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

MP4655 Rev 1.0

www.MonolithicPower.com

19

2/2/2016

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© 2016 MPS. All Rights Reserved.

stage is enabled and DIMO rises high. The LED

current is regulated through the LLC frequency

duty cycle control of the extra N-channel

MOSFET.

Standby Mode with Low Consumption Power

and No Audible Noise

The MP4655 features a standby mode with low

consumption power on the LLC power stage.

When PS_ON ≤ 0.8V, the MP4655 enters

standby mode, regardless of the status of the

LED driver stage control signals.

In standby mode, the LED stage is disabled,

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

The MP4655 takes the following actions to

decrease the system power consumption:

circuit works in soft-burst mode. The VFSET

voltage is clamped at 2.1V, so the switching

frequency at burst mode is not too high.

Power consumption is decreased in this

mode. A soft burst-on time and soft burst-off

time are added to the burst mode to eliminate

audible noise (see Figure 3).

2. The charge pump for VDR_DN works in burst

mode, and VDR_DN is controlled with a

larger ripple voltage. This decreases the IC

consumption current.

3. The internal logic circuit consumption current

is decreased.

First pulse GL: 50% width

+5%

-5%

First pulse GL: 50% width

+5%

94uA charge

94μA discharge

Figure 3: LLC Soft-Burst Operation at Standby

Mode

LED Current and System Voltage Regulation

The MP4655 accurately regulates both the LED

current and the output system voltage with only

one LLC power stage.

In PWM dimming off condition, the LED stage is

not enabled (PWMIN is low, EN_LED is not

enabled, or LED stage fault detected). The

MP4655 regulates the output system voltage

through the LLC frequency control, and the extra

N-channel MOSFET is on (see Figure 4A).

In PWM dimming on condition, the MP4655

regulates the LED current through the LLC

frequency control, and regulates the output

system voltage through the duty cycle control of

the extra N-channel MOSFET. The integrated

individual control for this extra N-channel

MOSFET achieves soft switching, and there is no

voltage spike (see Figure 4B).

Figure 4A: MP4655 Control Scheme at PWM

Dimming Off

Figure 4B: MP4655 Control Scheme at PWM

Dimming On

The regulation for the LED current and system

voltage at different conditions is as follows:

1. LED current regulation at PWM dimming on

(ICOMP loop)

For the LED current regulation loop with LLC

frequency control, the LED current is fed back to

IFB. The internal error amplifier regulates the

average value of IFB signal to the internal 199mV

reference voltage. Its output is connected to the

external current-loop compensation network on

ICOMP through an inner switch (S1).

During the PWM on interval, S1 is on, and the

output of the error amplifier is connected to the

external compensation network on ICOMP. The