Zxld1322 – Diodes ZXLD1322 User Manual
Page 11
ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
11 of 17
April 2010
© Diodes Incorporated
A Product Line of
Diodes Incorporated
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
ZXTN25015DFL. (Fig 2).
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
A PWM frequency of 200Hz, or lower is recommended, to minimize errors due to the rise and fall
times of the converter output.
Thermal Compensation of LED Current
High-luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and
reliable operation at high temperatures. This is usually achieved by reducing the LED current proportionally from its nominal
set value when the LED temperature rises above a predefined threshold. (Fig.3)
Fig. 3
The 'Thermal compensation current' generator inside the ZXLD1322 provides the necessary thermal compensation current
to meet this requirement, using an NTC thermistor and resistor. (Fig 4)
Fig. 4
ZXTN25015DFL