MP4051—NON-ISOLATED OFFLINE LED CONTROLLER WITH ACTIVE PFC

MP4051 Rev. 1.01

www.MonolithicPower.com

12

1/21/2014

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Output Over-Voltage Protection (OVP)

Output over voltage protection can prevent the

components from damage in the over voltage

condition. The positive plateau of auxiliary

winding voltage is proportional to the output

voltage, the OVP uses the auxiliary winding

voltage instead of directly monitoring the output

voltage, the OVP sample is shown in Figure 8.

Once the ZCD pin voltage is higher than 5.4V

after a 1.5us blanking time, the OVP signal will

be triggered and latched, the gate driver will be

turned off and the IC work at quiescent mode, the

VCC voltage dropped below the UVLO which will

make the IC shut down and the system restarts

again. The output OVP setting point can be

calculated as:

⋅⋅

=

+

AUX

ZCD2

OUT _ OVP

SEC

ZCD1

ZCD2

NR

V5.4V

NR

R

+

1.5µs

Blanking

Vcc

ZCD

Auxiliary Winding

OVP

signal

5.4V

Latch

Figure 8—OVP Sample Unit

To avoid the mis-trigger OVP by the oscillation

spike after the switch turns off, the OVP sampling

shown in Figure 9.

The current-limiting resistor between the output

of the aux-winding and the ZCD resistor divider

can also work as suppresser to avoid the OVP

mis-trigger.

0V

Sampling Here

Figure 9—ZCD Voltage and OVP Sample

Output Short Circuit Protection

The MP4051 clamps the CS pin voltage to less

than 2.5V to limit the available output power.

When the short circuit of the LED load occurs,

the voltage of the auxiliary winding will fall down

following the voltage of the Load and the VCC

drops to less than UV threshold and re-start the

system.

As supplementary, tie a resistor divider form CS

sensing resistor to ZCD pin, shown in Figure 10.

When the power MOSFET is turned on, the ZCD

pin monitors the rising inductor current, once the

ZCD pin reaches OCP threshold, typical 0.6V,

the gate driver will be turned off to prevent the

chip form damage and the IC works at quiescent

mode, the VCC voltage dropped below the UVLO

which will make the IC shut down and the system

restarts again. Please note that the value of the

should be much smaller than those of the ZCD

280ns

Blanking

ZCD

OCP

signal

0.6V

Latch

CS

GATE

PSR

control

Primary Winding

V

BUS

R

OCP2

R

CS

D

Figure 10—OCP Sample Unit