MIC2873 Datasheet, PDF (18/24 Pages) Micrel Semiconductor – 1.2A High-Brightness Flash LED Driver with Single-Wire Serial Interface

Micrel, Inc.
MIC2873
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, Ohm’s law type relationship between
thermal resistance, power dissipation and temperature
which are analogous to an electrical circuit:
Now replacing the variables in the equation for V_X, 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. 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:
T_J = P_DISS × (θ_JA ) + T_A
Eq. 3
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 1:
V_X = I_SOURCE × (R _XY + R _YZ ) + V_Z
Eq. 1
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2873 package, P_DISS can be calculated thus:
Linear Mode:
P_DISS
=
[P_OUT
1
×
^η

− 1]

− I_OUT²
× DCR
Eq. 4
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).
Boost Mode:
2
P_DISS
=
[P_OUT
×
 ¹
^η
− 1]

−
 ^IOUT
 ^{1− D}


× DCR
Eq. 5
Duty Cycle in Boost Mode:
D = ^{VOUT − VIN}
V_OUT
Eq. 6
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 data sheet.
July 17, 2014
18
Revision 1.0