OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 41)

41
For this kind of application, the choice of the NTC is funda-
mental to meet the requirements in terms of the Tset
TH
(temperature from which to start the derating) and the slope
rate (based on the NTC parameters B25/85 and B25/100) of
the characteristic Iout vs. Tset above the Tset
TH
. For the fi ne
tuning, the Rp resistor allows changing the Tset
TH
point to
reach the temperature required to start the derating.
Example:
The following example shows the result that can be achieved
by using a standard SMD NTC component from a well-known
NTC thermistor producer.
LEDset APPLICATIONS
Figure 41: Thermal derating – direct NTC connection (example).
Rs = 0 Ω (short circuit only)
R
NTC
= NCP15WM474J03RC
(470 kΩ 3 % B25/85 = 4582 - 0805 - Murata)
Two curves are shown to highlight the effect of the Rp resistor on the
determination of the Tset
TH
. The following Rp values have been used:
Rp
1
= 270 kΩ 0603 1 % (general purpose)
Rp
2
= 120 kΩ 0603 1 % (general purpose)