MIC2870 Datasheet, PDF (20/25 Pages) Micrel Semiconductor – 1.5A Synchronous Boost Flash LED Driver with I2C Interface

Micrel, Inc.
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:
MIC2870
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. 4
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 5:
Figure 3. 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 3:
V_X = I_SOURCE × (R _XY + R _YZ ) + V_Z
Eq. 3
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).
T_J = P_DISS × (θ_JA ) + T_A
Eq. 5
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2870 package, P_DISS can be calculated thus:
Linear
Mode:
P_DISS
= [P_OUT
1
×
^η
−

1]

−
I_OUT
2
× DCR
Eq. 6
2
Boost
Mode:
P_DISS
=
[P_OUT
×  ¹
^η
− 1]

−
 ^IOUT
 ^{1− D}


× DCR
Eq. 7
Duty Cycle in Boost Mode: D = ^{VOUT − VIN}
V_OUT
Eq. 8
Figure 4. 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.
Where the real board area differs from 1” square, θ_CA (the
PCB thermal resistance) values for various PCB copper
areas can be taken from Figure 5. Figure 5 is taken from
Designing with Low Dropout Voltage Regulators available
from the Micrel website.
September 5, 2013
20
Revision 1.0