MIC22405 Datasheet, PDF (18/30 Pages) Micrel Semiconductor – 4A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Figure 6. EN Turn-Off − 200mA Load
Current Limit
The MIC22405 uses a two-stage technique to protect
against overload. The first stage is to limit the current in
the P-channel switch; the second is over temperature
shutdown.
Current is limited by measuring the current through the
high-side MOSFET during its power stroke and
immediately switching off the driver when the preset limit
is exceeded.
The circuit in Figure 7 describes the operation of the
current limit circuit. Since the actual RDS_ON of the P-
channel MOSFET varies from part to part, and with
changes in temperature and with input voltage, simple IR
voltage detection is not employed. Instead, a smaller
copy of the Power MOSFET (Reference FET) is fed with
a constant current which is a directly proportional to the
factory set current limit. This sets the current limit as a
current ratio and thus, is not dependant upon the RDS_ON
value. Current limit is set to nominal value. Variations in
the scale factor K between the power PFET and the
reference PFET used to generate the limit threshold
account for a relatively small inaccuracy.
MIC22405
Thermal Considerations
The MIC22405 is packaged in a 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 at 4A 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 55°C/W. The
worst case Rθ_JC of the MLF 3mmx4mm is 25^oC/W.
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 25 + 30 = 55^oC/W
To calculate the junction temperature for a 50°C
ambient:
T_J = T_AMB+P_DISS . Rθ_JA
T_J = 50 + (0.89 x 55)
T_J = 98.95°C
This is below the maximum of 125°C.
Figure 7. Current-Limit Detail
June 2011
18
M9999-061511-A