MIC4802 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – High Efficiency 800mA Single Channel Linear WLED Driver with Ultra Fast PWM™ Control

Micrel Inc.
Figure 7. PWM Signal at 20% Duty Cycle (I_avg = 160mA)
MIC4802
Thermal Consideration
The MIC4802 thermal considerations involve calculating
the junction temperature based on the voltage drop
across the package and the LED current. The voltage
drop across the package is equal to the voltage at D1
with respect to ground times the LED current.
P_LOSS = I_LED * V_D1
The temperature rise (ΔT) is calculated:
ΔT = P_LOSS * θ_JA
Assuming the I_LED is 800mA and VD1 is 500mV at 20°C
room temperature, we can calculate the junction
temperature:
T_J = T_A + ΔT
T_J = 20°C + 0.4W * 41°C/W
T_J = 20°C + 16.4°C = 36.4°C
The junction temperature will be 36.4°C.
Figure 8. PWM Signal at 50% Duty Cycle (I_avg = 400mA)
Figure 9. PWM Signal at 80% Duty Cycle (I_avg = 640mA)
January 2011
9
M9999-013111-B