AL9901FDF-13 datasheet(13/20 Pages) DIODES

13 / 20 page

AL9901

Document number: DS37713 Rev. 1 - 2

13 of 20

www.diodes.com

March 2015

AL9901

Applications Information (cont.)

Output Open Circuit Protection

The non-isolated buck LED driver topology provides inherent protection against an open circuit condition in the LED string due to the LEDs being

connected in series with the inductor. Should the LED string become open circuit then no switching occurs and the circuit can be permanently left

in this state with damage to the rest of the circuit.

AC/DC Off-Line LED Driver

The AL9901 is a cost-effective off-line buck LED driver-converter specifically designed for driving LED strings. It is suitable for being used with

either a rectified AC line or any DC voltage between 15-500V. See figure 3 for typical circuit.

Figure 2 Typical Application Circuit (without PFC)

Buck Design Equations:

LEDs

osc

≥

LED

LEDs

)

(

−

SENSE

))

(

LED

Where ILED x 0.3 = IRIPPLE

Design Example

For an AC line voltage of 120V the nominal rectified input voltage is VIN = 120V x 1.41 = 169V. From this and the LED chain voltage, the duty

cycle can be determined:

D = VLEDs /VIN = 30/169 = 0.177

From the switching frequency, for example fOSC = 50 kHz, the required on-time of the internal MOSFET can be calculated:

tON = D/fOSC = 3.5 µs

The value of the inductor is determined as follows:

L = (VIN - VLEDs) x tON / (0.3 x ILED) = 4.6mH

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