FC9920 datasheet(4/7 Pages) FS

4 / 7 page

FC9920

2012. 10. 31

4/7

Revision No : 0

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When the switching MOSFET turns on, the

capacitance

CP is discharged into the DRAIN pin of

the IC. The discharge current is limited to about

210mA typically. However, it may become lower

at increased junction temperature. The duration of

the leading edge current spike can be estimated as:

TSPIKE = [(VIN · CP) / (ISAT)] +

trr

(4)

In order to avoid false triggering of the current sense

comparator, CP must be minimized in accordance

with the following expression:

CP < ISAT · (TBLANK(MIN) -

trr) / VIN(MAX)

(5)

where TBLANK(MIN) is the minimum blanking time of

200ns, and VIN(MAX) is the maximum instantaneous

input voltage.

Estimating Power Loss

Discharging the parasitic capacitance CP into the

DRAIN pin of the FC9920 is responsible for the bulk

of the switching power loss. It can be estimated using

the following equation:

PSWITCH = [(VIN2 · CP / 2) + VIN · ISAT ·

trr] · FS (6)

where FS is the switching frequency, ISAT is the

saturated DRAIN current of the FC9920. The

switching loss is the greatest at the maximum input

voltage. The switching frequency is given by the

following:

FS = (VIN – η-1 · VO) / VIN · TOFF

(7)

where η is the efficiency of the power converter.

When the FC9920 LED driver is powered from the

full-wave rectified AC input, the switching power

loss can be estimated as:

PSWIYCH ≈ [1 / (2·T

OFF)] · (VAC· P + 2·ISAT·trr) ·

(VAC - η-1 · VO)

(8)

VAC 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 FC9920 can be

calculated as:

PCOND = (D · IO2 · RON) + [IDD · VIN · (1–D)] (9)

where D = VO / (η · VIN) is the duty ratio, RON is

the on-resistance, IDD is the internal linear

regulator current.

When the LED driver is powered from the full-

wave rectified AC line input, the exact equation for

calculating the conduction loss is more

cumbersome. However, it can be estimated using

the following equation:

PCOND = (KC · IO2 ·RON) + (KD · IDD · VAC) (10)

where VAC is the input AC line voltage. The

coefficients KC and KD can be determined from the

minimum duty ratio of the FC9920.

Fig.1. Conduction Loss Coefficients KC and KD

EMI Filter

As with all off-line converters, selecting an input

filter is critical 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 current of the converter. As a

rule of thumb, this capacitor should be

approximately 0.1 - 0.2µF/W of LED output

power. A recommended input filter is shown in

Figure 2 for the following design example.

Design Example

Let us design an FC9920 LED lamp driver meeting

the following specification:

Input: Universal AC, 85-135VAC

Output Current: 100mA

Load: String of 24 LED (Power TOPLED

OSRAM

® VF = 2.5V max. each)

Step 1. Calculation L1.

The output voltage VO =24 x VF =60V (max.). Use

equation (1) assuming a 30% peak-to-peak ripple.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

KD (DM)

KC (DM)

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