MIC22205 Datasheet, PDF (19/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
Figure 7. Current-Limit Detail
Thermal Considerations
The MIC22205 is packaged in a MLF^® 3mm x 3mm – a
package that has excellent thermal-performance
equaling that of 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 at 2A load. 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.
• T_AMB is the operating ambient temperature.
Example:
The Evaluation board has two copper planes
contributing to an Rθ_JA of approximately 40ºC/W. The
worst case Rθ_JC of the MLF 3mm x 3mm is 28.7ºC/W.
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 28.7 + 11.3 = 40^ºC/W
MIC22205
To calculate the junction temperature for a 50°C
ambient:
T_J = T_AMB+P_DISS . Rθ_JA
T_J = 50 + (0.72 x 40)
T_J = 78.8°C
This is below the maximum of 125°C.
Thermal Measurements
Measuring the IC’s case temperature is recommended to
ensure it is within its operating limits. The most common
mistake made is to use the standard thermal couple that
comes with a thermal meter. This thermal couple wire
gauge is large, typically 22-gauge, and behaves like a
heat-sink, resulting in a lower case measurement.
Two better methods of temperature measurement are
using a smaller thermal couple wire or an infrared
thermometer. If a thermal couple wire is used, it must be
constructed of 36-gauge wire or higher (smaller wire
size) to minimize the wire heat-sinking effect. In addition,
the thermal couple tip must be covered in either thermal
grease or thermal glue to ensure the thermal couple
junction makes good contact with the case of the IC.
Omega brand thermal couple (5SC-TT-K-36-36) is
adequate for most applications.
Whenever possible, an infrared thermometer is
recommended. The measurement spot size of most
infrared thermometers is too large for an accurate
reading on a small form factor ICs. However, an IR
thermometer from Optris has a 1mm spot size, which
makes it a good choice for measuring the hottest point
on the case. Using a stand makes it easier to hold the
beam on the IC for long periods of time.
Sequencing and Tracking
There are four variations of sequencing and tracking that
are easily implemented using the MIC22205. The two
sequencing variations are Delayed and Windowed. The
two tracking variants are Normal and Ratio Metric. The
following diagrams illustrate methods for connecting two
MIC22205’s to achieve these requirements.
August 2011
19
M9999-082511-A