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FC9920 Datasheet(PDF) 3 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) 3 Page - First Silicon Co., Ltd

   
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FC9920
2012. 10. 31
3/7
Revision No : 0
Functional Description
Input Voltage Regulator
The FC9920 is a PWM peak current controller for
controlling a buck converter topology in continuous
conduction mode (CCM). The output current is
internally preset at 100mA.
When the input voltage of 20 to 400V appears at the
DRAIN pin, the internal high-voltage linear regulator
seeks to maintain a constant voltage 7.8VDC at the
VDD pin. Until this voltage exceeds the internally
programmed under-voltage threshold, the output
switching MOSFET is non-conductive. When the
threshold is exceeded, the MOSFET turns on. The
input current begins to flow into the DRAIN pin.
Hysteresis is provided in the under-voltage
comparator to prevent oscillation.
When the input current exceeds the internal preset
level, a current sense comparator resets an RS flip-
flop, and the MOSFET turns off. At the same time, a
one-shot circuit is activated that determines the
duration of the off-state (10.5µs typ.). As soon as this
time is over, the flip-flop sets again. The new
switching cycle begins.
A “blanking” delay of 300ns is provided that prevent
false triggering of the current sense comparator due
to the leading edge spike caused by circuit parasitics.
Application Information
The FC9920 is a low-cost off-line buck converter IC
specifically designed for driving multi-LED strings.
It can be operated from either universal AC line
range of 85 to 264VAC, or 20 to 400 VDC, and
drives up to tens of high brightness LEDs. All LEDs
can be run in series, and the FC9920 regulates at
constant current, yielding uniform illumination. The
FC9920 is compatible with triac dimmers. The output
current is internally fixed at 100mA. This part is
available in space saving TO-92 and SOT-89
package.
Selecting L1 and D1
There is a certain trade-off to be considered between
optimal sizing of the output inductor L1 and the
tolerated output current ripple. The required value of
L1 is inversely proportional to the ripple current ΔIO
in it.
L1 = (VO · TOFF) / ΔIO
(1)
where VO is the forward voltage of the LED string.
TOFF is the off-time of the FC9920.The output current
in the LED string (IO) is calculated then as:
IO = ITH - (ΔIO / 2)
(2)
where ITH is the current sense comparator threshold.
The ripple current introduces a peak-to-average error
in the output current setting that needs to be
accounted for. Due to the constant off-time control
technique used in the FC9920, the ripple current is
independent of the input AC or DC line voltage
variation. Therefore, the output current will remain
unaffected by the varying input voltage.
Adding a filter capacitor across the LED string can
reduce the output current ripple even further, thus
permitting a reduce value of L1. However, one must
keep in mind that the peak-to-average current error is
affected by the variation of TOFF. Therefore, the
initial output current accuracy might be sacrificed at
large ripple current in L1.
Another important aspect of designing an LED driver
with the FC9920 is related to certain parasitic
elements of the circuit, including distributed coil
capacitance of L1, junction capacitance and reverse
recovery of the rectifier diode D1, capacitance of the
printed circuit board traces CPCB and output
capacitance CDRAIN of the controller itself. These
parasitic elements affect the efficiency of the
switching converter and could potentially cause false
triggering of the current sense comparator if not
properly managed. Minimizing these parasitics is
essential for efficient and reliable operation of the
FC9920.
Coil capacitance of inductors is typically provided in
the manufacture s data books either directly or in
terms of the self-resonant frequency (SRF).
SRF = 1 / [2π · √ (L · C
L
)]
where L is the inductance value, and CL is the coil
capacitance.
Charging
and
discharging
this
capacitance every switching cycle causes high-
current spikes in the LED string. Therefore,
connecting a small capacitor CO (~10nF) is
recommended to bypass these spikes.
Using an ultra-fast rectifier diode for D1 is
recommended to achieve high efficiency and reduce
the risk of false triggering of the current sense
comparator. Using diodes with shorter reverse
recovery time
trr and lower junction capacitance CJ
achieves better performance. The reverse voltage
rating VR of the diode must be greater than the
maximum input voltage of the LED lamp.
The total parasitic capacitance present at the
DRAIN pin of the An9920 can be calculated as:
CP = CDRAIN + CPCB + CL + CJ
(3)


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