MP3394S—4-STRING, MAX 200mA/STRING WHITE LED DRIVER

MP3394S Rev. 1.03

www.MonolithicPower.com

9

3/21/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

Employing

the peak-current mode and

a

programmable constant frequency, the MP3394S

is a step-up LED driver with 4 channels or

regulated current sources to drive an array of up

to 4 strings of white LEDs.

Internal Regulator

The MP3394S includes an internal linear

regulator (VCC). When VIN is greater than 6V,

this regulator outputs a 5.8V power supply to the

external MOSFET switch’s gate driver and the

internal control circuitry. The VCC voltage drops

to 0V when the chip shuts down. The MP3394S

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 MP3394S checks the

topology connection first. The chip monitors the

over-voltage protection (OVP) pin to see if the

Schottky diode is connected or if the boost output

is shorted to GND. An OVP voltage of less than

75mV will disable the chip. The MP3394S 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.

It is recommended that the enable signal is given

after the establishment of the input voltage and

PWM dimming signal during the start-up

sequence.

Step-Up Converter

The

converter’s

operating

frequency

is

programmable (from 100kHz 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

output a signal to turn on the external MOSFET

(minimum turn on time is 100ns). 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

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 MP3394S 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