FC9931

2012. 10. 31

3/6

Revision No : 0

Functional Description

Power Topology

The FC9931 is optimized to drive

single-stage, single-switch, non-isolated topology, cascading

an input power factor correction (PFC) buck-boost stage

and an output buck converter power stage. This power

converter topology offers numerous advantages useful

for driving high-brightness light emitting diodes (HB LED).

These advantages include unity power factor, low harmonic

distortion of the input AC line current, and low output current

ripple. The output load is decoupled from the input voltage

with a capacitor making the driver inherently failure-safe for

the output load. The power converter topology also permits

reducing the size of a filter capacitor needed, enabling use

of non-electrolytic capacitors. The latter advantage greatly

improves reliability of the overall solution.

The FC9931 is a peak current-mode controller that is

specifically designed to drive a constant current buck-

boost-buck power converter. This patent pending control

scheme features two identical current sense comparators

for detecting negative current signal levels. One of the

comparators regulates the output LED current, while the

other is used for sensing the input inductor current. The

second comparator is mainly responsible for the converter

start-up. The control scheme inherently features low inrush

current and input under-voltage protection. The FC9931 can

operate with programmable constant frequency or constant

off-time. In many cases, the constant off-time operating mode

is preferred, since it improves line regulation of the output

current, reduces voltage stress of the power components

and simplifies regulatory EMI compliance.

Input Voltage Regulator

The FC9931 can be powered directly from its VIN pin, and

takes a voltage from 8V to 450V. When a voltage is applied

at the VIN pin, the FC9931 seeks to maintain a constant

7.5V at the VDD pin. The V

reference for the current sense comparators. The regulator

is equipped with an under-voltage protection circuit which

shuts off the FC9931 when the voltage at the VDD pin falls

below 6.2V.

The VDD pin must be bypassed by a low ESR capacitor

(≥ 0.1µF) to provide a low impedance path for the high

frequency current of the output GATE driver.

The FC9931 can also be operated by supplying a voltage

at the VDD pin greater than the internally regulated voltage.

This will turn off the internal linear regulator and the FC9931

will function by drawing power from the external voltage

source connected to the VDD pin.

PWM Dimming and Wall Dimmer Compatibility

PWM Dimming can be achieved by applying a TTL-

compatible square wave signal at the PWMD pin. When the

PWMD pin is pulled high, the GATE driver is enabled and the

circuit operates normally. When the PWMD pin is left open

or connected to GND, the GATE driver is disabled and the

external MOSFET turns off. The FC9931 is designed so that

the signal at the PWMD pin inhibits the driver only, and the

IC need not go through the entire start-up cycle each time

ensuring a quick response time for the output current.

The power topology requires little filter capacitance at

the output, since the output current of the buck stage is

continuous, and since AC line filtering is accomplished

through the middle capacitor rather than the output one.

Therefore, disabling the FC9931 via its PWMD or VIN pins

can interrupt the output LED current in accordance with

the phase-controlled voltage waveform of a standard wall

dimmer.

Oscillator

Connecting an external resistor from RT pin to GND programs

switching frequency:

Connecting the resistor from the RT pin to the GATE

programs constant off-time: