MIC22700_10 Datasheet, PDF (16/21 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Current Limit
The MIC22700 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 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.
Figure 4. Current Limit Detail
Thermal Considerations
The MIC22700 is packaged in the 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
MIC22700
Where:
• P_DISS is the power dissipated within the MLF^®
package and is typically 1.5W at 7A load. This
has been calculated for a 1µH inductor and
details can be found in table 1 below 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.
VINÆ
VOUT
@7AÈ
0.7V
1.2V
1.8V
2.5V
3.3V
2.6V
2.073
2.133
2.207
—
—
3.3V
1.884
1.896
1.934
1.953
—
3.6V
1.836
1.853
1.873
1.881
1.423
4.5V
1.784
1.786
1.814
1.797
1.79
5.5V
1.791
1.796
1.826
1.803
1.789
Table 1. Power Dissipation (W) for 7A output
• 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 4x4 is 14^oC/W.
Rθ_JA = Rθ_JC + Rθ_CA
Rθ_JA = 14 + 25 = 39^oC/W
To calculate the junction temperature for a 50°C
ambient:
T_J = T_AMB+P_DISS . Rθ_JA
T_J = 50 + (1.5 x 39)
T_J = 109°C
This is below the maximum of 125°C.
March 2010
16
M9999-031810-B