mic94044 Micrel Semiconductor, mic94044 Datasheet - Page 6

• Manufacturer Part Number: mic94044
• Description: 28m? Rdson 3a High Side Load Switch In 1.2mm X 1.2mm Mlf Package
• Manufacturer: Micrel Semiconductor

Application Information
Power Switch SOA
The safe operating area (SOA) curve represents the
boundary of maximum safe operating current and
maximum safe operating ambient temperature.
The curves above show the SOA for various values of
V
board.
Power Dissipation Considerations
As with all power switches, the current rating of the
switch is limited mostly 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 V
I
the equation:
Thermal circuits can be considered using these same
rules and can be drawn similarly by replacing current
sources with power dissipation (in Watts), resistance
with thermal resistance (in
with temperature (in
SOURCE
Micrel, Inc.
March 2011
IN,
mounted on a typical 1 layer, 1 square inch copper
, V
V
Z
X
I =
Figure 2: Simple Electrical Circuit
and the resistor values, R
SOURCE
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-40 -20
Figure 1: SOA Graph
Maximum Switch Current
vs. Ambient Temperature
o
(1" Square Copper)
C).
AMBIENT TEMPERATURE (°C)
0
V
(
R
IN
20
= 3.6V
XY
V
40
IN
+
o
= 2.5V
C/W) and voltage sources
R
60
YZ
V
IN
80 100 120 140
)
= 5V
+
V
V
IN
Z
= 4.5V
XY
and R
X
if we know
YZ
using
6
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
P
the operating ratings section of the datasheet and Rθ
(the PCB thermal resistance) values for various PCB
copper areas is discussed in the document “Designing
with Low Dropout Voltage Regulators” available from the
Micrel website (LDO Application Hints).
DISS
is calculated as I
T
J
= P
Figure 3: Simple Thermal Circuit
DISS
x (Rθ
SWITCH
JC
CA
+ Rθ
) and the package (Rθ
2
x R
CA
) + T
SWmax
A
. Rθ
M9999-032311-A
J
) from power
MIC94044/5
JC
is found in
JC
X
).
, we
CA