MIC22705_11 Datasheet, PDF (18/29 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Figure 6. EN Turn-Off − 200mA Load
Current Limit
The MIC22705 is protected against overload in two
stages. The first 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 part-to-part, over 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.
MIC22705
Thermal Considerations
The MIC22705 is packaged in a MLF^® 4mm 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 7A 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 25°C/W. The worst case
Rθ_JC of the MLF^® 4mm x 4mm is 14^oC/W.
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 14 + 25 = 39°C/W
To calculate the junction temperature for a 50°C
ambient:
T_J = T_AMB + P_DISS × Rθ_JA
T_J + 50 + (1.8 × 39)
T_J = 120°C
Figure 7. Current-Limit Detail
March 2011
18
M9999-033111-A