MIC9416X Datasheet, PDF (10/12 Pages) Micrel Semiconductor – 3A High-Side Load Switch with Reverse Blocking

Micrel, Inc.
MIC94161/2/3/4/5
junction temperature of 125°C, in this case can be 30%
higher (see the “On Resistance vs. Temperature” graph
in the Typical Characteristics section):
R_SW(MAX) = 27mΩ
Eq. 3
Figure 5. Simple Thermal Circuit
Now replacing the variables in the equation for V_X, one
can find the junction temperature (T_J) from power
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rθ_CA) and the package (Rθ_JC).
P_DISS is calculated as I_SW2 × R_SW(MAX). Rθ_JC is found in the
operating ratings section of the datasheet and Rθ_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.
Example:
A switch is intended to drive a 3A load and is placed on a
printed circuit board which has a ground plane area of at
least 25mm × 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
80°C.
Summary of variables:
• I_SW = 3A
• V_IN = 3V to 4.2V
• T_A = 80°C
• Rθ_JA = 108°C/W
• P_DISS = I_SW2 x R_SW
The worst case switch resistance (R_SW) at the lowest V_IN
of 3V is not available in the datasheet, so the next lower
value of V_IN is used, as shown in Equation 2
R_SW @ 2.7V = 21mΩ Eq. 2
Therefore:
T_RISE = (3A² × 27mΩ) × 108°C/W = 26.2°C
T_J = T_RISE + T_A = 26.2°C + 80°C = 106.2°C
This is below the maximum 125°C.
If this were a figure for worst case R_SW for 25°C, an
additional consideration is to allow for the maximum
March 14, 2014
10
Revision 1.1