FC9922

2012. 10. 31

5/8

Revision No : 0

current is limited to about 150mA typically. However, it

may become lower at increased junction temperature. The

duration of the leading edge current spike can be estimated

as:

T

(4)

In order to avoid false triggering of the current sense

comparator, C

following expression:

C

(T

(5)

V

IN(MAX)

where T

V

Estimating Power Loss

Discharging the parasitic capacitance C

the FC9921 is responsible for the bulk of the switching power

loss. It can be estimated using the following equation:

P

2

• C

(6)

where F

DRAIN current of the FC9921. The switching loss is the

greatest at the maximum input voltage.

The switching frequency is given by the following:

F

-1

• V

(7)

where

η

When the FC9921 LED driver is powered from the full-wave

as:

P

(8)

1

(V

-1

• V

2 • T

OFF

V

The switching power loss associated with turn-off transitions

of the DRAIN pin can be disregarded. Due to the large amount

of parasitic capacitance connected to this switching node,

the turn-off transition occurs essentially at zero-voltage.

Conduction power loss in the FC9921 can be calculated as:

P

2

• R

(9)

where D = V

I

When the LED driver is powered from the full-wave

conduction loss is more cumbersome. However, it can be

estimated using the following equation:

P

2

• R

(10)

where V

AC

C

and K

the FC9921.

Fig. 1.

EMI Filter

As w

to obtaining good EMI. A switching side capacitor, albeit of

small value, is necessary in order to ensure low impedance

to the high frequency switching currents of the converter. As

a rule of thumb, this capacitor should be approximately 0.1-

shown in Figure 2 for the following design example.

Design Example

Let us design an FC9922 LED lamp driver meeting the

Input:

Universal AC, 85-265VAC

Output Current: 20mA

Load:

String of 10 LED (LW541C by OSRAM

V

0

0.1

0.2

0.3

0.4

0 5

0.6

0.7

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Kd(Dm)

Kc(Dm)

Dm