MIC22950 Datasheet, PDF (16/22 Pages) Micrel Semiconductor – 10A Integrated Switch Synchronous Buck Regulator with Frequency Programmable to 2MHz

Micrel, Inc.
Current Limit
The MIC22950 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 4 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 I.R 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.
Figure 4. Current Limit Detail
Thermal Considerations
The MIC22950 is packaged in the MLF^® 5mm x 5mm, 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_A + P_DISS × Rθ_JA
MIC22950
Where:
P_DISS is the power dissipated within the MLF^® package
and is typically 1.5W at 10A load. This has been
calculated for a 1µH inductor and details can be found in
Table 1.
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_A is the Operating Ambient temperature.
V_IN
V_OUT
@ 10A
1
1.2
1.8
2.5
3.3
3
3.5
4
4.5
5
1.66 1.67 1.68 1.7 1.73
1.68 1.69 1.71 1.72 1.74
1.76 1.76 1.77 1.78 1.8
1.85 1.84 1.84 1.85 1.86
-
1.92 1.91 1.91 1.92
Table 1. Power Dissipation (W) for 10A Output
Example
The Evaluation board has two copper planes
contributing to an Rθ_JA of approximately 25^oC/W. The
worst case Rθ_JC of the MLF^® is 11^oC/W. If we look at a
typical application of 3.6V to 1.8V @ 10A, the estimated
Power dissipation in the MLF^® package taken from Table
1 will be 1.76W:
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 11 + 25 = 36^oC/W
To calculate the junction temperature for a 50^oC
ambient:
T_J = T_A+P_DISS. Rθ_JA
T_J = 50 + (1.76 x 36)
T_J = 113^oC
This is below our maximum of 125^oC.
February 2010
16
M9999-021910-B