MP4034 – OFFLINE LED DRIVER

MP4034 Rev. 1.02

www.MonolithicPower.com

11

12/4/2013

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© 2013 MPS. All Rights Reserved.

The relationship of peak current at ON period

and OFF period is:

P

PK _ S

PK

S

N

II

N

=×

Where

P

turns, and

S

winding turns.

The MP4034 detects the secondary side diode

duty cycle by sampling the auxiliary winding

voltage and generates a ZCD signal as shown

in Figure 6.

Figure 6: VFB and ZCD Waveforms

When the FB voltage is high—which means

that the current is flowing through secondary-

side

diode—the

ZCD

signal

goes

high.

Conversely, when the FB voltage is low—which

means no current flows through the secondary-

side diode—the ZCD signal goes low, meaning

S_ ON

S

S_ ON

S_ OFF

T

D

TT

=

+

Then the average output current is:

OUT

PK _ S

S

S_ ON

P

PK

SS _ ON

S _ OFF

1

II

D

2

T

N

1

I

2N

T

T

=×

×

=×

×

×

+

The MP4034 keeps

S

D at 0.4. Thus the output

current is:

PP

OUT

PK

S

PK

SS

NN

11

II

D

I

2N

5 N

=×

×

×

=×

×

This provides enough information to design the

transformer turn ratio.

Leading-Edge Blanking

Turning the power switch on induces a spike on

the sense resistor. To avoid falsely terminating

the switching pulse, the MP4034 includes a

300ns leading-edge blanking period. During this

blanking period, the current sense comparator

is disabled and the gate driver can not switch

off.

DCM Detection

The MP4034 is designed to operate in

discontinuous conduction mode (DCM). To

avoid operating in continuous conduction mode

(CCM), the MP4034 detects the falling edge of

the FB input voltage with each cycle. If the chip

does not detect a 120mV falling edge, it will

stop switching.

Over Voltage Protection

The MP4034 has two levels of over-voltage

protection based on the FB voltage.

In normal operation, MP4034 samples the FB

pin voltage 3.5μs after the primary switch turns

off, as shown in Figure 6.

Figure 7: Auxiliary Voltage Waveform

P_ AU

DOWN

FB

O

D

SUP

DOWN

N

R

V(V

V )

NR

R

=×

×

+

+

voltage.

When the MP4034 detects that the FB voltage

equals 4.0V, the first level OVP triggers. The

switching frequency drops to maintain the

voltage exceeds 6.35V for 3.5μs, it will shut

down immediately and discharge the VCC