MP4034 – OFFLINE LED DRIVER

MP4034 Rev. 1.02

www.MonolithicPower.com

14

12/4/2013

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spike and better system operation.

The power dissipated in the snubber circuit is

approximately:

2

SN

SN

K

PK

S

SN

PS

O

V

1

PL I

f

2V

N

V

=⋅

⋅

⋅

×

−×

Where:

side.

The power consumed in the snubber resistor

2

SN

SN

SN

V

R

P

=

The maximum ripple of the snubber capacitor

voltage is:

SN

SN

SN

SN

S

V

V

CR

f

Δ=

⋅⋅

Generally, a 15% ripple is reasonable. Use the

The damping resistor (R) in series with the RCD

has a relatively large value to prevent any

excessive ringing voltage that can affect the EMI.

Use a damping resistor of about 200Ω to 500Ω to

limit the drain voltage ringing.

Resistor Divider

For better application performance, use a resistor

divider with values in the range of 10kΩ to 100kΩ

to limit noise from adjacent components on the

FB pin. Connect a resistor with a value ranging

from 1kΩ to 2kΩ from the FB pin to the resistor

divider to limit substrate injection current effects,

as shown in Figure 12.

Figure 12: FB Pin in Series with ON Resistor

Dummy Load

A dummy load is required in open-output

applications for good over-voltage protection.

Use a dummy load of ~10mW for good load

regulation.

Maximum Switching Frequency

Because of the parameter tolerance of the

sampling

detecting

time

and

inductance

tolerances,

select

a

secondary-side-diode

conduction time that exceeds 5.4µs as follows.

SM

S _ ON

PK

PO

D

NL

TI

5.4 s

N(V

V )

⋅

=

⋅>

μ

⋅+

For high or low temperature operation, select a

maximum switching frequency below 75kHz.