mic94082yft Micrel Semiconductor, mic94082yft Datasheet - Page 13

• Manufacturer Part Number: mic94082yft
• Description: 67m? Rdson 2a High Side Load Switch In 0.85mm X 0.85mm Thin 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 junction temperature.
The curves above show the SOA for various V
mounted on a typical 1 layer, 1 square inch copper
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 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 by replacing current
sources with power dissipation (in Watts), resistance
with thermal resistance (in ºC/W) and voltage sources
with temperature (in ºC).
Micrel, Inc.
December 2008
Vx
=
Isource
Figure 2. Simple Electrical Circuit
⋅
(
Rxy
Figure 1. SOA Graph
+
Ryz
)
+
Vz
IN
’s
13
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
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).
Example:
A switch is intended to drive a 1A load and is placed on
a printed circuit board which has a ground plane area of
at least 25mm by 25mm (625mm
is a Li-ion battery with a lower operating threshold of 3V
and the ambient temperature of the assembly can be up
to 50ºC.
Summary of variables:
DISS
T
I
V
T
Rθ
Rθ
SW
J
A
IN
is calculated as I
= P
JC
CA
= 50
= 1A
= 3V to 4.2V
= 85ºC/W
= 53ºC/W Read from Graph in Figure 4
DISS
Figure 4. Excerpt from the LDO Book
o
C
Figure 3. Simple Thermal Circuit
x (Rθ
JC
+ Rθ
SWITCH
CA
CA
) and the package (Rθ
) + T
2
x R
A
2
SWmax
). The Voltage source
MIC94080/1/2/3/4/5
M9999-120208-B
. Rθ
J
) from power
JC
is found in
JC
).
CA