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

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

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

in it.

(1)

(2)

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

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

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

)]

capacitance.

Charging

and

discharging

this

capacitance every switching cycle causes high-

current spikes in the LED string. Therefore,

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

achieves better performance. The reverse voltage

maximum input voltage of the LED lamp.

The total parasitic capacitance present at the

DRAIN pin of the An9920 can be calculated as:

(3)