ZXLD1321

Issue 1 - January 2008

12

www.zetex.com

© Zetex Semiconductors plc 2008

Fig 1

PWM control

A wider dimming range can be achieved by applying a PWM control signal to the ADJ pin to turn

the device on and off, giving an average output current proportional to the duty cycle of the

control signal. The ADJ pin can be driven directly from the open drain NMOS output of a

microcontroller, or indirectly with a low saturation voltage NPN transistor such as the Zetex

ZXTN25015DFH. (Fig 2).

Fig 2

In the circuit of Fig 4, the average LED output current will be

Where duty cycle

A PWM frequency of 200Hz, or lower is recommended, to minimize errors due to the rise and fall

times of the converter output.

ZXLD1321

ADJ

ZXTN25015DFH

D

nom

I

avg

I

LED

LED

*

)

(

)

(

=

)

2

1

(

2

T

T

T

D

+

=