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FC9921 Datasheet(PDF) 4 Page - First Silicon Co., Ltd
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FC9921 Datasheet(HTML) 4 Page - First Silicon Co., Ltd
/ 8 page
2012. 10. 31
Revision No : 0
The FC9921 is a PWM peak current controller for controlling
a buck converter topology in continuous conduction mode
(CCM). The output current is internally preset at 20mA.
When the input voltage of 20 to 400V appears at the
DRAIN pin, the internal high-voltage linear regulator seeks
to maintain a voltage of 7.5VDC 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
is provided in the under-voltage comparator to prevent
When the input current exceeds the internal preset level,
MOSFET turns off. At the same time, a one-shot circuit is
activated that determines the duration of the off-state (10.5µs
The new switching cycle begins.
A “blanking” delay of 300ns is provided that prevents false
triggering of the current sense comparator due to the leading
edge spike caused by circuit parasitics.
The FC9921 is a low-cost off-line buck converter IC
be operated from either universal AC line range of 85 to
265VAC, or 20 to 400VDC, and drives up to tens of high
brightness LEDs. All LEDs can be run in series, and the
FC9921 regulates at constant current, yielding uniform
illumination. The FC9921 is compatible with triac dimmers.
available in space saving TO-92 and SOT-89 packages.
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 ∆I
L1 = (V
) / ΔI
is the forward voltage of the LED string. T
is the off-
time of the FC9921. The output current in the LED string (I
is calculated then as:
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 FC9921,
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.
the output current ripple even further, thus permitting a
reduced value of L1. However, one must keep in mind that
the peak-to-average current error is affected by the variation
. Therefore, the initial output current accuracy might
Another important aspect of designing an LED driver with
the FC9921 is related to certain parasitic elements of the
circuit, including distributed coil capacitance of L1, junction
capacitance of the printed circuit board traces C
of the controller itself. These parasitic
could potentially cause false triggering of the current sense
comparator if not properly managed. Minimizing these
Coil capacitance of inductors is typically provided in the
manufacturer’s data books either directly or in terms of the
self-resonant frequency (SRF).
SRF = 1 / (2π √(L • C
where L is the inductance value, and C
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 C
recommended to bypass these spikes.
of the current sense comparator. Using diodes with shorter
reverse recovery time t
and lower junction capacitance C
achieves better performance. The reverse voltage rating V
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 FC9921 can be calculated as:
When the switching MOSFET turns on, the capacitance C
is discharged into the DRAIN pin of the IC. The discharge
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