MP4651—OFF LINE LED DRIVER

MP4651 Rev.1.0

www.MonolithicPower.com

8

1/13/2011

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© 2011 MPS. All Rights Reserved.

DESIGN INFORMATION

Steady State and Enable Control

The MP4651 is a fixed operating frequency off-

line LED driver, specifically designed for the high

power isolated applications. Powered by 9V to

30V input supplies, the MP4651 outputs two 180

degree phase shifted driving signals for the

external power stages. Its enhanced 9V gate

driver provides adequate driving capability to the

external MOSFETs and directly drives the

external gate driving transformer.

The MP4651 utilizes Pulse Width Modulation

control to the system. The operating frequency is

set by an external resistor connected from FSET

pin to GND. The LED current is fed back to FB

pin and compared with internal 0.6V reference

voltage.

Together

with

the

integrator

compensation network on COMP pin, which is

connected to the output of the error amplifier, the

output LED current is accurately regulated. The

voltage on COMP pin is compared with the

internal oscillator and generates duty cycle

modulated signals to control the external power

switches.

The system power is controlled by EN pin. When

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

powered up and the internal circuit starts.

Brightness Control

MP4651 implements burst dimming (digital

brightness) of the LED. The MP4651 has a built-

in burst oscillator which can generate a triangle

waveform on the BFS pin. Burst dimming can be

achieved by either a DC voltage input or external

PWM signal.

When burst dimming with a DC input voltage,

add a capacitor in parallel with a resistor on BFS

pin to set the burst frequency and apply the DC

voltage on the PWMIN pin to program the burst

duty cycle.

The burst frequency can be synchronized to an

external frequency. Applying a synchronizing

frequency signal with narrow pulse on SYNC pin

can synchronize the burst frequency. The

synchronizing frequency should be higher than

the burst frequency set by the BFS pin. Please

refer to SYNC pin description for details.

When burst dimming with external PWM signal,

pull up BFS pin to VCC through a 20kΩ resistor

and apply the PWM signal on PWMIN pin.

Fast and High Contrast Ratio PWM Dimming

The MP4651 implements fast and high contrast

ratio PWM dimming to the WLED. The PWM

dimming signal (controlled by a DC input voltage

or direct PWM signal) is outputted on PWMOUT

pin to drive the external MOSFET in series with

the LEDs, therefore the LED current rises up

immediately when PWM dimming signal is

effective.

The PWM dimming signal is also used to

disconnect the compensation network (on comp

pin, a capacitor or a R-C network) from the error

amplifier at PWM off interval, and so that the

compensation network voltage is hold at this

interval and gets nearly immediately to the

steady state value when PWM signal is effective.

It eliminates the control loop response time and

realizes fast dimming.

The MP4651 strictly controls the sequence of the

driving signals. Both the sequence of GL and GR

signals and the delay time between PWM

dimming signal and driving signals are accurately

fixed. Therefore, for each time of PWM dimming,

the driving signals are exactly the same and so

does the output power delivered to the load. It

eliminates the possibility of flicker at small PWM

dimming pulse and thus realizes the high

contrast ratio PWM dimming.

PWMOUT

PWMIN

COMP

Figure 2—Fast and High Contrast Ratio PWM

Dimming