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FC9920 Datasheet(PDF) 4 Page - First Silicon Co., Ltd

No. de Pieza. FC9920
Descripción  3-Pin Switch-Mode LED Lamp Driver
Descarga  7 Pages
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Fabricante  FS [First Silicon Co., Ltd]
Página de inicio  http://www.firstsilicon.co.kr/
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FC9920 Datasheet(HTML) 4 Page - First Silicon Co., Ltd

   
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FC9920
2012. 10. 31
4/7
Revision No : 0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
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
DM
KD (DM)
KC (DM)


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