MP4031—PRIMARY-SIDE–CONTROLLED, OFFLINE LED CONTROLLER WITH PFC

MP4031 Rev.1.03

www.MonolithicPower.com

13

5/8/2013

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Where:

• N is the turn ratio of the primary side to the

secondary side,

(typically 0.4), and

MOSFET source and GND.

Power-Factor Correction

The MULT pin connects to the tap of a resistor

divider from the rectified instantaneous line

voltage. The multiplier output also has a

sinusoidal shape. This signal provides the

reference for the current comparator against the

primary-side–inductor current, which shapes the

primary-peak current into a sinusoid with the

same phase as the input line voltage. This

achieves a high power factor.

Inductor current

Multiplier output

Figure 4: Power-Factor Correction

The multiplier’s maximum output voltage to the

current comparator is clamped to 2.3V to limit the

cycle-by-cycle current. The multiplier’s minimum

output voltage is clamped to 0.1 to ensure a turn-

on signal during the TRIAC dimming OFF interval,

which pulls down the rectifier input voltage and

accurately detects the dimming phase.

VCC Timing Sequence

Initially, VCC charges through the internal

charging circuit from the AC line. When VCC

reaches 10V, the internal charging circuit stops

charging, the control logic initializes and the

internal main MOSFET begins to switch. Then

the auxiliary winding takes over the power supply.

However, the initial auxiliary-winding positive

voltage may not be large enough to charge VCC,

causing VCC to drop. Instead, if the VCC voltage

drops below the 9V threshold, the internal

charging circuit triggers and charges VCC to 10V

again. This cycle repeats until the auxiliary

winding voltage is high enough to power VCC.

If any fault occurs during this time, the switching

and the internal charging circuit will stop and

latch, and VCC drops. When VCC decreases to

7V, the internal charging circuit re-charges for

auto-restart.

10V

9V

Vcc

Internal

Charging Circuit

And Regulates the VCC

Auxiliary Winding Takes Charge

Fault happens

Gate

Switching Pulses

7V

Figure 5: VCC Timing Sequence

Auto Start

The MP4031 includes an auto starter that starts

timing when the MOSFET turns off. If ZCD fails

to send a turn-on signal after 122µs, the starter

will automatically sends a turn-on signal to avoid

unnecessary IC shutdowns if ZCD fails.

Minimum OFF Time

The MP4031 operates with a variable switching

frequency; the frequency changes with the

instantaneous input-line voltage. To limit the

maximum

frequency

and

get

good

EMI

performance, the MP4031 employs an internal

minimum OFF-time limiter of 5µs, as shown in

Figure 6.

ZCD

GATE

5.6us

Figure 6: Minimum OFF time