AL9910/AL9910A

UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED

DRIVER

AL9910/AL9910A

Document number: DS 35103 Rev. 2 - 2

7 of 14

www.diodes.com

March 2011

© Diodes Incorporated

Setting Operating Frequency

The AL9910 is capable of operating over a 25 and 300 kHz switching frequency range. The switching frequency is

25

22

+

osc

R

µs

efficiency.

When operating in buck mode the designer must keep in mind that the input voltage must be maintained higher than 2 times

the forward voltage drop across the LEDs. This limitation is related to the output current instability that may develop when the

AL9910 operates at a duty cycle greater than 0.5. This instability reveals itself as an oscillation of the output current at a sub-

CS

LD

PWM_D

GND

R

OSC

V

DD

V

IN

Q1

R

OSC

V

IN

CS

LD

PWM_D

GND

R

OSC

V

DD

V

IN

Q1

R

OSC

V

IN

Figure 2. Constant off-time configuration

When using this mode the nominal switching frequency is chosen and from the nominal input and output voltages the off-time

can be calculated:

OSC

nom

IN

nom

OUT

OFF

f

V

V

t

1

1

)

(

)

(

∗

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

=

(

)

)

k

(

22

25

)

µs

(

t

R

OFF

OSC

Ω

−

∗

=