FC9910B

2013. 08. 29

7/10

Revision No : 1

7. Inductor Design

Reffering to t he typical application circuit below the

value can be calculated from the desired peak to peak

LED ripple current in the inductor.

Typically such ripple current is selected to be 30% of

the nominal LED current. In the example given here,

the nominal current I

is 350mA.

The next step is determining the total voltage drop

across the LED string. For example, when the string

consists of 10 High Brightness LEDs and each didode

has a forward voltage drop of 3.0V at its nominal current

, the total LED voltage V

is 30V.

Knowing the nominal rectified input voltage

Vin = 120V*1.41 = 169V, the switching duty ratio can

be determined as:

D = V

/ Vin = 30 / 169 = 0.177

8. Input Bulk Capacitor

Then given the switching frequency , in the example

transistor can be calculated:

T

The required value of the inductor is given by

L = (V - V

)* T

/ (0.3*I

) = 4.6mH

An input filter capacitor should be designed to hold

the rectified AC voltage above twice the LED string

voltage throughout the AC line cycle.

Assuming 15% relative voltage ripple across the

capacitor, a simplified formula for the minimum value

of the bulk input capacitor is given by:

A passive PFC circuit at the input requires using two

series connected capacitors at the place of calculated

Each of these identical capacitors should be

rated for 1/2 of the input voltage and have twice as

much capacitance.

The FC9910B can be turned off by pulling the PWM_D

pin to ground. When disabled, the FC9910B draws

quiescent current of less then 1mA.

LED

LED

LED

ON

IN

LEDS

ON

LED

C

= I

* V

* 0.06 / V

LED

MIN

LEDS

N

2

C

= 22uF, a value 22uF / 250V can be used.

MIN

9. Enable

6. Power Factor Correction

When the input power to the LED driver does not

exceed 25W, a simple passive power factor

correction circuit can be added to the FC9910B

application circuit of Figure 4 in order to pass the

AC line harmonic limits of the EN61000-3-2

standard for Class C equipment. The typical

application circuit diagram shows how this can be

done without affecting the rest of the circuit

significantly. A simple circuit consisting of 3 diodes

and 2 capacitors is added across the rectified AC

line input to improve the line current harmonic

distortion and to achieve a power factor greater

than 0.85.