AL9902

Document number: DS37878 Rev. 1 - 2

11 of 18

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July 2015

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AL9902

Input Bulk Capacitor

For offline lamps, an input bulk capacitor is required to ensure that the rectified AC voltage is held above twice the LED string voltage throughout

the AC line cycle. The value can be calculated from:



IN

C

max

_

DC

L

min

_

Line

ch

in

V

f

2

V

2

)

D

1

(

P













Where

ch

D

L

max

_

DC

V



min

_

Line

V

2

If the capacitor has a 15% voltage ripple then a simplified formula for the minimum value of the bulk input capacitor approximates to:

2

IN

LEDs

LED

V

0.06

V

I





Power Factor Correction

If power factor improvement is required then for the input power less than 25W, a simple method for improving the power factor can be

implemented by potential dividing down the rectified mains voltage (resistors R1 and R2 in Figure 2) and feeding it into the LD pin. The current

drawn from the supply voltage will follow an approximate half sine wave. A filter across the LEDs reduces the potential for flicker. This circuit also

significantly reduces the size of input capacitors.

HD06

1

2

4

Z1

602V

C4

Rsense

Rosc

D4

GND

AL9902

PWM

VDD

LD

CS

Rosc

SO

SW

VIN

L2

R2

R1

C1

C2

C3

L1

AC+

AC-

LED+

LED-

Figure 2 Typical Application Circuit with Simple PFC

Passive power factor correction using 3 high voltage diodes and 2 identical capacitors can be implemented. For further design information please

see AN75 from the Diodes website.

DC-DC Buck LED Driver

The design procedure for an AC input buck LED driver outlined in the previous chapters equally applies to DC input LED drivers.