Diodes ZXLD1370 - Manual (Page 29)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
29 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Thermal Control of LED Current
For thermal control of the LEDs, the ZXLD1370 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 TADJ directly 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
= TH at T
threshold
Figure 34 Thermal Feedback Network
The thermistor resistance, R
T
, at a temperature of T degrees Kelvin is given by
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
T
e R R
R
1
T
1
B
R T
Where
R
R
is the thermistor resistance at the reference temperature, T
R
T
R
is the reference temperature, in Kelvin, normally 273 + 25 = 298K (+25°C)
B is the “beta” value of the thermistor.
For example,
1) Temperature threshold T
threshold
= 273 + 70 = 343K (+70°C)
2) TH1 = 10kΩ at +25°C and B = 3900 Æ R
T
= 1.8kΩ @ +70°C
3) R
th
= R
T
at T
threshold
= 1.8kΩ
Over-Temperature Shutdown
The ZXLD1370 incorporates an over-temperature shutdown circuit to protect against damage caused by excessive die temperature. A warning
signal is generated on the STATUS output when die temperature exceeds +125°C nominal and the output is disabled when die temperature
exceeds +150°C nominal. Normal operation resumes when the device cools back down to +125°C.