MIC33030-4YHJ MICREL [Micrel Semiconductor], MIC33030-4YHJ Datasheet - Page 13

• Manufacturer Part Number: MIC33030-4YHJ
• Description: 8MHz 400mA Internal Inductor Buck Regulator with HyperLight Load
• Manufacturer: MICREL [Micrel Semiconductor]

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:
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
with temperature (in
Now replacing the variables in the equation for Vx, we
can find the junction temperature (T
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rθ
As can be seen in the diagram, total thermal resistance
Rθ
Micrel Inc.
February 2011
JA
= Rθ
Vx
T
T
J
JC
J
=
=
=
+ Rθ
P
P
Isource
DISS
DISS
CA
⋅
⋅
. Hence this can also be written:
(
(
R
R
º
C):
⋅
θ
θ
(
Rxy
JC
JA
+
)
CA
+
+
R
) and the package (Rθ
T
Ryz
θ
º
AMB
C/W) and Voltage sources
CA
)
)
+
+
Vz
T
AMB
J
) from power
JC
):
13
Since effectively all of the power loss in the converter is
dissipated within the MIC33030 package, P
calculated thus:
Where
Rθ
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
Summary of variables:
I
V
V
T
Rθ
η
Typical Characteristics Efficiency vs. Load graphs)
The worst case switch and inductor resistance will
increase at higher temperatures, so a margin of 20% can
be added to account for this:
Therefore:
T
T
This is well below the maximum 125
OUT
AMB
J
J
OUT
IN
@ 300mA = 75% (worst case with V
= 0.216W. (76
= 66
JC
JA
= 3V to 4.2V
= 0.3A
= 1.8V
= 50
= 76
and Rθ
º
η
C
P
P
P
= Efficiency taken from efficiency curves
º
º
DISS
DISS
DISS
C
C/W from Datasheet
JA
= 0.18 x 1.2 = .216W
=
=
are found in the operating ratings section
P
1
.
OUT
º
8
C/W) + 50
⋅
0
⋅
.
3
(
η
1
⋅
(
−
0
º
C.
1
.
1
75
)
º
C
−
1
)
= 0.18W
º
C.
M9999-020311-C
IN
=4.2V from the
MIC33030
DISS
can be