MIC9404X Datasheet, PDF (11/13 Pages) Micrel Semiconductor – 28mΩ RDSON 3A High Side Load Switch in 1.2mm x 1.2mm MLF® package

Micrel, Inc.
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:
Figure 1. Simple 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:
Vx = Isource ◊ (Rxy + Ryz)+ Vz
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).
MIC94040/1/2/3
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 (625mm²). 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^oC.
Summary of variables:
I_SW = 2A
V_IN = 3V to 4.2V
T_A = 50^oC
Rq_JC = 90^oC/W from Datasheet
Rq_CA = 53^oC/W Read from Graph in Figure 3
Figure 2. Simple Thermal Circuit
Now replacing the variables in the equation for Vx, we
can find the junction temperature (T_J) from power
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rq_CA) and the package (Rq_JC).
T_J = P_DISS x (Rq_JC + Rq_CA) + T_A
P_DISS is calculated as I_SWITCH2 x R_SWmax. Rq_JC is found in
the operating ratings section of the datasheet and Rq_CA
(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).
Figure 3. Excerpt from the LDO Book
P_DISS = I_SW2 x R_SWmax
The worst case switch resistance (R_SWmax) at the lowest
V_IN of 3V is not available in the datasheet, so the next
lower value of V_IN is used.
R_SWmax @ 2.5v = 90mW
If this were a figure for worst case R_SWmax for 25^oC, an
additional consideration is to allow for the maximum
junction temperature of 125^oC, the actual worst case
resistance in this case can be 30% higher (See R_DSON
variance vs. temperature graph). However, 90mΩ is the
maximum over temperature.
Therefore:
T_J = 2² x 0.090 x (90+53) + 50
T_J = 101^oC
This is below the maximum 125^oC.
May 2008
11
M9999-053008-A