AL9902

Document number: DS37878 Rev. 1 - 2

12 of 18

www.diodes.com

July 2015

© Diodes Incorporated

AL9902

DC-DC Boost LED Driver

Due to the topology of the AL9902 LED driver-converter, it is capable of being used in boost configurations as shown in Figure 3

– at reduced

accuracy. The accuracy can be improved by measuring the LED current with an op amp and use the op amp’s output to drive the LD pin.

A Boost LED driver is used when the forward voltage drop of the LED string is higher than the input supply voltage. For example, the Boost

topology can be appropriate when input voltage is supplied by a 48V power supply and the LED string consists of twenty HB LEDs, as the case

may be for a street light.

VIN

C3

Rsense

Rosc

L1

GND

AL9902

LD

PWM

VDD

CS

Rosc

SO

SW

VIN

D1

C1

C2

Figure 3 Boost LED driver

In a Boost converter, when the internal MOSFET is ON, the energy is stored in the inductor which is then delivered to the output when the internal

MOSFET switches OFF. If the energy stored in the inductor is not fully depleted by the next switching cycle (continuous conduction mode) the DC

conversion between input and output voltage is given by:

D

1

V

V

IN

OUT





,

OUT

IN

OUT

V

V

V

D





OSC

ON

f

D

t



From this the required inductor value can be determined by:

LED

ON

IN

I

3

.

0

t

V

L







The Boost topology LED driver requires an output capacitor to deliver current to the LED string during the time that the internal MOSFET is on.

In boost LED driver topologies, if the LEDs should become open circuit damage may occur to the power switch and so some form of detection

should be present to provide overvoltage detection/protection.