mic4801 Micrel Semiconductor, mic4801 Datasheet - Page 9

• Manufacturer Part Number: mic4801
• Manufacturer: Micrel Semiconductor

Micrel Inc.
January 2011
Figure 7. PWM Signal at 20% Duty Cycle (I
Figure 8. PWM Signal at 50% Duty Cycle (I
Figure 9. PWM Signal at 80% Duty Cycle (I
avg
avg
avg
= 120mA)
= 300mA)
= 480mA)
9
Thermal Consideration
The MIC4801 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
The temperature rise (ΔT) is calculated:
ΔT = P
Assuming the I
room temperature, we can calculate the junction
temperature:
T
T
T
The junction temperature will be around 49.7°C.
J
J
J
LOSS
= T
= 20°C + 0.3W * 98.9°C/W
= 20°C + 29.7°C = 49.7°C
= I
A
LOSS
+ ΔT
LED
* θ
* V
JA
D1
LED
is 600mA and V
D1
is 500mV at 20°C
M9999-011811-A
MIC4801