mic94041yfl Micrel Semiconductor, mic94041yfl Datasheet - Page 11

• Manufacturer Part Number: mic94041yfl
• Description: 28m? Rdson 3a High Side Load Switch In 1.2mm X 1.2mm Mlf Package
• Manufacturer: Micrel Semiconductor

Application Information
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
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 (Rq
P
the operating ratings section of the datasheet and Rq
(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).
Micrel, Inc.
May 2008
DISS
T
Vx
J
is calculated as I
= P
=
Isource
DISS
Figure 1. Simple Electrical Circuit
Figure 2. Simple Thermal Circuit
x (Rq
◊
(
Rxy
JC
o
C).
+ Rq
+
SWITCH
Ryz
CA
CA
) and the package (Rq
)
) + T
o
2
+
C/W) and voltage sources
x R
Vz
A
SWmax
. Rq
J
) from power
JC
is found in
JC
).
CA
11
Example:
A switch is intended to drive a 2A 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 50
Summary of variables:
The worst case switch resistance (R
V
lower value of V
If this were a figure for worst case R
additional consideration is to allow for the maximum
junction temperature of 125
resistance in this case can be 30% higher (See R
variance vs. temperature graph). However, 90mΩ is the
maximum over temperature.
Therefore:
This is below the maximum 125
IN
of 3V is not available in the datasheet, so the next
I
V
T
Rq
Rq
P
R
T
T
SW
o
A
J
J
IN
DISS
C.
SWmax
JC
CA
= 2
= 101
= 50
= 2A
= 3V to 4.2V
= 90
= 53
= I
2
o
Figure 3. Excerpt from the LDO Book
x 0.090 x (90+53) + 50
@ 2.5v = 90mW
C
SW
o
C
o
o
C/W from Datasheet
2
C/W Read from Graph in Figure 3
x R
IN
is used.
SWmax
o
C, the actual worst case
o
C.
2
). The Voltage source
SWmax
SWmax
M9999-053008-A
MIC94040/1/2/3
) at the lowest
for 25
o
C, an
DSON