MIC2874 Datasheet, PDF (18/22 Pages) Micrel Semiconductor – 1.2A High-Brightness Flash LED Driver with Single-Wire Serial Interface

Micrel, Inc.
MIC2874
T_J = P_DISS × (θ_JC + θ_CA ) + T_A
Eq. 2
As can be seen in the diagram, total thermal resistance
θ_JA = θ_JC + θ_CA. Hence this can also be written as in
Equation 3:
Where the real board area differs from 1 inch square, θ_CA
(the PCB thermal resistance) values for various PCB
copper areas can be taken from Figure 7. Figure 7 is taken
from Designing with Low Dropout Voltage Regulators
available from the Micrel website (“LDO Application
Hints”).
T_J = P_DISS × (θ_JA ) + T_A
Eq. 3
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2874 package, P_DISS can be calculated thus:
Linear Mode:
P_DISS
=
[P_OUT
×  ¹
^η
−
1]

−
I_OUT
2
× DCR
Boost Mode:
2
P_DISS
=
[P_OUT
× 

1
η
− 1]

−
 ^IOUT
 ^{1− D}


× DCR
Eq. 4
Duty Cycle in Boost Mode:
D = ^{VOUT − VIN}
V_OUT
Eq. 5
Eq. 6
where:
η = Efficiency taken from efficiency curves and DCR =
inductor DCR. θ_JC and θ_JA are found in the operating
ratings section of the data sheet.
Figure 7. Graph to Determine PC Board Area for a Given
PCB Thermal Resistance
Figure 7 shows the total area of a round or square pad,
centered on the device. The solid trace represents the
area of a square, single sided, in horizontal orientation,
solder masked, copper PC board trace heat sink,
measured in square millimeters. No airflow is assumed.
The dashed line shows PC boards trace heat sink covered
in black oil-based paint and with 1.3m/sec (250 feet per
minute) airflow. This approaches a “best case” pad heat
sink. Conservative design dictates using the solid trace
data, which indicates that a maximum pad size of 5000
mm² is needed. This is a pad 71mm by 71mm (2.8 inches
per side).
August 26, 2014
18
Revision 1.0