TLE4309 Datasheet, PDF (10/12 Pages) Infineon Technologies AG – Adjustable Linear Low Dropout LED Driver

TLE4309
4. Calculating the maximum power dissipation:
For calculating the maximum power dissipation, the minimum forward voltage sum of the
LEDs connected in series V_F,MIN,total (= No. of LEDs * V_F,MIN) , the maximum supply
voltage V_I,MAX , the IC current consumption I_q,MAX , as well as the the maximum output
current I_Q,MAX needs to be considered. Hence, the maximum power dissipation P_D,MAX
calculates:
P_D,MAX = (V_I,MAX - V_F,MIN,total) * I_Q,MAX + V_I,MAX * I_q,MAX
With assuming a maximum current consumption of 15 mA @ I_Q = 260 mA, the
maximum power dissipation for the example is 1.28 W.
5. Thermal Resistance needed:
The thermal resistance from junction to ambient R_th,j-a calculates:
R_th,j-a = (T_j,max - T_a,max) / P_D,MAX
With allowing a junction temperature of 150 °C, the R_th,j-a needed in our example would
be 50 K/W.
In case some copper area on the PCB is used as a heat sink, the area needed is
approximately 6 cm² (board in horizontal position, no airflow). If the area is not available,
several via holes to the GND-layer or to a heatsink area on the PCB backside help to
distribute the heat.
For additional information on the thermal resistance see Infineon’ s special subject book
“Thermal Resistance - Theory and Practice” including extended package information.
Datasheet
10
Rev. 1.0, 2007-03-20