signal and Iset is the

LED current amplitude.

For analog dimming, either a PWM signal or DC

signal can be applied to ADIM pin.

When a PWM signal is applied to ADIM pin, this

signal will be filtered by the internal RC filter.

The

LED

current

amplitude

is

equal

to

Iset*D_Dim where D_dim is the duty cycle of

PWM dimming signal and Iset is the LED

current amplitude. 20kHz or higher PWM signal

is recommended to get the better filtering

performance.