ZXLD1321 datasheet(13/20 Pages) DIODES

13 / 20 page

ZXLD1321

Issue 1 - January 2008

www.zetex.com

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 ZXLD1321 provides the necessary thermal

compensation current to meet this requirement, using an NTC thermistor and resistor. (Fig 4)

Fig 4

The TADJ pin of the device has a voltage threshold of 75mV nominal, which is derived from the

reference voltage VREF. If the voltage (VTADJ) on the TADJ pin is held above the threshold, the

thermal compensation current will be zero and no thermal compensation is applied. However, if

VTADJ falls below the threshold, a thermal compensation current (ITC) is produced that is

proportional to VTADJ. ITC is injected into the control loop in such a way as to reduce the demand

current IADJ, causing the control loop to decrease the LED current. The LED current will be

reduced to less than 10% of the set value when VTADJ falls below 50mV.

The threshold voltage has been chosen to set a nominal threshold of 105

°C and the device has

been optimized to operate with a standard 103KT1608 thermistor and 5k resistor in the potential

divider. Circuit details are given in the application notes. Alternative thermistor/resistor networks

can be used providing the input resistance presented to the device at the TADJ pin is similar at

the threshold temperature. If no LED thermal compensation is required, the TADJ pin should be

connected to VREF to disable this function.

LED

LED temperature

ZXLD1321 Datasheet(PDF) 13 Page - Diodes Incorporated