mic33153yhj Micrel Semiconductor, mic33153yhj Datasheet - Page 13

• Manufacturer Part Number: mic33153yhj
• Description: 4mhz Pwm 1.2a Internal Inductor Buck Regulator With Hyperlight Load And Power Good
• Manufacturer: 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, Ohm’s law type of relationship between thermal
resistance, power dissipation and temperature which is
analogous to an electrical circuit:
From this simple circuit, one can calculate V
knows I
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 V
can find the junction temperature (T
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rθ
Micrel Inc.
September 2010
SOURCE
V
T
J
X
=
=
P
I
, V
DISS
SOURCE
Z
×
and the resistor values, R
(
º
Rθ
C):
×
JC
(
R
CA
+
XY
) and the package (Rθ
Rθ
º
C/W) and voltage sources
+
CA
R
YZ
)
+
)
T
+
AMB
V
Z
J
) from power
XY
X
and R
JC
X
if one
, one
):
YZ
13
As can be seen in the diagram, total thermal resistance
Rθ
Since effectively all of the power loss in the converter is
dissipated within the MIC33153 package, P
calculated thus:
Where:
η
Rθ
of the datasheet.
Example:
A MIC33153 is intended to drive a 1A 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
= Efficiency taken from efficiency curves
@ 1A = 80% (worst case with V
= 0.54W x (55
= 79.7
JA
JC
JA
= 3V to 4.2V
= 1A
º
= Rθ
= 1.8V
= 50
= 55
and Rθ
C.
P
P
P
T
º
º
º
DISS
C
DISS
DISS
JC
J
C
C/W from Datasheet
=
+ Rθ
JA
P
= 0.45 x 1.2 = .54W
DISS
=
=
are found in the operating ratings section
1.8
P
CA
º
C/W) + 50
OUT
. Hence this can also be written:
×
⋅
1
(
Rθ
×
×
(
(
0.80
JA
1
η
1
−
)
+
1)
º
C
−
T
1)
AMB
= 0.45W
º
C.
IN
M9999-092910-A
=4.2V from the
MIC33153
DISS
can be