MIC33030 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – 8MHz 400mA Internal Inductor Buck Regulator with HyperLight Load

Micrel Inc.
MIC33030
Power Dissipation Considerations
As with all power devices, the ultimate current rating of
the output is limited by the thermal properties of the
package and the PCB it is mounted on. There is a
simple, ohms law type relationship between thermal
resistance, power dissipation and temperature which are
analogous to an electrical circuit:
From this simple circuit we can calculate Vx if we know
Isource, Vz and the resistor values, Rxy and Ryz using
the equation:
Vx = Isource ⋅ (Rxy + Ryz) + Vz
Thermal circuits can be considered using these same
rules and can be drawn similarly replacing current
sources with Power dissipation (in Watts), Resistance
with Thermal Resistance (in ^ºC/W) and Voltage sources
with temperature (in ^ºC):
Now replacing the variables in the equation for Vx, we
can find the junction temperature (T_J) from power
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rθ_CA) and the package (Rθ_JC):
( ) T_J = P_DISS ⋅ Rθ_JC + Rθ_CA + T_AMB
As can be seen in the diagram, total thermal resistance
Rθ_JA = Rθ_JC + Rθ_CA. Hence this can also be written:
( ) T_J = P_DISS ⋅ Rθ _JA + T_AMB
Since effectively all of the power loss in the converter is
dissipated within the MIC33030 package, P_DISS can be
calculated thus:
P_DISS
= P_OUT
⋅(¹
η
− 1)
Where η = Efficiency taken from efficiency curves
Rθ_JC and Rθ_JA are found in the operating ratings section
of the datasheet.
Example:
A MIC33030 is intended to drive a 300mA load at 1.8V
and is placed on a printed circuit board which has a
ground plane area of at least 25mm square. The Voltage
source is a Li-ion battery with a lower operating
threshold of 3V and the ambient temperature of the
assembly can be up to 50^ºC.
Summary of variables:
I_OUT = 0.3A
V_OUT = 1.8V
V_IN = 3V to 4.2V
T_AMB = 50^ºC
Rθ_JA = 76^ºC/W from Datasheet
η @ 300mA = 75% (worst case with V_IN=4.2V from the
Typical Characteristics Efficiency vs. Load graphs)
P_DISS
= 1.8 ⋅ 0.3 ⋅ ( ¹ − 1)
0.75
= 0.18W
The worst case switch and inductor resistance will
increase at higher temperatures, so a margin of 20% can
be added to account for this:
P_DISS = 0.18 x 1.2 = .216W
Therefore:
T_J = 0.216W. (76 ^ºC/W) + 50^ºC
T_J = 66^ºC
This is well below the maximum 125^ºC.
February 2011
13
M9999-020311-C