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FC9921 Datasheet(PDF) 5 Page - First Silicon Co., Ltd

No. de Pieza. FC9921
Descripción  3-Pin Switch-Mode LED Lamp Driver IC
Descarga  8 Pages
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Fabricante  FS [First Silicon Co., Ltd]
Página de inicio  http://www.firstsilicon.co.kr/
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FC9921 Datasheet(HTML) 5 Page - First Silicon Co., Ltd

   
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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:
T
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:
C
P < ISAT
(T
BLANK(MIN) - trr )
(5)
V
IN(MAX)
where T
BLANK(MIN) is the minimum blanking time of 200ns, and
V
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:
P
SWITCH = [(VIN
2
• 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:
F
S = (VIN - η
-1
• V
O) / VIN • TOFF
(7)
where
η
When the FC9921 LED driver is powered from the full-wave
as:
P
SWITCH
(8)
1
(V
AC • CP + 2 • ISAT • trr )(VAC - η
-1
• V
O )
2 • T
OFF
V
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:
P
COND = (D • IO
2
• R
ON) + [IDD • VIN • (1 - D)]
(9)
where D = V
O /ηVIN is the duty ratio, RON is the on-resistance,
I
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:
P
COND = (KC • IO
2
• R
ON ) + (KD • IDD • VAC )
(10)
where V
AC
C
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
V
F = 4.1V max. each)
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


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