MIC2871 Datasheet, PDF (18/24 Pages) Micrel Semiconductor – 1.2A High-Brightness LED Flash Driver with Single-Wire Serial Interface

Micrel, Inc.
MIC2871
Power Dissipation Consideration
As with all power devices, the ultimate current rating of the
output is limited by the thermal properties of the device
package and the PCB on which the device is mounted.
There is a simple, Ω’s law type relationship between
thermal resistance, power dissipation and temperature
which are analogous to an electrical circuit:
Now replacing the variables in Equation 2, we can find the
junction temperature (T_J) from the power dissipation,
ambient temperature and the known thermal resistance of
the PCB (θ_CA) and the package (θ_JC).
T_J = P_DISS × (θ_JC + θ_CA ) + T_A
Eq. 3
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 4:
T_J = P_DISS × (θ_JA ) + T_A
Eq. 4
Figure 5. Series Electrical Resistance Circuit
From this simple circuit we can calculate V_X if we know
I_SOURCE, V_Z and the resistor values, R_XY and R_YZ using
Equation 2:
V_X = I_SOURCE × (R _XY + R _YZ ) + V_Z
Eq. 2
Thermal circuits can be considered using this same rule
and can be drawn similarly by replacing current sources
with power dissipation (in watts), resistance with thermal
resistance (in °C/W) and voltage sources with temperature
(in °C).
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2871 package, P_DISS can be calculated thus:
Linear
Mode:
P_DISS
=
[P_OUT
×  ¹
^η
−
1]

−
I_OUT
2
× DCR
Eq. 5
2
Boost
Mode:
P_DISS
=
[P_OUT
×  ¹
^η
− 1] −  ^IOUT 
 ^1−D
× DCR
Eq. 6
Duty Cycle in Boost Mode: D = ^{VOUT − VIN}
V_OUT
Eq. 7
Figure 6. Series Thermal Resistance Circuit
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 datasheet.
May 29, 2013
18
052913-1.0 (while in progress)
Revision 1.0 (final document)