Power dissipation
AN2531
12/40
5 Power
dissipation
The maximum power dissipation can be calculated with ambient temperature and thermal
resistance of the chip. The thermal resistance depends on the type of package and can be
found together with maximum junction temperature in the datasheet. The maximum
allowable power consumption without a heatsink is calculated as follows:
Equation 6
P
d max
……. maximum power dissipation [W]
T
a
……….…. ambient temperature [°C]
T
j max
……... maximum junction temperature [°C]
R
thja
………. junction to ambient thermal resistance [°C/W].
A high power RGB LED is in fact driven in linear mode with STP LED driver family. The
current flowing through each channel of the LED driver is constant and so power dissipation
depends on the voltage on each channel, which is the difference between the supply voltage
(DC bus) and the forward voltage drop on the LEDs. Therefore it is recommended to keep
the supply voltage as low as possible, but always above the maximum LED forward voltage.
shows the RGB LED connection to the driver. Total power dissipation in this case is
calculated using the following equation:
Equation 7
P
tot
……….…….power dissipation on chip [W]
I…………………constant LED current set by external resistor [A]
V
c
………………LED supply voltage [V]
V
f_red
………….red LED forward voltage [V]
V
f_blue
….…….blue LED forward voltage [V]
V
f_green
……....green LED forward voltage [V].
thja
a
jmax
dmax
R
T
–
T
P
=
(
) (
)
(
)
f_green
C
f_blue
C
f_red
C
tot
V
–
V
I
*
2
V
–
V
*
I
V
–
V
*
I
P
+
+
=
