MP4027 – PRIMARY-SIDE CONTROL, OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4027 Rev. 1.02

www.MonolithicPower.com

13

12/9/2014

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Vbus

Vcomp

Vgate

VCC

Brown out

happen

Brown out

detected

Brown

out

recover

Figure 10: Brown-Out Protection Waveforms

NTC Thermal Protection

The NTC provides LED thermal protection. A

NTC resistor to monitor the LED temperature

can be connected to this pin directly. The

internal

pull-up

resistor

generates

a

corresponding voltage on the external NTC

resistor, and the LED current changes as NTC

voltage changes. Figure 11 shows the NTC

curve.

Io

Iset/3

0

Iset

Figure 11: NTC Curve

LED current drops to minimum output, the

minimum output current is determined by gate

minimum on time. (equal to 400ns LEB time)

PWM Dimming

The MP4027 can accept direct PWM dimming

signal. Applying a PWM signal (>200Hz) on

NTC pin can achieve dimming performance.

Following the NTC curve of Figure 11, if the

high level of the PWM signal is higher than

scale. And if the low level of the PWM signal is

internal reference voltage of EA can be

modulated as the external PWM dimming signal

to capture the duty cycle information.

COMP

EA

CS

Current

Caculation

To Multiplier

PWM Signal

NTC

Figure 12: PWM Dimming

With large COMP cap, the loop response is

slow. As long as the PWM frequency is higher

than 200Hz, the duty cycle information can be

filtered and averaged by COMP. Then, by the

close loop control, the output LED current

linearly changes with dimming duty from

maximum to minimum.

IC Thermal Shut Down

To prevent thermal damage to the system and

IC, if internal temperatures exceed 150°C, the

MP4027 stops switching and the IC is latched

off until VCC drops below UVLO and restarts.

Design Example

For the design example, please refer to MPS

application note AN076 for the detailed design

procedure and information.