ST STEVAL-ILL009V1, Application Note, Page: 23 / 40

AN2531 STEVAL-ILL009V1 reference board
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8.5 Design calculation
8.5.1 LED supply voltage
In order to have low power dissipation on STP04CM596 LED drivers it was chosen to have
LED supply voltage 4 V. The maximum current flowing through LEDs is 2.8 A (0.35 A x 8).
Therefore L4973D3.3 DC-DC converter with output power capability 12 W - 4 V at 3 A was
designed. The output voltage is calculated in
Equation 8 :
Equation 8
Where:
V
F
……. Converter feedback input - > 3.3 V
V
d
…… LED supply voltage --> 4 V
From Equation 9 below resulting R9 = 1300
Ω
(R11 is chosen 6.2 k
Ω
)
Equation 9
8.5.2 Temperature protection
Using STLM20 temperature sensor
The STLM20 is a precise analog temperature sensor for low current applications. It operates
over a –55 to 130 °C (Grade 7) or –40 to 85 °C (Grade 9) temperature range. The power
supply operating range is 2.4 to 5.5 V. The accuracy of the STLM20 is ± 1.5 °C, at an
ambient temperature of 25 °C. More information about the STLM20 is described in the
datasheet (refer to
Chapter 12 , point 9 ).
A simple linear transfer function, with good accuracy near 25 °C is expressed as:
Equation 10
If the sensor temperature is 50 °C, the output voltage is 1.263 V (resulting from
Equation 10 ). This analog voltage is then sensed by the 8-bit ADC with an input voltage
range 0 to 5 V inside the microcontroller. This number is used by software to limit the
temperature. The software also includes the table with pre-calculated integer numbers for
temperatures of 60, 70 and 80 °C and so it is very easy to change temperature limits (see
Table 2 ).
Note: Temperature limit set to 50 °C was chosen in order to demonstrate temperature limitation
feature (it takes a long time to heat LEDs assembled on heatsink to high temperature). In
final application higher temperature limit can be set according the LEDs used and their
maximum operating temperature.
9 11
11
d F
R R
R
V V
+
=
1.3K
Ω
3.3
3.3 – 4
6.2
V
V – V
R R
F
F d
11 9
= × = =
1.263V 1.8528 50 10 –11.79 1.8528V T C –11.79mV/ V
–3
o
= + × × = + × ° =