FC9921 datasheet(5/8 Pages) FS

5 / 8 page

FC9921

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:

SPIKE = ((VIN • CP) / (ISAT)) +trr

(4)

In order to avoid false triggering of the current sense

comparator, C

P must be minimized in accordance with the

following expression:

P < ISAT •

BLANK(MIN) - trr )

(5)

IN(MAX)

where T

BLANK(MIN) is the minimum blanking time of 200ns, and

IN(MAX) is the maximum instantaneous input voltage.

Estimating Power Loss

Discharging the parasitic capacitance C

P into the DRAIN pin of

the FC9921 is responsible for the bulk of the switching power

loss. It can be estimated using the following equation:

SWITCH = [(VIN

• C

P ) / 2 + VIN • ISAT • trr ] • FS

(6)

where F

S is the switching frequency, ISAT is the saturated

DRAIN current of the FC9921. The switching loss is the

greatest at the maximum input voltage.

The switching frequency is given by the following:

S = (VIN - η

-1

• V

O) / VIN • TOFF

(7)

where

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

as:

SWITCH ≈

(8)

AC • CP + 2 • ISAT • trr )(VAC - η

-1

• V

O )

2 • T

OFF

AC is the input AC line voltage.

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:

COND = (D • IO

• R

ON) + [IDD • VIN • (1 - D)]

(9)

where D = V

O /ηVIN is the duty ratio, RON is the on-resistance,

DD is the internal linear regulator current.

When the LED driver is powered from the full-wave

conduction loss is more cumbersome. However, it can be

estimated using the following equation:

COND = (KC • IO

• R

ON ) + (KD • IDD • VAC )

(10)

where V

and K

d can be determined from the minimum duty ratio of

the FC9921.

Fig. 1.

C and Kd

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 FC9921 LED lamp driver meeting the

Input:

Universal AC, 85-265VAC

Output Current: 20mA

Load:

String of 10 LED (LW541C by OSRAM

F = 4.1V max. each)

0.1

0.2

0.3

0.4

0 5

0.6

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Kd(Dm)

Kc(Dm)

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