Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 32
ZXLD1374
Document number: DS35032 Rev. 3 - 2
32 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Thermal Control of LED Current
For thermal control of the LEDs, the ZXLD1371 monitors the voltage on the TADJ pin and reduces output current if the voltage on this pin falls
below 625mV. An external NTC thermistor and resistor can therefore be connected as shown below to set the voltage on the TADJ pin to
625mV at the required temperature threshold. This will give 100% LED current below the threshold temperature and a falling current above it as
shown in the graph. The temperature threshold can be altered by adjusting the value of Rth and/or the thermistor to suit the requirements of the
chosen LED.
The Thermal Control feature can be disabled by connecting T
ADJ
to REF.
Here is a simple procedure to design the thermal feedback circuit:
1. Select the temperature threshold T
THRESHOLD
at which the current must start to decrease
2. Select the Thermistor TH1 (both resistive value at +25°C and beta)
3. Select the value of the resistor R
TH
as R
TH
= TH1 at T
THRESHOLD
Figure 43 Thermal Feedback Network
For example,
1.
Temperature threshold T
THRESHOLD
= +70°C
2.
TH1 = 10kΩ at 25˚C and beta= 3500 Æ TH1 = 3.3kΩ at +70°C
3. R
TH
= TH1 at T
THRESHOLD
= 3.3kΩ