MP4655–PURE, SINGLE-STAGE, LLC, LED CURRENT AND SYSTEM VOLTAGE CONTROLLER

MP4655 Rev 1.0

www.MonolithicPower.com

20

2/2/2016

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© 2016 MPS. All Rights Reserved.

LED current is regulated by this control loop and

the LLC operating frequency is programmed by

the ICOMP voltage.

During the PWM off interval, S1 is turned off, and

the

compensation

network

on

ICOMP

is

disconnected from the error amplifier and holds

its value until the next PWM on interval.

The MP4655 integrates burst mode for the LED

current regulation. When the IFB voltage is

higher than 1.08 times its reference voltage and

the ICOMP voltage is low enough (which means

it is at its highest operating frequency), the IC

skips some switching cycles until the IFB voltage

decreases sufficiently.

2. Output system voltage regulation at PWM

dimming off (VCOMP loop)

For the system voltage regulation loop, the

output system voltage is fed back to VFB. During

the PWM off interval, the MP4655 regulates the

system voltage through the LLC frequency

control, and the extra N-channel MOSFET is on.

The internal voltage-loop error amplifier regulates

the average value of the VFB voltage to the

reference voltage (VREF). Its output is connected

to

the

external

voltage-loop

compensation

network on VCOMP through an inner switch S2.

During the PWM off interval, S2 is on, and the

output of the voltage-loop error amplifier is

connected to the external compensation network

on VCOMP. The output system voltage is

regulated by this control loop and the LLC

operating frequency is programmed by VCOMP.

During the PWM on interval, S2 is turned off, and

the compensation network on VCOMP is

disconnected from the error amplifier and holds

its value until the next PWM off interval.

The MP4655 also integrates burst mode for

voltage regulation with the LLC frequency control

through the VCOMP loop. When VFB is higher

than 1.05 times the reference voltage and

VCOMP is sufficiently low (which means a high

LLC operating frequency), the IC skips some

switching cycles until VFB voltage decreases

sufficiently.

The LLC operating frequency is controlled by the

output of the current loop error amplifier on

ICOMP at PWM dimming on and is controlled by

the voltage loop error amplifier on VCOMP at

PWM dimming off. A higher compensation output

voltage results in a lower operating frequency.

3. Output system voltage regulation at PWM

dimming on (DCOMP loop)

During the PWM on interval, the LLC frequency

control regulates the LED current, and the output

system voltage is regulated through the duty

cycle of the extra N-channel MOSFET. The

output system voltage is fed back on VFB, and

the internal error amplifier regulates its average

value to its reference voltage (VREF). The output

of this error amplifier is connected to DCOMP

through an inner switch (S3). S3 is on during the

PWM on interval, and the output of this error

amplifier

is

connected

to

the

DCOMP

compensation network. Together with slope

compensation, the DCOMP voltage is compared

with the current through the output system

voltage stage, which is fed back to VOCP and

determines the duty cycle of the extra N-channel

MOSFET. The duty cycle of this extra N-channel

MOSFET can achieve a 0 to 100% range. The

MP4655 integrates individual controls on the

extra N-channel MOSFET and achieves soft

switching with no voltage spike.

During the PWM off interval, S3 is disconnected

and DCOMP holds its voltage until the next PWM

on interval. The extra N-channel MOSFET is

forced on during the PWM off interval.

Both the LED current and output system voltage

are accurately regulated at PWM dimming on

and PWM dimming off. ICOMP, VCOMP, or

DCOMP holds its voltage when its loop is not

effective. This achieves a fast transition between

different compensation loops at PWM dimming.

Together with the external dimming MOSFET,

which holds the LED output voltage during the

PWM off interval, both the LED current and

output system voltage are regulated accurately

and smoothly at PWM dimming. No voltage ripple

or LED current overshoot or undershoot is

caused by PWM dimming.

Dimming Control

The MP4655 provides two dimming methods:

PWM dimming mode and analog dimming mode.

Applying a digital PWM signal on PWMIN allows

for PWM dimming. The brightness of the LED

string is proportional to the duty cycle of the

external PWM signal. A driving signal on DIMO is