Diodes ZXLD1320 - Manual (Page 13)

ZXLD1320
Issue 1 - January 2008 13 www.zetex.com
© Zetex Semiconductors plc 2008
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 ZXLD1320 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 (V
TADJ
) on the TADJ pin is held above the threshold, the
thermal compensation current will be zero and no thermal compensation is applied. However, if
V
TADJ
falls below the threshold, a thermal compensation current (ITC) is produced that is
proportional to V
TADJ
. I
TC
is injected into the control loop in such a way as to reduce the demand
current I
ADJ
, causing the control loop to decrease the LED current. The LED current will be
reduced to less than 10% of the set value when V
TADJ
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.
I
LED
LED temperature