ZXLD1322

ZXLD1322

Document number: DS32166 Rev. 3 - 2

11 of 17

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April 2010

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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:

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