FC9922

2012. 10. 31

4/8

Revision No : 0

Functional Description

The FC9922 is a PWM peak current controller for controlling

a buck converter topology in continuous conduction mode

(CCM). The output current is internally preset at 50mA.

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

oscillation.

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.

Application Information

The FC9922 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

FC9922 regulates at constant current, yielding uniform

illumination. The FC9922 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

(1)

V

time of the FC9921. The output current in the LED string (I

is calculated then as:

I

(2)

where I

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 FC9922,

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

of T

Another important aspect of designing an LED driver with

the FC9922 is related to certain parasitic elements of the

circuit, including distributed coil capacitance of L1, junction

capacitance of the printed circuit board traces C

capacitance C

could potentially cause false triggering of the current sense

comparator if not properly managed. Minimizing these

the FC9922.

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

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

achieves better performance. The reverse voltage rating V

R

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:

C

(3)

When the switching MOSFET turns on, the capacitance C

P

is discharged into the DRAIN pin of the IC. The discharge

50mA