OSRAM OPTOTRONIC Constant current LED power supplies with 3DIM - Manual (Page 43)

43
Note:
The DAC interface can be implemented in various ways. The only
thing to take into account is that the DAC circuit must be able to
sink the Iset current (274 µA) imposed by the Vset connection.
In the MCU memory (ROM or EEPROM if present), the NTC
divider characteristic can be stored as a lookup table. Here
are some notes on this:
• The higher the number of the stored points, the more ac-
curate is the output of the measurement.
• If the measured temperature is between two points, an in-
terpolation can be used.
• The number of lookup table points should be higher in the
region of interest (i.e. if the overtemperature is 70 °C, the
number of points should be higher between 60 and 80 °C).
• A higher AD resolution improves the accuracy of the mea-
surement but also affects the memory resources.
• A little digital fi lter should be applied on the AD raw values
if no external hardware fi lter is used on this input (RC fi lter).
Once the fi ltered AD
1
count value is translated into a tempera-
ture value by the stored temperature lookup table, the Vset
reference value must be generated. For converting the tem-
perature value into a Vset value, another lookup table (e.g.
Vset lookup table) has to be used.
The output result is used as a reference to generate the duty
cycle of the PWM channel corrected by the feedback com-
ing from the AD
2
channel.
On the output side, the PWM output used to generate the
Vset is connected to a fi lter (active or passive) to rectify the
signal. In the example (fi gure 42), only a general DAC inter-
face block is shown. DAC output is sensed by an A/D chan-
nel of the MCU, thus controlling the Vset output in a closed-
loop system, which ensures the highest accuracy of the output.
LEDset APPLICATIONS
Figure 42: Thermal derating – MCU solution.
A simple voltage regulator is required to regulate the +12Vset to 5 V which
are needed to supply the MCU.
The voltage divider R
2
/NTC is connected to the same 5 V supply as the MCU.
The middle point of the divider is connected to an A/D channel of the MCU.