MIC22400_11 Datasheet, PDF (18/28 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
Thermal Considerations
The MIC22400 is packaged in the MLF^® 3mm x 4mm, a
package that has excellent thermal performance
equaling that of the larger TSSOP packages. This
maximizes heat transfer from the junction to the exposed
pad (ePAD) which connects to the ground plane. The
size of the ground plane attached to the exposed pad
determines the overall thermal resistance from the
junction to the ambient air surrounding the printed circuit
board. The junction temperature for a given ambient
temperature can be calculated using:
T_J = T_AMB + P_DISS · Rθ_JA
where:
• P_DISS is the power dissipated within the MLF^®
package and is typically 0.89W at 3A load. This has
been calculated for a 1µH inductor and details can
be found in Table 3 for reference.
• Rθ_JA is a combination of junction to case thermal
resistance (Rθ_JC) and case-to-ambient thermal
resistance (Rθ_CA), since thermal resistance of the
solder connection from the ePAD to the PCB is
negligible; Rθ_CA is the thermal resistance of the
ground plane to ambient, so Rθ_JA = Rθ_JC + Rθ_CA.
V_OUT
@3AÈ
1
V_INÆ
1.2
1.8
2.5
3.3
3
0.732
0.741
0.825
0.894
–
3.5
0.689
0.691
0.764
0.813
0.817
4
0.672
0.668
0.732
0.776
0.816
4.5
0.668
0.662
0.720
0.762
0.801
5
0.670
0.665
0.720
0.765
0.800
Table 3. Power Dissipation (W) for 4A Output
MIC22400
• T_AMB is the Operating Ambient temperature.
Example:
To calculate the junction temperature for a 50°C
ambient:
T_J = T_AMB+P_DISS . Rθ_JA
T_J = 50 + 0.894 x 45
T_J = 90.2°C
This is below the maximum of 125°C.
June 2011
18
M9999-061511-F