MP4652—HIGH PERFORMANCE OFF-LINE TV LED DRIVER

MP4652 Rev.1.01

www.MonolithicPower.com

9

9/23/2011

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

© 2011 MPS. All Rights Reserved.

OPERATION

Steady State and Enable Control

The MP4652 is a high-performance, off-line LED

driver specifically designed for high-power

isolated applications such as LED backlighting

for TVs. Powered by a 9V to 30V input supply,

the MP4652 outputs two 180-degree phase-

shifted gate driver signals for external power

stages. Its enhanced 9V gate driver provides

adequate driver capability to the external

MOSFETs and directly drives the external

MOSFETs through a gate drive transformer. The

MP4652 can be used to control LLC, half-bridge,

flyback, and other power stages.

The MP4652 can accurately regulate the LED

output current using both PWM control and a

compensation network on the COMP pin. PWM

control uses an external resistor connected from

FSET pin to GND to set the operating frequency.

The LED current feeds back to the FB pin with a

sample-hold circuit and compared against an

internal

0.6V

reference

voltage.

The

compensation network on the COMP pin, which

connects to the output of the error amplifier, then

accurately regulates the output LED current. The

voltage on COMP pin is compared with the

internal oscillator and generates duty cycle

modulated signals to control the external power

switches. This PWM control makes MP4652

suitable for half-bridge, flyback, and other power

stages.

The MP4652 FSET pin can also take voltage

feedback from a frequency-controlled external

circuit to adjust the device frequency. Connect

this feedback circuit to FSET using a resistor.

This frequency control makes MP4652 suitable

for LLC and other frequency-controlled power

stages.

The system power is controlled by EN pin. When

the chip is enabled, the built-in regulator for VCC

powers up the internal circuit. When VCC exceeds

its UVLO point, IC starts to operate and outputs the

gate drive signals.

Brightness Control

MP4652 implements PWM dimming on the LED

current by using either a DC input voltage or a

direct PWM input signal. The MP4652 has a

built-in burst oscillator that can generate a

triangle waveform on the BFS pin.

When using a DC input voltage for PWM

dimming, connect a capacitor in parallel to a

resistor on BFS pin to set the burst frequency

and apply the DC voltage to the PWMIN pin to

program the PWM dimming duty cycle.

The burst frequency can also be synchronized to

an

external

frequency

by

applying

a

synchronizing narrow-pulse signal on the SYNC

pin. The synchronizing frequency should be

higher than the burst frequency set by the BFS

pin. Please refer to SYNC pin description for

details.

When using a direct PWM input signal for PWM

dimming, use a 20kΩ pull-up resistor between

the BFS pin to VCC and apply the PWM signal

on PWMIN pin.

Continuous Fast PWM Dimming

The MP4652 implements fast and continuous

PWM dimming on the LED current, as shown in

Figure 2. The PWM dimming signal (controlled by

a DC input voltage or a direct PWM signal)

outputs from the PWMOUT pin to drive the

external dimming MOSFET in series with the

LED string. Therefore, the LED current quickly

rises when the PWM dimming signal goes high,

and quickly falls when PWM dimming signal falls.

This fast PWM dimming feature helps the

MP4652 achieve a high PWM dimming ratio.

The MP4652 provides continuous PWM dimming

to the system. It outputs continuous gate driver

signals to the power stage during both PWM on

and PWM off intervals. This makes the power

flow continuous for magnetic components—such

as transformers and inductors—which can

eliminate audible noise.

With this fast and continuous PWM dimming

feature, the MP4652 can achieve 1000:1 high