ZXLD1320
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
ZXTN25020DFL. (Fig 2).
ZXTN25015DFL
ADJ
ZXLD1320
Fig 2
In the circuit of Fig 4, the average LED output current will be
I LED ( avg ) = I LED ( nom ) * D
Where duty cycle
D =
T 2
( T 1 + T 2 )
A PWM frequency of 200Hz, or lower is recommended, to minimize errors due to the rise and fall
times of the converter output.
Issue 1 - January 2008
? Zetex Semiconductors plc 2008
12
www.zetex.com
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