MP4000 – OFFLINE HIGH BRIGHTNESS WLED DRIVER CONTROLLER

MP4000 Rev. 1.3

www.MonolithicPower.com

8

11/7/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

LED Current Regulation and Zero-Current

Detection

With a floating buck converter configuration, as

shown in the typical application circuit, the

MP4000 turns off the MOSFET S1 with a peak

current control. The peak current is sensed with

peak current is regulated as:

REF

L _ PEAK

sense

V

I

R

=

.

In normal operation, MP4000 turns on S1 when

the current in the freewheeling diode goes to

zero. As a result, the average LED current is

well regulated as

REF

LED

sense

V

I

2R

=

⋅

.

The zero-current detection is realized at DRIVE

pin by sensing the MOSFET drain dv/dt current

through the S1’s miller cap. When the

freewheeling diode current goes to zero, S1

caused by the inductor and the parasitic caps.

dv/dt current through the miller cap changes

from negative to zero. At this time, the MP4000

turns on S1. As a result, the MP4000 turns on

S1 when the inductor current goes to zero and

S1 drain voltage is at minimum. MP4000

controls the buck converter operating in current

boundary conduction mode.

A cap Cout can be used in parallel with the LED

string to reduce the current ripple.

Such boundary operation mode can minimize

the S1 turn-on loss and eliminate the

freewheeling diode reverse recovery loss so

that high switching frequency is possible to

reduce passive components’ size. Furthermore,

the required inductance value is small, which

can help further inductor size reduction.

Brightness Dimming Control

The MP4000 employs a burst dimming control

scheme so that an external PWM signal can

control the dimming by varying the duty cycle,

or a DC signal can control the dimming by

varying the DC voltage level.

For the PWM input dimming control, a 100Hz to

2kHz PWM signal is recommended to connect

to the DIM pin. BOS pin is connected to GND

through a 300K

Ω resistor, setting about 1.2V

reference for the PWM input logic signal. PWM

input high will make the IC switching. PWM

input low turns off the IC.

For the DC input dimming control, a voltage

from 0V to 2.4V is recommended to connect to

the DIM pin. 2.4V and above is for the 100%

brightness. 120mV and below is for the 5%

from BOS pin to GBD to program the burst

BOS

BOS

BOSH

DIM

i

C

Vf

=

⋅

.

For applications that do not need burst dimming

control, open DIM pin and short BOS pin to

GND.

For some case that needs high dimming

resolution, where the dimming off time would be

smaller than the switching period, dimming ON

signal will force the MOSFET to turn on even

when the inductor current is not zero. If this

condition would happen in the application, ultra

fast recovery diode is recommended for the

freewheeling diode.

Frequency Setting and Inductor Design

In case the zero-current detection circuit fails,

which can happen in starting up with a large

output cap and in output short condition, a

maximum off time of about 1.7ms is applied to

make sure the device is still in operation and

the short current doesn’t run away.