AL9910/AL9910A/AL9910-5

UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED DRIVER

AL9910/AL9910A/AL9910-5

Document number: DS35103 Rev. 7 - 2

12 of 16

www.diodes.com

February 2012

© Diodes Incorporated

DC-DC Buck LED Driver

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

When driving long LED chains care should be taken not to induce SBO – maximum LED chain voltage should be less half of

DC-DC Boost LED Driver

Due to the topology of the AL9910 LED driver-controller it is capable of being used in boost configurations – 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.

Fig. 5 Boost LED Driver

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

output when the external 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

external 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 Over-voltage detection/protection.

R

SENSE

AL9910/A

GATE

CS

LD

PWM_D

GND

R

OSC

V

DD

V

IN

Q1

L1

D1

C1

R

OSC

C2

C3

V

IN